Dcn lecture1 01 (11 files merged) (2 files merged).pdf

Data Communication and Networks By Malashri S TextBook : Data Communications and Networks ,By Achyut S Godbole, Atul Kahate Unit 1: Introduction to Data Communication and networking  What is a network? A network is a set of devices(often refered to as nodes) connected by co...

Data Communication and Networks By Malashri S TextBook : Data Communications and Networks ,By Achyut S Godbole, Atul Kahate Unit 1: Introduction to Data Communication and networking  What is a network? A network is a set of devices(often refered to as nodes) connected by communication links. A node can be a computer , printer or any other device capable of sending and/ or receiving data in the network.  What is data communication? The word data refers to information presented in any form agreed upon by parties creating and using it. Communication is exchange of information between two humans. Data communication means exchange of information between two or more computers.  What is an IP Address? An IP address is a unique address that identifies a device on the internet. IP addresses are the identifier that allows information to be sent between devices on a network: they contain location information and make devices accessible for communication An IP address is a 32-bit address. The IP addresses are unique. The address space of IPv4 is 232 or 4,294,967,296. Dotted-decimal notation What is MAC address?  A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment.  A network interface card (NIC) is a hardware component without which a computer cannot be connected over a network. It is a circuit board installed in a computer that provides a dedicated network connection to the computer. It is also called network interface controller, network adapter or LAN adapter.  A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment.  A MAC address is given to a network adapter when it is manufactured. It is hardwired or hard-coded onto your computer’s network interface card (NIC) and is unique to it.  MAC addresses are stored in PROM and its address length is 48bits  The MAC address is a 12 digit hexadecimal number that is most often displayed with a colon or hypen separating every two digits (an octet), making it easier to read. Example: A MAC address of 2c549188c9e3 is typically displayed as 2C:54:91:88:C9:E3 or 2c-54-91-88-c9-e3. Simplest form of data communication Transmission media sender reciever Fig: simplest data Communication  D 10bytes A 5bytes B E 20bytes C Assume that capacity of each link is 100bytes and source A -----reciever D , Source B------receiver E, source C-------recieverD Real-life data communication D M E U M L U modem T L modem I T P I L Transmission medium P modem E L X E E X R E modem R modem Fig: Real life data communication sysytems  Real-life data communication process involves many hardware devices and software techniques.  The above figure shows a real-life data communication system that contains various components as well as other aspects as follows:  Modem  Multiplexer and Demultiplexer  Transmission medium  Topology  Routing  Protocol  Types of network depending on the available physical area  Modem:It is a hardware device that converts data into a format suitable for a transmission medium so that it can be transmitted from one computer to another (historically along telephone wires).  Multiplexer and Demultiplexer  Multiplexing in computer networks increases the amount of data that can be transmitted in a given time-span over a given bandwidth.  The main aim is to share the resources.  Multiplexer combines multiple data streams from multiple sources into one composite signal and transmits it over shared medium.  Demultiplexer performs exactly reverse process of multiplexer, it splits the composite signal into multiple single data streams and delivers it to desired destination respectively.  Transmission medium:  Transmission media is a communication channel that carries the information from the sender to the receiver.  Transmission media is of two types are wired media and wireless media.  Wired media-examples: copper and fiber-optic cable  Wireless media-examples: radio waves  Topology: A Network Topology is the arrangement with which computer systems or network devices are connected to each other either physically or logically. Example : star, mesh, ring ,bus etc  Protocol: Computers cannot communicate with each other arbitrarily ,like humans. For instance, when we dial someone’s phone number , we first greet the person and inform who is calling and then proceed. Computers also need to follow certain protocol during data communication. A protocol is a standard set of rules that allow electronic devices to communicate with each other. These rules include what type of data may be transmitted, what commands are used to send and receive data, and how data transfers are confirmed.  Types of computer network Based on available physical area there are 3 types of networks:Local Area Network( LAN), Metropolian Area Network(MAN) and Wide Area Network(WAN)  Data communication Transmission media sender reciever Data communication involves exchange of data between two computers. Computer works with a binary language of zeroes and ones One computer generates a stream of bits of zeroes and ones and sends it to another computer to which it is connected in some fashion. Connection can be wired or wireless These two computers need not be close to each other , they can be in same room, different cities, states, countries and even continents. The magic of the data communication is that enabling the exhange of bits from one computer to another. Characteristics of data communication  In any data communication systems , the following three characteristics are desired:  Correct delivery-the data must reach only intended recipient and not someone else  Accurate delivery-there must not be any alteration in data while it is in transit  Timely delivery-data must travel from sender to receiver in a finite amount of time Unit1 continued… Protocols A protocol defines 3 elements: ฀ Syntax- what is to be communicated? ฀ Semantics – how it is to be communicated? ฀ Timing- when it is to be communicated? Standards Standards are necessary in almost every business and public service entity. Networking standards exist to help ensure products of different vendors are able to work together in a network without risk of incompatibility. There are two types of standards: de Jure and de Facto A formal or de Jure standard are the standards developed by an official industry or government body. For example, there are formal standards for applications such as Web browsers (e.g., HTTP, HTML), for network layer software (e.g., IP), data link layer software (e.g. , Ethernet IEEE 802.3), and for physical hardware (e.g., V.90 modems). Formal standards typically take several years to develop, during which time technology changes, making them less useful. De facto standards are those that emerge in the marketplace and are supported by several vendors but have no official standing. For example, Microsoft Windows is a product of one company and has not been formally recognized by any standards organization, yet it is a de facto standard. Signal Propagation Note: The fundamental basis for any data communication to take place is signal propagation. Example: A X B Metal rod A X B Metal rod Sinusoidal signal Amplitude (A): Defn: Amplitude is defined as the vertical distance of a signal from horixontal x-axis. Measured in ฀ volts(representing voltage) ฀ ampere(representing current) ฀ watt(representing power) Period (T): Defn: time that signal takes to complete one cycle is called period of a signal. Measured in seconds or smaller units such as milliseconds(ms), microseconds(µs), nanoseconds(ns) and picoseconds(ps). Frequency (f): Frequency is defines as no. of cycles a signal completes in one second. Measured in Hertz (Hz) Frequency is inversely proportional to period of a signal. i.e, f=1/T - Phase(P); Defn: it is defined as position of a waveform relative to time zero. It is measured in degrees or radians. Bandwidth In terms of analog signal, the bandwidth of a signal is defined as the difference between the upper and lower frequencies of a signal generated (measured in Hz) In terms of digital signal, bandwidth of the channel is the maximum bit rate supported by the channel.(measured in bps) A channel is the medium through which the input signal passes. The bandwidth of the medium should always be greater than the bandwidth of the signal to be transmitted else loss of information takes place. Analog and Digital Transmission Methods Depending upon the type of signal and type of transmission media used there are four possible types of transmission ฀ Analog signal, Analog transmission ฀ Digital signal, Digital transmission ฀ Digital Signal ,Analog transmission ฀ Analog signal, Digital transmission Analog Signal Analog Transmission As the signal moves across the distance, it loses power and becomes impaired by factors such as moisture in the cable, dirt on a contact somewhere in the network. This is called attenuation. Amplifiers are used to overcome this issue. By the time the signal arrives at the amplifier, it is not only attenuated, it is also impaired and noisy. One of the problems with a basic amplifier is that it is a dumb device. All it knows how to do is to add power, so it takes a weak and impaired signal, adds power to it, and brings it back up to its original power level. But along with an increased signal, the amplifier passes  Digital signal Digital Transmission Repeaters are network devices operating at physical layer of the OSI model that amplify or regenerate an incoming signal before retransmitting it. They are incorporated in networks to expand its coverage area. They are also known as signal boosters. Noise is an unwanted electrical signal that is added with the transmitted signal while passing through the communication channel. The noise interferes with the signal and may produce distortions to the signal Digital Signal , Analog Transmission Modem: it is derived from two components, a modulator and a demodulator Modulator uses some conventions or a coding scheme and converts a digital signal to an analog signal of bandwidth that can be transmitted through that channel. Demodulator converts the analog signal back to digital signal. Note: Derived Analog signals must have a bandwidth less than that of a channel of telephone conversation i.e, 4KHz  Modulationtechniques The process of imposing an input signal onto a carrier wave by varying its one or more properties is called. modulation Based on which property of the carrier signal is altered there are three types of modulation techniques: ฀ ฀ ฀ Note: Carrier signal a high frequency signal on which the message signal is imposed. The frequency of the carrier signal must be with in 0Hz and 4KHz. Amplitude Shift Keying(ASK) In ASK , frequency and phase of the carrier signal is kept as it is and amplitude is varied to represent bit 0 and bit 1 In the below example higher amplitude wave represents bit 1 and lower amplitude wave represents bit0. Bit string 1010101 is modulated as a analog signal. At the receiving end , the modem measures amplitude at regular intervals to decode them as 0s and 1s and stored at destination node. Normally used over optical fibers. Over other transmission medium it is less efficient due to noise. Disadvantages of Amplitude shift keying ASK is Highly susceptible to noise and other external factors. Applications of Amplitude shift keying 1.Digital data through an optical fiber is transmitted using ASK technique. 2.The technique was widely used in traditional telephone modems. Frequency shift keying(FSK) In FSK , amplitude and phase of the carrier signal is kept as it is and amplitude is varied to represent bit 0 and bit 1 In the below example high frequency wave represents bit 1 and frequency wave represents bit0. Bit string 10011101 is modulated as a analog signal. At the receiving end , the modem measures frequency at regular intervals to decode them as 0s and 1s and stored at destination node. Advantages of frequency shift keying 1.FSK provides better noise immunity. 2.The signal transmission through FSK is quite simple. 3.It is suitable for long-distance data transmission. Disadvantages of frequency shift keying 1.It utilizes more bandwidth as compared to ASK and PSK thus is not bandwidth efficient. Phase Shift Keying(PSK) In FSK , amplitude and phase of the carrier signal is kept as it is and phase is varied to represent bit 0 and bit 1 In the below example 0degree phase wave represents bit 1 and 180 degree wave represents bit0. Bit string 10010110 is modulated as a analog signal. At the receiving end , the modem measures phase at regular intervals to decode them as 0s and 1s and stored at destination node. Analog Signal Digital Transmission Here analog signals are converted into digital bits and then transmitted as digital signal. The most popular technique used to di this is Pulse Code Modulation technique. A special hardware device called codec(coder/decoder) It is also called A/D converter and D/A converter Basic steps in PCM: At source: 1. Sample the analog signal at regular intervals say t sec -sampling 2. Convert these analog signals into discrete values.-Quantization 3. Convert these values into binary numbers by assigning fixed number of bits for each value 4. Convert the binary numbers as digital signal by concatenating all these binary numbers. At Destination 1. Convert the digital signal into binary numbers 2. Separate out the discrete values of signals by using the number of bits for each discrete value 3. Reconstruct the original analog signal Note: the received analog signal is now slightly different from the original signal due to approximation carried out at source, this difference is known as Quantization Noise or Quantization Error, However we have saved a lot in number of bits that needs to be transmitted, thus there is tradeoff between accuracy and cost or speed. The aim of any PCM strategy would be to reduce the quantization noise to a negligible level without increasing the load on the network significantly.. Modes of Data Transmission  Digital data can be transmitted in number of different ways from source to destination. These different modes are outlines as follows: Simplex, Half-Duplex and Full Duplex Communication Parallel and Serial Communication Synchronous and Asynchronous Communication  Simplex, Half-Duplex and Full Duplex Communication This classification is based on which of the communicating device is allowed to send data and at what point of time. Simplex Communication Simplex communication is a communication channel that sends information in one direction only. This is similar to one-way street, where vehicles are allowed to drive only in one direction. Example:Radio, TV etc  Half-Duplex Communication In a half-duplex system, both parties can communicate with each other, but not simultaneously; the communication is one direction at a time. Bandwidth of the link is completely utilized for sending the data, i.e entire BW is reserved for the device sending data. An example of a half-duplex device is a walkie-talkie two-way radio that has a "push-to-talk" button; when the local user wants to speak to the remote person they push this button, which turns on the transmitter but turns off the receiver, so they cannot hear the remote person. To listen to the other person they release the button, which turns on the receiver but turns off the transmitter. Full Duplex Transmission Full-duplex is a type of communication in which data can flow two ways at the same time. Full duplex devices, therefore, can communicate back and forth simultaneously. For example, two computers connected via an Ethernet cable can send and receive data at the same time. Bandwidth of the link is divided into two channels for carrying data in either direction. Parallel transmission In data transmission, parallel communication is a method of conveying multiple binary digits (bits) simultaneously. For example, an 8-bit parallel channel will convey eight bits (or a byte) simultaneously. n wires are required to transmit n bits. Advantages: This is very fast method of transmitting data. Most suitable for short distance communication. Disadvantages: It is an expensive method as several wires are required for transmission. Not suitable for long distance as it does not guarantee high accuracy over long distance "Skew" can develop in parallel transmission due to slightly different properties in each parallel wire which may not be exactly identical, which could result in different bits travel at different speeds. Note: When the problem of skew happens, different bits travel at different speeds but destination with measure the signal to identify whether it was bit 1 or bit 0 at the same time. Therefore the problem of skew result in an in accurate interpretation of bits. Applications: Parallel transmission are usually used for short distance there too with all the wires absolutely identical especially where the speed rather than cost is of great importance. Foe example data transmission from CPU registers to main memory and vice versa SERIAL COMMUNICATION serial communication is the process of sending data one bit at a time, sequentially, over a communication channel. Most suitable for long distance communication. Because computers generates data in terms of bytes, they need to be separated out and sent bit by bit. Thus a conversion equipment parallel to serial converter is required to convert bytes to stream of bits, i.e convert data from parallel to serial at the sender. At the destination all the bits are collected, measured and out together as bytes in the memory. To do so serial to parallel converter is used at the receiver end. In serial communication the measurement of the signal is done in the middle of the bit duration. Because if the values are measured at edges, the reading will be indeterminate. synchronization in serial communication In serial communication , we need to identify the start of every character transmitted and also identify the middle position of each bit interval to measure the bit value. Normally sender and receiver have two different clocks at their respective ends. The point is to synchronize the clock of the receiver exactly with that of the transmitter so that correct readings are taken. Consider the below example to illustrate synchronization in serial communication:  In order to achieve proper synchronization, clocks in the electronics of sender and receiver must be adjusted and synchronized initially. This can be done using 3 approaches Asynchronous Communication Synchronous communication Isochronous communication Interface is used to achieve such synchronizations. Electronic devices such as Universal Asynchronous Receiver Transmitter(UART) and Universal Synchronous Asynchronous Receiver Transmitter(USART) Asynchronous communication In asynchronous communication the time when character will be sent cannot be predicted. In such scenarios when data is transmitted as characters and at unpredictable pace, we have synchronize the source and destination for each character. This is called asynchronous communication Here the character are preceded with a start bit and succeeded with 1, 1.5 or 2 stop bits as shown in the below fig. NRZ –L signaling is used for encoding the signal. According to NRZ convention negative voltage denotes a binary 1 and positive voltage denotes binary 0. When the line is idle no character is sent over the line , a constant negative voltage signifying binary1 is being sent. When the character is being sent, first a start bit 0 is sent followed by 5 to 8 bits of a character followed by a parity bit if used and then follows a stop bit Synchronous communication In synchronous transmission, the whole block of data bits is transferred instead of one character at a time. The block of bits may or may not contain different characters, it may also contain digitized image. It offers real-time communication between linked devices. Bit oriented protocol is used between sender and receiver. Because there are no beginning and end bits, the data transfer rate is quicker but there’s an increased possibility of errors occurring. Over time, the clocks will get out of sync, and the target device would have the incorrect time, so some bytes could become damaged on account of lost bits. To resolve this issue, it’s necessary to regularly re-synchronise the clocks  An example of synchronous transmission would be the transfer of a large text file. Before the file is transmitted, it is first dissected into blocks of sentences. The blocks are then transferred over the communication link to the target location.  If the block of bits transferred contain different characters the receiver needs to know the beginning of the first character. it will then start clock and start sampling to determine bit value. To perform this synchronization, each data block is preceded with unique bit pattern called as SYN character. It has a bit pattern 00101101.this bit pattern is usually not used in normal communication. Therefore SYN character is reserved to indicate the start of each block for synchronization. However there is a problem in this scheme. However , the bit pattern of two characters could be such that , when sent one after the another it might constitute a SYN character thus fooling the receiver. Therefore normally two SYN characters are sent consecutively. The bit pattern of two SYN characters i.e, 0010110100101101 cannot be obtained by concatenating any characters. Therefore receiver is asked to look for two SYN characters before starting the clock for sampling  When the clock starts measuring the bit values, the counter within the receiver is incremented for every bit received measured and pushed into the character assembler. Once the character is assembled, the character is moved to the separate buffer and bit counter is set to 0 to prepare for the next character as shown in the below fig.. To convert the data from parallel to serial at the sender and serial to parallel at the reciever, the equipment USART is used. It is also responsible to generate SYN characters at the beginning of every data block before transmission, recognizing them at the receiving end.  Transmission Errors: Detection and corrections Error classification It is virtually impossible to transmits signals(analog or digital) without any distortion even in a most perfect condition. This is basically due to various impairments that occurs during the process of transmissions as a result of imperfect medium and/or environment The errors are classified into three categories: Delay distortion Attenuation Noise  Delay distortion: It is caused because signals at different frequencies travel at different speeds along the medium The property of signal propagation is such that the speed of travel of signal of frequency at the center of bandwidth is highest and low at the both ends of bandwidth Therefore signal with different frequencies will arrive at the destination at different times. Hence at the receiving end, if the receiving signals are measured at specific time, they will not measure up to original signal resulting in its misinterpretations.  Attenuation: attenuation is another form of distortion. As signal travels through medium its strength decreases over the distance. Attenuation is very small at short distance and increases with the distance. This is because some of the signal energy is absorbed by the medium. Attenuation is higher at higher frequencies. Amplifiers can be used to boost the signal strength but it not only boost the strength of signal it also boost the accompanying noise Noise: some electromagnetic energy gets inserted somewhere during the transmission which is normally called noise. Apart from delay distortion and attenuation, noise is a major limiting factor in the performance of any communication system. Types of Errors If the signal carries binary data , there are two types of errors: Single bit error and burst errors Single bit error: A single bit of the data unit changes i.e from 0 to 1 or 1 to 0. they are more likely to occur in parallel transmission because it is more likely that one wire among eight wires carry bits has become noisy resulting in corruption of single bit for each byte. Burst errors: if multiple bits(2 or more) of a data unit changes during transmission, it is called as burst errors. It is more likely to occur in serial transmission as the duration of noise is more than a single bit duration. These bits need not be adjacent. Error detection Checksum: checksum is a fixed length data that is a result of performing certain operation on data to transmitted from sender to the receiver. The sender runs appropriate checksum algorithm to compute the checksum of data, appends it as a field in the packet that contains the data to be sent. When the receiver receives the data, it runs the same checksum algorithm on the received data and generate a new checksum and compares it with the checksum appended by the sender with data. Id it matches, it is assured that data is received correctly or else data received is erroneous data.  Various checksum algorithms are popular. Most common examples are parity check, modular sum, position dependent checksum etc Modular sum: In modular sum the data that sender is sending is arranged in smaller blocks called words. All words are added and the 2’s complement of sum of words is computed. The resultant stream of bits is a checksum that will be appended along with the data to be transmitted. The receiver computes the fresh checksum on received data and compares it with the received checksum to detect error.  Vertical redundancy check: It is also known as parity check. In this method the sender appends single additional bit called parity bit. There are 2 schemes to compute parity bit: even parity scheme and odd parity scheme. Even parity: the parity bit is added such a way that no. of 1’s inclusive parity bit must be even. 11110001 1 Odd parity: the parity bit is added such a way that no. of 1’s inclusive parity bit must be odd.1110110 0 Longitudinal redundancy check: A block of bits to be transmitted are arranged row wise. For example: if we want to send 32 bits we arrange them into a list of 4 rows each consisting of 8bits. Then parity bit is computed for every column and a new row of 8bits will be created. This becomes a parity bit for the whole block. MULTIPLEXING Multiplexing and Demultiplexing Multiplexing divides the physical line or medium into logical segmets called channels. Different channels can carry data simultaneously over the same physical medium. Hardware equipment called multiplexer is used at the sender side to combine inputs from different sources and loads them on different channels of a medium. The combined data(composite signal) traverses over the medium simultaneously. At the destination demultiplexer(mux) separates the composite signal into multiple signals meant for different destinations and delivers them appropriately. Mux is responsible for both multiplexing and demultiplexing. Therefore it has to be present at the both ends of the medium. Multiplexing allows us to add new channels in the same medium, but because the capacity of a medium is limited, increase in the number of channels reduces the capacity of the channel TYPES OF MULTIPLEXING There are two ways in which multiplexing can be achieved: Frequency Division Multiplexing Time Division multiplexing Frequency division multiplexing frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands called channels, each of which is used to carry a separate signal. Though the ultimately carried composite signal by the medium is of type analog, the input signals can be canalog or digital.  If the input signals are analog , multiplexers at both the ends of medium are sufficient as shown below:  If the input signals are digital, such as from computers, they need to be passed through modem first to convert digital data to analog signal and then fed to multiplexers. At the receiver end demultiplexer separates the signals meant for different destinations and then given as an input to modem connected to them, where analog signals are converted back to digital data and then delivered to destination as shown in the below fig. FDM and Analog Telephony system In the analog telephone system, the voice signals are carried over twisted copper wire pairs from your home to the nearest exchange called last mile No conversion from analog to digital equivalents takes place at this stage of transmission Frequency of human voice that are carried over telephone lines are in the range of 0-4000Hz, therefore bandwidth of 4KHz is sufficient to carry signal conversation But we know that capacity of twisted pair cables are far higher than 4KHz, that is where FDM comes in handy which enables multiple conversations to be carried over single wire. In order to utilize the underlying infrastructure such as transmission equipments and switches to maximum extent, telephone companies make use of FDM Analog telephone hierarchy The structure used by AT&T in the US is shown below:  At first level 12 voice channels each of bandwidth 4KHz are multiplexed to form a group. This group has a bandwidth of 12*4=48KHz At the next level, 5 such groups are multiplexed to form a supergroup. Thus the bandwidth of a supergroup is 5*48=240KHz. A supergroup can carry 12*5=60 voice signals at a time. At the third level, 10 supergroups are multiplexed to form a mastergroup. Thus mastergroup bandwidth will be 240*10=2.52MHz. Thus mastergroup can carry 10*60=600 voice signals at a time. At the last level 6 mastergroup channels are multiplexed to form a jumbogroup of bandwidth 2.52*6=16.984MHz. Thus a jumbogroup can carry 600*6=3600 voice signals at a time. Time Division Multiplexing(TDM) TDM is a technique used for digital transmission only. In TDM unlike FDM frequency bandwidth of a medium is not divide, instead transmission time is divided into number of time slices and each time slice is allotted to different source nodes who wants to transmit data. The time slice during which source node is sending data, entire bandwidth belong to the source node. Because the data rate is directly proportional to bandwidth , the source node which is sending the data during its time slice can send data at a high rate. However because the transmission time is divided into multiple time slices and allotted to different source nodes, over all data rate reduces for each source node.  There are two ways of allocating time slices at various nodes: Synchronous TDM (also known as TDM) Statistical TDM(STDM) Synchronous TDM In synchronous TDM, the transmission time is divided into no, of time slices of same size. Time slices are allotted to every source node connected to the line in a round robin fashion regardless of whether the source node wants to send data or not. At the receiver, the position of data on the data frame specifies which source node has sent the data. Statistical TDM(STDM) It is a variation of TDM technique and is more intelligent. It monitors which node sends data more frequently and in more quantity and allocates the time slices more often to those nodes. Relatively inactive nodes gets the time slices less often and the completely idle node may not get the time slice at all. Because the statistics is maintained about the activities of various source nodes , hence the name statistical TDM Here the position of the data in the data frame does not determine which source has sent the data , so control information needs to be sent along with the data during transmission. Control information include sender and receiver address and no. of bytes transmitted. 1. DS-0 is a single digital channel with a capacity of 64Kbps which can carry one voice conversation. 2. At the next level 24 DS-0 channels are multiplexed to form DS-1 service with a capacity of 24*6=1.544Mbps which includes 8Kbps overhead bits. This can carry 24 signals simultaneously. 3. At the third level, 4DS-1 channels are multiplexed to form DS-2 channel with a capacity of 1.544*4=6.312 Mbps including overhead of 168Mbps and can carry 24*4=96 conversation simultaneously. 4. Next level 7DS-2 channels are multiplexed to form a DS-3 channel with a capacity of 7*6.312=44.376Mbps including overhead bits of 1.368Mbps. This can carry 7*96=672 voice conversation simultaneously. 5. At the last level 6DS-3 channels are multiplexed to form a DS-4 channel with a capacity of 7*44.376=274.176Mbps including 16.128Mbps. This can carry 6*672=4032 voice conversation simultaneously. Digital Telephone Hierarchy Just like FDM, TDM also helps in the digital telephone hierarchy. This is called Digital Signal(DS) Service. Wavelength Division Multiplexing(WDM) If ur transmission medium is not copper wire but is a optical fiber , a variation of FDM is used called WDM. Here transmissions from multiple optical fibres are combined and sent together. Multiple transmissions have different wavelength and they are combined and sent together via single and more powerful optical fiber. At the receiver side individual transmissions are retrieved by filtering out based on their wavelengths. It has very high accuracy compared to FDM Modern research allows as many as 200 different channels to share a single high capacity optical fiber line. In such case we refer it as DWDM( Dense WDM)  The concept of WDM is shown in below fig: ERROR RECOVERY  Stop and wait It is the simplest flow control method. In this, the sender will send one frame at a time to the receiver. The sender will stop and wait for the acknowledgment from the receiver. This time(i.e. the time between message sending and acknowledgement receiving) is the waiting time for the sender and the sender is totally idle during this time. When the sender gets the acknowledgment(ACK), then it will send the next data packet to the receiver and wait for the acknowledgment again and this process will continue as long as the sender has the data to send. This can be understood by the diagram below:  Suppose if any frame sent is not received by the receiver and is lost. So the receiver will not send any acknowledgment as it has not received any frame. Also, the sender will not send the next frame as it will wait for the acknowledgment for the previous frame which it had sent. So a deadlock situation arises here. To avoid any such situation there is a time-out timer. The sender waits for this fixed amount of time for the acknowledgment and if the acknowledgment is not received then it will send the frame again.  Consider a situation where the receiver has received the data and sent the acknowledgment but the ACK is lost. So, again the sender might wait till infinite time if there is no system of time- out timer. So, in this case also, the time-out timer will be used and the sender will wait for a fixed amount of time for the acknowledgment and then send the frame again if the acknowledgement is not received.  Advantages of Stop and Wait Protocol It is very simple to implement. The main advantage of this protocol is the accuracy. The next frame is sent only when the first frame is acknowledged. So, there is no chance of any frame being lost. Disadvantages of Stop and Wait Protocol We can send only one packet at a time. After every transmission, the sender has to wait for the acknowledgment and this time will increase the total transmission time. This makes the transmission process slow. Go-back-N In go-back-n multiple frames are transmitted at a time and receiver sends a acknowledgement at one shot for all the frames received. It is a mechanism to detect and control the error in datalink layer. sender receiver frame0 frame1 frame2 ACK2 frame3 frame4 frame5 ACK5  If a receiver receives a damaged frame or if an error occurs while receiving a frame then, the receiver sends the NAK ( negative acknowledgement) for that frame along with that frame number, that it expects to be retransmitted. After sending NAK, the receiver discards all the frames that it receives, after a damaged frame.  If a receiver receives a damaged frame, it sends the NAK for the frame in which error or damage is detected. The NAK number, like in go-back-n also indicates the acknowledgement of the previously received frames and error in the current frame. The receiver keeps receiving the new frames while waiting for the damaged frame to be replaced. The frames that are received after the damaged frame are not be acknowledged until the damaged frame has been replaced. The selective repeat retransmits only that frame which is damaged or lost. Sliding window In sliding window method, multiple frames are sent by sender at a time before needing an acknowledgment. Multiple frames sent by source are acknowledged by receiver using a single ACK frame Sliding window refers to an imaginary boxes that hold the frames on both sender and receiver side. The windows have a specific size in which the frames are numbered modulo- n, which means they are numbered from 0 to n-l. For e.g. if n = 8, the frames are numbered 0, 1,2,3,4,5,6, 7, 0, 1,2,3,4,5,6, 7, 0, 1, …. When the receiver sends an ACK, it includes the number of next frame it expects to receive. For example in order to acknowledge the group of frames ending in frame 4, the receiver sends an ACK containing the number 5. When sender sees an ACK with number 5, it comes to know that all the frames up to number 4 have been received. Sliding Window on Sender Side At the beginning of a transmission, the sender’s window contains nframes. As the frames are sent by source, the left boundary of the window moves inward, shrinking the size of window. This means if window size is w, if four frames are sent by source after the last acknowledgment, then the number of frames left in window is w-4. When the receiver sends an ACK, the source’s window expand i.e. (right boundary moves outward) to allow in a number of new frames equal to the number of frames acknowledged by that ACK.  At the beginning of transmission, the receiver’s window contains n-1 spaces for frame but not the frames. As the new frames come in, the size of window shrinks. As soon as acknowledgment is sent, window expands to include the number of frames equal to the number of frames acknowledged. Therefore, the sliding window of sender shrinks from left when frames of data are sending. The sliding window of the sender expands to right when acknowledgments are received. The sliding window of the receiver shrinks from left when frames of data are received. The sliding window of the receiver expands to the right when acknowledgement is sent. Suppose that we have sender window and receiver window each of size 4. So the sequence numbering of both the windows will be 0,1,2,3,0,1,2 and so on. The following diagram shows the positions of the windows after sending the frames and receiving acknowledgments. Message Switching Message switching is a network switching technique in which data is routed in its entirety from the source node to the destination node, one hop at a time. During message routing, every intermediate switch in the network stores the whole message. Before sending the message, the sender node adds the destination address to the message. It is then delivers entirely to the next intermediate switching node. The intermediate node stores the message in its entirety, checks for transmission errors, inspects the destination address and then delivers it to the next node. The process continues till the message reaches the destination. In the switching node, the incoming message is not discarded if the required outgoing circuit is busy. Instead, it is stored in a queue for that route and retransmitted when the required route is available.This is called store and forward network.  Advantages It reduces network congestion due to store and forward method. Any switching node can store the messages till the network is available. Messages of unlimited sizes can be sent. It does not have to deal with out of order packets or lost packets as in packet switching. Disadvantages In order to store many messages of unlimited sizes, each intermediate switching node requires large storage capacity. Store and forward method introduces delay at each switching node. This renders it unsuitable for real time applications. Transmission Medium Defn: Transmission media are the physical infrastructure component, which carry data from one node to another. Example: telephone wires that connect telephones to the central offices(telephone exchanges), coaxial cable that carry cable TV transmissions at home. They always need not be in the form of physical wire , they can be invisible also.  Transmission media can be divided into 2 main categories: Guided media and Unguided media Guided media Guided media are typically based on physical cable. Twisted pair wires and coaxial cables carry data in the form of electric current signals where as optical fibres carry data in the form of light. Twisted pair wire There are two classes of twisted pair wire: Unshielded Twisted Pair(UTP) It is the most commonly used media due to its usage in telephone system. It can carry both data and voice. It consist of two conductors usually copper. These copper conductors are covered with PVC or some other insulators.  Earlier these wires were kept parallel but it resulted in greater level of noise. hence the wires were normally twisted. Twisting the wires did not completely eliminate the noise problem completely but it the reduced the noise level to greater extent. Advantages: They are flexible , cheap and easy to install. EIA developed various standards of UTP cables for specific purpose as shown in the below table: Shielded Twisted Pair(STP) The structure of STP wire is similar to that of UTP, except that twisted pair wires are itself carried by metal shield as shown in below fig.  The metal shield prevents penetration of electromagnetic noise. It also helps to eliminate crosstalk. Crosstalk: an effect where one wire picks up the signals travelling in another wire. This effect can be felt sometimes when we hear another conversation in the background during our call. The metal shield prevents such sounds. Shield reduces noise and cross talk , however they are expensive than UTP. Coaxial cable (coax) They are most commonly used by cable companies to carry cable transmission It consists of an inner conductor surrounded by insulating sheath, which is in turn enclosed in an outer conductor(shield). This outer conductor acts as an second conductor and also act as an shield against noise. Outer conductor is covered by plastic cover which completes the circuit.  Comparison with UTP: it is very expensive, less flexible and more difficult to install in a building where no. of twist and turns are required. It is much more reliable and carry for higher data rates. There are various categories of coaxial cables depending upon thickness and size of the shield, insulators and the outer coating. Examples: RG-8, RG-9, RG-11, RG-58 and RG-59 Optical Fiber In optical fibres , data is transmitted in terms of light and not as electrical signals. Optical fibers are made up of glass fibers that are enclosed in a plastic jacket. This allows fibres to bend and not break. A transmitter at the sender using either LED or Laser send pulses of light across fiber. A receiver at the other end use light sensitive transistor to detect absence or presence of light to indicate 0 or 1.  In the below fig. shown below consists of fibre which is covered with buffer layer which protects it from moisture. An outer jacket surrounds the entire cable., the outer jacket is made of either Teflon, plastic or metal. Teflon is more expensive than plastic but do not provide structural strength. Metal provides structural strength but it is too expensive and heavy. The choice depend on these factors. In the fiber, the cladding covers the core depending upon size. The core and cladding must be very pure and should not vary in size or shape.  For data transmission to occur, the sender device must be capable of inducing bits 0 and 1 in the light source and receiver must have photosensitive cell called photodiode which will translate light back to data bits. There are two types of light sources: Light Emitting Diode- it is cheaper but it provides unfocussed light that hits the boundaries of core with different angles and gets diffused over distances. Hence it is used for short distances Laser Diode- they are expensive but provides more focused beam ad retains its characters for a long distance.  Light trav4ls at a speed of 300,000Km/sec in a straight direction as long as it moves through a uniform substance. But as we change the medium , the direction as well as the speed changes suddenly. Due to the same reason when a spoon immersed in glass of water appears bent. This phenomenon is called refraction As shown in below figure it can be observed that as we change the direction of incoming light, the direction of refracted light also changes. At one stage the refracted light becomes horizontal. After this stage if the angle of incident light still changes, the light is not refracted , instead it hits back i.e reflected. Optical fibers use reflection to guide the light through the optical fiber. The densities of core and cladding of fiber is adjusted in such a way that when light pulse is induced , it wont refract , instead it reflects. Modes of propogation There are different modes of propagation depending on the physical characteristics of the fiber and light source used: Multimode singlemode Multimode Here LED light is used as source. Therefore multiple beams of light pass through the core of a fiber. The structure of the core decides the way the light travels through it. There are two types of multimode propagation: Step Index Graded Index Step Index Here the core and cladding has different densities. Therefore at the interface there is a sudden change in direction of a light. That is why it is called step index. Multiple beams take different paths on reflection as shown in the fig. below. The beam which strikes the core at smaller angle gets reflected more number of times than the beam that hits the core with grater angle. This means , at the destination , all the beams do not reach at destination at the same time, which creates diffusion and confusion in terms of interpretation. Due to this reason it is used for shorter distance. Graded Index Here core itself is made of varying densities. The density is hihest at the core and gradually decreases towards the edge due to which the beam first goes through the gradual refraction giving rise to a curve then reflects as shown in below fig. Single mode Here highly focused light beam is used and travels more or less horizontally. The diameter of the core is much smaller than multimode fiber due to which the angle at which the light is projected(critical angle) will be close to 90 degree and the light travels closely horizontally. As the propagation of different beams of light is more or less similar , the delays are negligible and hence reconstruction of the signal is easier. Advantages and Disadvantages Unguided Medium Cellular(mobile) Telephones  The emergence of first mobile telephone in 1946 was difficult to operate and had a single channel for both sending and receiving communication.  You had a push a button to enable the transmitter and disable the receiver, this is push-to-talk system.  The second development took place in 1960s called Improved Mobile Telephone System(IMTS). strong transmitter was used to cover a large area. two frequencies were used for communication, one for sending and other for receiving. This full duplex system had only 23 channels, but however users had to wait for a long time get dial tone.  The third development is AMTS (Advanced Mobile Phone System).  A large geographical area is divided into small regions called cells.  Each cell has an antenna and cell office to control that cell.  All such cells are controlled and coordinated by MTSO(Mobile telephone switching office) as shown in below diagram  MTSO is also responsible for coordination communications between cells and TCO(Telephone Central Office) that is connected to landline telephone systems  Typical radius of cell is 0-12 miles, depending upon population of area  Frequency modulation is used for communication between the mobile phone and landline system.  Normally two frequency bands are used for this purpose, one for communication initiated by mobile phones and another band for landline systems. The two bands are typically 824-849MHz and 869-894MHz  The available frequencies are divided into two types: control channels and voice channels  Control channels are used for setting up a call and if the call is established successfully voice channels are allocated for the communication Whenever user is allocated a voice channel, it is done in such a way that adjacent channels are not allocated to prevent interference.  Since the number of available frequencies are limited, the same frequencies can be reused in the neighboring cell so that large no. of users are served.  The frequency reuse is the greater improvement over IMTS. cluster2  In the below diagram fixed number of cells are grouped to form a cluster, here the cluster size is 7. cells labelled as A,B,C,D,E,F and G belong to one cluster1.  Such clusters are replicated as many no. of times to cover a large geographical area.  The cells labelled A in both the cluster is allocated same set of frequencies.  The distance between these cells is such that there is no chance of interference. cluster1 Scenario1: Scenario2 Scenario 3: Satellite Communication  A satellite is a physical object that revolves around the earth at a known height.  The satellite performs the function of an antenna and repeater together.  ground station is an equipment installed on the earth’s surface that enables communications over one or more satellites.  Following fig. illustrates ground station A can send information to ground station B via satellite.  However this possess a problem, if earth along with its ground stations is revolving and the satellite is stationary, the sending and receiving stations and the satellite will be out of synchronous as time passes by. Therefore normally geosynchronous satellites are used, which moves at the same revolution per minute as the earth and in the same direction, there fore movement of earth does not matter to communicating stations based on earth.  Using satellites, we can communicate between any two parts of the world. Minimum three satellites are required to cover the earth’s surface entirely  super high frequency ranging from 3GHz to 30GHz is used for satellite communication.  Two frequency bands are used, one band for sending the signals from earth station to satellite(uplink) and another for sending signals from satellite to earth station(downlink)  There are three methods of communication using satellite: 1. Frequency division multiple access(FDMA)- it puts transmission on separate frequency 2. Time division multiple access(TDMA)-it assigns each transmission a proportion of time 3. Code division multiple access(CDMA)- it assigns unique code for each transmission and spreads it over the available set of frequencies Frequency Division Multiple Access  It is the simplest and most established technique employed in satellite communications.  FDMA splits the total bandwidth into multiple channels.  Each ground station on earth is allocated a particular frequency group(range of frequencies)  With in ground station, different frequencies are allocated to different channels, which are used by different stations connected to ground station.  Before transmitting, ground station scans for a free channel and once it finds an empty channel , it gets allocated to the transmitting station. Time Division Multiple Access There is no modulation of frequencies here. Earth station transmits data in the form of packets one by one Bit rate of 10Mbps to 100Mbps are common for TDMA transmission Code Division Multiple Access(CDMA) CDMA is different from both TDMA and FDMA It allows any transmitter to transmit in any frequency and at any time CDMA allows multiple users to simultaneously use a common channel for transmission of information. A CDMA transmitter will code its information with a unique code agreed upon by transmitter and the receiver before transmission begins. The transmitter sends the coded signal to the receiver. Using the same code sequence, the receiver decodes the received signal. Here the coding scheme is nothing but a unique frequency with which the original signal is modulated to form a codified signal that is to be transmitted Internetworking Devices  The connecting devices are classifies into networking and internetworking devices. Each of them has another level of classification: Repeaters A repeater is also called regenerator , is an electronic device which simply regenerates a signal. It works at the physical layer of the OSI model Signal travelling across the physical wire becomes weak over distance or may get corrupted as they get interfered with other signals or noise, the integrity of data is in danger, Repeaters receives such signal as input and regenerates a new signal. Repeaters has intelligence to realize that it is still bit 1 or 0 and therefore recreates the bit pattern of signal say 5v for bit1 and 0v for bit 0. Bridges A bridge is a computer that has its own processor, memory and two NIC to connect to two portions of network. It operates at both physical as well as data link layer. The main idea of using a bridge is to divide a big network into smaller sub networks, called segments as shown in the below fig. due to bridge multiple segments act a part of single network. Bridge appears to be same as repeater but it is more intelligent than bridge as it forwards the augment only to the concerned segment to which the destination host is attached thus prevents excess traffic. Types of bridges Every bridge maintains a table of host address versus segment number to which they belong as shown below:  Bridges are classified into three categories based on how many segments it connects to and how it creates the mapping table: Simple bridge Learning bridge Multiport bridge Simple bridge:  A simple bridge connects to only two segments and the mapping table entries are done manually by the operator.  If a new host is added or if existing host is replaced or deleted, the table has to be manually updated again and again, for these reasons simple bridges are the cheapest. Learning bridge: It is also called adaptive bridge It does not have to be programmed manually unlike a simple bridge.  Multiport bridge: when a simple or learning bridge connects more than two network segments, it is called multiport bridge. Routers A router operates at the physical, data link and network layer. A router is termed as an intelligent device and its capabilities are more than those of a repeater or a bridge. A router is useful for interconnecting two or more networks. Router forwards packets across different network types that uses same transmission protocol(such as TCP/IP) How does a router work??? In the following figure there is a token ring network A and an Ethernet network B bases on bus architecture. A router connects to token ring at point X and to Ethernet at point Y, since the same router connects to both the network , router must have two NICs. i.e, router is capable of working as a special host on a token ring as well as Ethernet network. Each of the router NIC is specific to one network type i.e, NIC at point X is a token ring NIC and NIC at point Y is a Ethernet NIC GATEWAYS Gateway operates at all layers of OSI model.  Routers are capable of forwarding fames between the different network types for ex: token ring, Ethernet, X.25 etc.. But all these network use a common transmission protocol for communication such as TCP/IP. If the different network use different protocols for communication then gateways are use to relay frames between them For ex, if network A is using TCP/IP and network B using Novell Network, a gateway can relay frames between them. This means gateways are not only ability of translating between frame formats but also different protocols. Clearly, gateways are very powerful computers compared to bridge and router, typically used to connect huge network. Gateways take care of different frame sizes, data rates, formats, acknowledgement schemes , priority schemes etc. LAN,MAN and WAN  There are basically three categories of computer networks: Local Area Network It is generally a privately owned network within a single office, campus, building covering a distance of few kilometers. The main reason for designing a LAN is to share resources such as disks, printers, programs and data. It also enables exchange of information. Classical LAN’s data rates are 4-16Mbps, later 100Mbps LAN’s were introduced. LAN’s use bus, ring and star topology. Ethernet and Token ring are most popular LAN’s. Ethernet use bus topology and Token ring use ring topology. Metropolian Area Network It is a network designed to cover an entire city Suppose san organization wants to connect computers in its three branches of a city, organization could obviously lay its own private network. If every organization does that then there would be wire everywhere around public area. Instead services of existing telephone networks can be used. Wide Area Network It is huge compare to LAN and MAN It spams across city, state country and even continent boundaries. Foe example a WAN could be made up of LAN in India, another LAN is US and third LAN in Japan, all connected to each other to form a big network of networks Local Area Networks LAN’s are usually broadcast networks. All nodes in a LAN share a same transmission medium Broadcast networks can be static or dynamic. Static method: Each host is given a fixed time slot to transmits its frame similar to TDM. If host does not have anything to do then that time slice is wasted and that is why this method is not so popular. Dynamic method: A host can send frame any time but if two hosts send frame at the same time, collision will occur. To resolve this issue MAC protocol is used which decides which node can transmit frame and when. This is a part of data link layer of OSI model. Dynamic method is further classified as centralized and Decentralized. Centralized method: Here a single entity called bus arbitration unit decided who should send the data next. This is typically a master slave method. In star topology hub can perform the role of bus arbitration unit. Decentralized method: here no external arbitrator is required. This requires all hosts to follow certain discipline and so this method is more efficient and more popular. Decentalized method can be implemented in two ways: Carrier sensing- Ethernet using bus topology Token passing- Token Ring and FDDI using ring topology Ethernet Ethernet is the popular packet switching LAN technology. It is the technology used by several thousands of LANs around the world. Ethernet uses bus topology where single coaxial cable is used as a transmission medium and all the hosts in the Ethernet connects to this bus as shown in below fig:  A device called transceiver is used to establish a connection between computer and Ethernet. Physically a small hole is made on the outer layer of the coaxial cable so that transceiver can attach to the cable() medium carrying the signals. Transceiver is responsible for sensing voltages on the cable for interpreting the signals. Transceiver contains analog circuits to interface with the medium and digital circuit for interfacing with the computer. Based on the signals on the cable, transceiver determines other host is using the cable i.e, cable is busy. At any instant of time the cable(bus) can be in any of the following three states:  Idle state  Busy carrying a legitimate signal  Busy carrying erratic signal generated by a collision  Transceiver does not connect to the host directly. Instead it connects to a Network Interface Card(NIC) which is plugged into motherboard of the hosts which functions like a small computer. It has its own memory, CPU and limited instruction set which perform all the network related operations on behalf of host, such as validating the incoming frame etc. Each NIC is assigned a unique code called as hardware address or physical address by the manufacturer of NIC. It uniquely identifies the host. If NIC of a host is changed to new NIC, physical address of host changes to address of new NIC.  Logical architecture of a NIC is shown below: Following steps are carried out while transmitting a file/message by a host to another host on the bus(ethernet) Properties of Ethernet Ethernet uses bus topology and has a transmission speed of 10Mbps It has following properties: Broadcast network Best-effort delivery Decentralized access control Carrier sense multiple access with collision detection(CSMA/CD) Every node in the Ethernet before transmission , through the transceiver determines if the bus is idle or not by looking for the presence/absence of carrier wave on bus i.e, it performs carrier sensing. If the bus is idle , then it sends a limited amount of data so that other hosts also get chance for transmission. But signal transmitted does not reach all the parts of the network immediately, it takes some finite time. Untill the signal reaches the another part, the host in that part might want to transmit data and thus senses bus idle and so it transmits a frame. When trhis happens , the electrical signals from two transmissions intermingle and neither remains a meaningful transmission. Such incidents are refered as collision which produces erratic signal.  To resolve collision , while host transmits data, transceiver of the hosts continues to listen to the bus to see if collision has occurred. If collision has occurred then transceiver inform its NIC about it which stops the further transmission. It also generates one jamming signal in the bus to inform other nodes in the bus about the collision so that the node trying to send the data also detects jamming signal and back off. Now all the nodes wanting to send a data wait for a while. How long should the node wait???????? Ethernet uses binary exponential back-off policy where sender waits for a random amount of time after first collision is detected, twice as long if a retransmission also results into collision and for time as long if the re- retransmission also results in collision and so on. By doubling the waiting period and by making it random, chances of another collision after the first one is not very high. Token Ring Unlike Ethernet which uses bus topology, token ring network is based on ring topology A token ring network employs a mechanism called token passing All hosts on the token ring shares the same physical medium and the hosts here are arranged to form a ring. When the hosts on the network wants to transmit data, it cannot send immediately, it needs permission to do so. Once a node gets permission for data transmission, it is guaranteed that no other hosts will be allowed to transmit data at the same time, thus a host sending a data has an exclusive control over the transmission medium, having right to do so. The sending computer transmits a frame. Which travels across the ring. Each host on the network must accept it, check the destination address in the frame If it is not meant for it, forward it to the next host on the ring. Only the actual destination node makes a copy of frame while other nodes just forward it along. At the destination node, before forwarding it checks the CRC to ensure there is no error in received frame and then only makes a copy of it and also changes a flag bit in the frame to indicate the receipt of a correct frame. The frame comes back to the sender covering the entire ring. Sender checks the flag bit to verify whether it was received successfully by the destination or if there were any errors during the transmission. This is hoe the acknowledgement scheme is implemented in token ring.  How exactly permission is given to host for data transmission????? Token ring hardware uses a special 3-byte frame called as token The token is a permission for data transmission. Token ring hardware makes sure there is one and only one token frame on the medium which keeps circulating over the ring from one host to another continuously. The host wanting to transmit data , removes and captures the token frame from the medium and transmits its data frame over the network. Once the data frame comes back to it by completing its full journey, it then sends back the captured token frame on the transmission medium so that any other node wanting to send data can get its turn for data transmission. The circulating mechanism of token frame ensures that every host in the network is given equal chance for data transmission  Data frame in token ring: Fiber Distributed Data Interface(FDDI) It is an alternative to Ethernet and token ring architecture ands supports the data rate of 100Mbps Originally FDDI was developed with optical fiber as transmission medium because only optical fiber could support such data rate those days. These days even copper wires support such data rate and a copper version of FDDI was developed and is called Copper Distributed Data Interface(CDDI) Properties of FDDI The main properties of FDDI are Token passing for media access control Self healing mechanism Operation of FDDI FDDI operates exactly like token ring with only one difference. Token ring uses a single wire through all the hosts in the network whereas, FDDI uses two wires. FDDI uses two independent wires to connect to every host  FDDI uses only one ring for data transmission and works exactly like Token ring. i.e, NIC of each host examines all the frames that circulate around the ring, comparing the destination address in the frame with its own. Like token ring, destination nodes here also keeps the copy of a frame only if the address in the frame matches with its own or else it simply forwards it along the ring. What is the need of the second ring then??? Self-healing mechanism The self-healing mechanism of the FDDI network is made possible using the second ring. When a network error occurs or when a host is down, the NIC of the host realizes that it cannot communicate with its neighboring node. In such cases NIC uses the second ring which is used as a backup for such failures. This is called loopback.  Fig. above shows one such instance where a host is down due to some hardware failure. In such case, to bypass that host without affecting the rest of the network, the second ring(backup) is used. When the host on the network is down in the first ring, the second ring is used to create a new closed loop between all the other host bypassing the host with fault using loopback mechanism. Fault ring can be easily isolated and repaired Integrated Services Digital Network(ISDN) ISDN Architecture The fundamental concept in ISDN is digital bit pipe. This is a conceptual pipe through which bits flow between the end use and the CO(ISDN exchange) The bit pipe is bidirectional. The bits corresponds to the contents of a telephone call , fax message, an email sent through a computer or anything else There are 2 primary set of users of ISDN services: home users and business users The bandwidth needs of home users are far smaller than business users. Two separate standards were developed to address two user types A low bandwidth standard was defined for home users and high bandwidth standard was defined for business users. Business users can have multiple bit pipes in case they need more bandwidth than provided. ISDN channel types B Channel A Bearer Channel (B Channel) is defined at a rate of 64 kbps. It is the basic user channel and can carry any type of digital information in full duplex mode as long as the required transmission rate does not exceed 64 kbps. It can be used to carry digital data, digital voice and any other low data rate information.  A data channel (D channel) carry control signal for bearer channels. This channel does not carry data and can be either 16 or 64 kbps, depending upon the user’s need. If we are using in-band signaling, the same cable carries data as well as control signals. If we are using out-of-band signaling (as in ISDN), then two different cables carry data and control information.  H-Channel A Hybrid channel (H-channel) is provided for user information at higher bit rates. There are three types of H-Channels depending on the data rates: H0 with data rate of384 kbps H11 with data rate of 1536 kbps H12 with data rate of 1920 kbps Hybrid channels are used for high data rate applications such as video, teleconferencing. ISDN interfaces There are two ways in which ISDN can be used. These two interfaces supported by ISDN are Basic Rate Interface(BRI) mainly used by home users and Primary Rate Interface(PRI) mainly used by corporate customers. Basic Rate Interface(BRI) BRI specifies a digital pipe that contains 2 B channels and one Data channel. The 2B+D channels are called BRI in ISDN terminology 2B channels require 64Kbps each. The D channel demands 16Kbps and BRI service itself needs 48Kbps for operating overhead. Therefore BRI needs a total of 192Kbps bandwidth. A BRI channel is most suitable for residential and small office subscribers. For example a home user can use one B channel for internet browsing, another B channel for telephone call at the same time. If high speed connection to internet is required then two B channels can be combined. Primary Rate Interface(PRI) PRI has capacity more than BRI A PRI has 23 B channels instead of twpo and one D channel of datarate 64Kbps. This makes PRI a 23B+D channel. This gives a total bandwidth of 1.536Mbps PRI operation itself needs 8Kbps overhead. Thus the total capacity of PRI is 1.544Mbps TCP/IP  The OSI Model we just looked at is just a reference/logical model. It was designed to describe the functions of the communication system by dividing the communication procedure into smaller and simpler components. But when we talk about the TCP/IP model, it was designed and developed by Department of Defense (DoD) in 1960s and is based on standard protocols. It stands for Transmission Control Protocol/Internet Protocol. The TCP/IP model is a concise version of the OSI model. TCP/IP, the protocol stack that is used in communication over the Internet and most other computer networks, has a five-layer architecture. The TCP/IP model consists of five layers: the application layer, transport layer, network layer, data link layer and physical layer  Application Layer – This layer performs the functions of top three layers of the OSI model: Application, Presentation and Session Layer. It is responsible for node-to-node communication and controls user-interface specifications. Some of the protocols present in this layer are: HTTP, HTTPS, FTP, TFTP, Telnet, SSH, SMTP, SNMP, NTP, DNS, DHCP Transport layer(Host-to-Host layer) This layer is analogous to the transport layer of the OSI model. It is responsible for end-to-end communication and error-free delivery of data. It shields the upper-layer applications from the complexities of data. The two main protocols present in this layer are : Transmission Control Protocol (TCP)(connection oriented) – It is known to provide reliable and error-free communication between end systems. It performs sequencing and segmentation of data. It also has acknowledgment feature and controls the flow of the data through flow control mechanism. It is a very effective protocol but has a lot of overhead due to such features. Increased overhead leads to increased cost. User Datagram Protocol (UDP) – On the other hand does not provide any such features. It is the go-to protocol if your application does not require reliable transport as it is very cost-effective. Unlike TCP, which is connection-oriented protocol, UDP is connectionless.  Internet Layer – This layer parallels the functions of OSI’s Network layer. It defines the protocols which are responsible for logical transmission of data over the entire network. The main protocols residing at this layer are : IP – stands for Internet Protocol and it is responsible for delivering packets from the source host to the destination host by looking at the IP addresses in the packet headers. IP has 2 versions: IPv4 and IPv6. IPv4 is the one that most of the websites are using currently. But IPv6 is growing as the number of IPv4 addresses are limited in number when compared to the number of users. ICMP – stands for Internet Control Message Protocol. It is encapsulated within IP datagrams and is responsible for providing hosts with information about network problems. ARP – stands for Address Resolution Protocol. Its job is to find the hardware address of a host from a known IP address. ARP has several types: Reverse ARP, Proxy ARP, Gratuitous ARP and Inverse ARP.  Network Access Layer – A network layer is the lowest layer of the TCP/IP model. A network layer is the combination of the Physical layer and Data Link layer defined in the OSI reference model. It defines how the data should be sent physically through the network. This layer is mainly responsible for the transmission of the data between two devices on the same network. The functions carried out by this layer are encapsulating the IP datagram into frames transmitted by the network and mapping of IP addresses into physical addresses. Transmission Control Protocol(TCP) TCP stands for Transmission Control Protocol. It is a transport layer protocol that facilitates the transmission of packets from source to destination. It is a connection-oriented protocol that means it establishes the connection prior to the communication that occurs between the computing devices in a network. This protocol is used with an IP protocol, so together, they are referred to as a TCP/IP. The main functionality of the TCP is to take the data from the application layer. Then it divides the data into a several packets, provides numbering to these packets, and finally transmits these packets to the destination. The TCP, on the other side, will reassemble the packets and transmits them to the application layer. As we know that TCP is a connection-oriented protocol, so the connection will remain established until the communication is not completed between the sender and the receiver. Working of TCP In TCP, the connection is established by using three-way handshaking. The client sends the segment with its sequence number. The server, in return, sends its segment with its own sequence number as well as the acknowledgement sequence, which is one more than the client sequence number. When the client receives the acknowledgment of its segment, then it sends the acknowledgment to the server. In this way, the connection is established between the client and the server. TCP packet format The length of TCP header is minimum 20 bytes long and maximum 60 bytes  Source port: It defines the port of the application, which is sending the data. So, this field contains the source port address, which is 16 bits. Destination port: It defines the port of the application on the receiving side. So, this field contains the destination port address, which is 16 bits. Sequence number: This field contains the sequence number of data bytes in a particular session. Acknowledgment number: When the ACK flag is set, then this contains the next sequence number of the data byte and works as an acknowledgment for the previous data received. For example, if the receiver receives the segment number 'x', then it responds 'x+1' as an acknowledgment number. HLEN: It specifies the length of the header indicated by the 4-byte words in the header. The size of the header lies between 20 and 60 bytes. Therefore, the value of this field would lie between 5 and 15. Reserved: It is a 4-bit field reserved for future use, and by default, all are set to zero. Flags There are six control bits or flags: URG: It represents an urgent pointer. If it is set, then the data is processed urgently. ACK: If the ACK is set to 0, then it means that the data packet does not contain an acknowledgment. PSH: If this field is set, then it requests the receiving device to push the data to the receiving application without buffering it. RST: If it is set, then it requests to restart a connection. SYN: It is used to establish a connection between the hosts. FIN: It is used to release a connection, and no further data exchange will happen  Window size It is a 16-bit field. It contains the size of data that the receiver can accept. This field is used for the flow control between the sender and receiver and also determines the amount of buffer allocated by the receiver for a segment. The value of this field is determined by the receiver. Checksum It is a 16-bit field. This field is optional in UDP, but in the case of TCP/IP, this field is mandatory. Urgent pointer It is a pointer that points to the urgent data byte if the URG flag is set to 1. It defines a value that will be added to the sequence number to get the sequence number of the last urgent byte. Options It provides additional options. The optional field is represented in 32-bits. If this field contains the data less than 32-bit, then padding is required to obtain the remaining bits User Datagram Protocol(UDP) UDP stands for User Datagram Protocol. UDP is a simple protocol and it provides nonsequenced transport functionality. UDP is a connectionless protocol. This type of protocol is used when reliability and security are less important than speed and size. UDP is an end-to-end transport level protocol that adds transport-level addresses, checksum error control, and length information to the data from the upper layer. The packet produced by the UDP protocol is known as a user datagram  The user datagram has a 16-byte header which is shown below: Where, Source port address: It defines the address of the application process that has delivered a message. The source port address is of 16 bits address. Destination port address: It defines the address of the application process that will receive the message. The destination port address is of a 16-bit address. Total length: It defines the total length of the user datagram in bytes. It is a 16-bit field. Checksum: The checksum is a 16-bit field which is used in error detection Disadvantages of UDP protocol UDP provides basic functions needed for the end-to-end delivery of a transmission. It does not provide any sequencing or reordering functions and does not specify the damaged packet when reporting an error. UDP can discover that an error has occurred, but it does not specify which packet has been lost as it does not contain an ID or sequencing number of a particular data segment.