Unit 3 - Transmission Media PDF

University of Technology, Jamaica School of Computing and Information Technology Transmission Media Handout #3 Bandwidth The maximum amount of data transmitted over an internet connection in a given amount...

University of Technology, Jamaica School of Computing and Information Technology Transmission Media Handout #3 Bandwidth The maximum amount of data transmitted over an internet connection in a given amount of time. Bandwidth is often mistaken for internet speed when it's actually the volume of information that can be sent over a connection in a measured amount of time – calculated in megabits per second (Mbps). Bandwidth vs Speed Bandwidth is how much information you receive every second, while speed is how fast that information is received or downloaded. Let's compare it to filling a bathtub. If the bathtub faucet has a wide opening, more water can flow at a faster rate than if the pipe was narrower. Think of the water as the bandwidth and the rate at which the water flows as the speed. Bandwidth vs Latency Latency is sometimes referred to as delay or ping rate. It's the lag you experience while waiting for something to load. If bandwidth is the amount of information sent per second, latency is the amount of time it takes that information to get from its source to you. Bandwidth vs Throughput Throughput is how much information actually gets delivered in a certain amount of time. So if bandwidth is the max amount of data, throughput is how much of that data makes it to its destination – taking latency, network speed, packet loss and other factors into account. Transmission Media Transmission media is a communication channel that transmits information from the source/transmitter to the receiver. It is a physical path for data transfer through electromagnetic signals. Information is carried over in the form of bits through LAN. It can mediate the propagation of signals for telecommunication. Signals are imposed on a wave that is suitable for the chosen medium. These media lie underneath the physical layer that regulates them. There are two main types of transmission media in computer networks. These are guided and unguided media. While guided media requires physical medium, unguided media requires air for communication. 1 Guided Transmission Media Guided media are also known as wired or bounded media. These media consist of wires through which the data is transferred. Guided media is a physical link between transmitter and recipient devices. Signals are directed in a narrow pathway using physical links. These media types are used for shorter distances since physical limitation limits the signal that flows through these transmission media. Twisted Pair Cable In this type of transmission media, two insulated conductors of a single circuit are twisted together to improve electromagnetic compatibility. These are the most widely used transmission medium cables. These are packed together in protective sheaths. They reduce electromagnetic radiation from pairs and crosstalk between the neighboring pair. Overall, it improves the rejection of external electromagnetic interference. These are further subdivided into unshielded and shielded twisted pair cables. Unshielded Twisted Pair Cable(UTP): These consist of two insulated copper wires that are coiled around one another. These types of transmission media block interference without depending on any physical shield. The unshielded twisted pair are very affordable and are simple to set up. These provide a high-speed link. Shielded Twisted Pair (STP): This twisted cable consisted of a foil shield to block external interference. The insulation within these types of the twisted cable allow greater data transmission rate. These are used in fast-data-rate Ethernet and in data and voice channels of telephone lines. Optical Fibre Cable Also known as fiber optic cable, these are thin strands of glass that guide light along their length. These contain multiple optical fibers and are very often used for long-distance communications. Compared to other materials, these cables can carry huge amounts of data and run for miles without using signal repeaters. Due to lesser requirements, they have less maintenance costs and it improves the reliability of the communication system. These can be unidirectional as well as bidirectional in nature. 2 Coaxial Cable These guided transmission media contain an insulation layer that transmits information in baseband mode and broadband mode. Coaxial cables are made of PVC/Teflon and two parallel conductors that are separately insulated. Such cables carry high frequency electrical signals without any big loss. The dimension of cable and connectors are controlled to give them constant conductor spacing for efficient functioning as a transmission line. Unguided Transmission Media Also known as unbounded or wireless media, they help in transmitting electromagnetic signals without using a physical medium. Here, air is the medium. There is no physical connectivity between transmitter and receiver. These types of transmission media are used for longer distances however they are less secure than guided media. There are three main types of wireless transmission media. Radio Waves Radio waves are transmitted in every direction throughout free space. Since these are omnidirectional, sent waves can be received by any antenna. These waves are useful when the data is to multicasted from one sender to multiple receivers. Radio waves can cover large areas and even penetrate obstacles such as buildings and walls. The frequency of these waves ranges between 3 kHz to 1GHz. Due to its omnidirectional nature, issues such as interference might arise when another signal with the same bandwidth or frequency is sent. Infrared These waves are useful for only very short distance communication. Unlike radio waves, they do not have the ability to penetrate barriers. Their range varies between 300GHz – 400THz. Since they have larger bandwidth, the data rate is very high for infrared waves. These have less interference and are more secure. Microwaves For these waves, it is important for the transmitter and receiver antenna to be aligned. This is why it is known as line-of-sight transmission. Due to this, they are suitable for shorter distances. They comprise of electromagnetic waves with frequencies ranging between 1-400 GHz. Microwaves provide bandwidth between the range of 1 to 10 Mbps. Distance covered by the signal is proportional to the height of the antenna. For travelling to longer distances, the height of the tower should be increased. These are further sub categorized as terrestrial and satellite type microwave transmission. Type Advantages Disadvantages Unshielded Twisted Pair Less expensive, Attenuation leads to short-distance Easy to install, communication, High speed Susceptible to external interference Shielded Twisted Pair Reduced crosstalk, Bulky and expensive, Faster than UTP Difficult to install 3  Optical Fibre Cable Increased bandwidth, High-cost, Fragile Immunity to interference Coaxial Cable High bandwidth, Complete disruption due to single Noise immunity cable failure Radio Easy to generate, More interference Can penetrate obstacles Infrared Less interference Cannot penetrate obstacles Media Access Control In IEEE 802 LAN/MAN standards, the Medium Access Control (MAC) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublayer and the logical link control (LLC) sublayer together make up the data link layer. The LLC provides flow control and multiplexing for the logical link (i.e. EtherType, 802.1Q VLAN tag etc), while the MAC provides flow control and multiplexing for the transmission medium. Code Division Multiple Access (CDMA) CDMA (Code-Division Multiple Access) refers to any of several protocols used in second- generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra- high-frequency (UHF) cellular phone systems in the 800 megahertz (MHz) and 1.9 gigahertz (GHz) bands. CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern code. This enables the signal to be intercepted only by a receiver whose frequency response is programmed with the same code, following along with the transmitter frequency. There are trillions of possible frequency sequencing codes, which enhances privacy and makes cloning difficult. Carrier Sense Multiple Access (CSMA) CSMA is a mechanism that senses the state of the shared channel to prevent or recover data packets from a collision. It is also used to control the flow of data packets over the network so that the packets are not get lost, and data integrity is maintained. In CSMA, when two or more data packets are sent at the same time on a shared channel, the chances of collision occurred. Due to the collision, the receiver does not get any information regarding the sender's data packets. And the lost information needs to be resented so that the receiver can get it. Therefore, we need to sense the channel before transmitting data packets on a network. It is divided into two parts, CSMA CA (Collision Avoidance) and CSMA CD (Collision Detection). 4 CSMA/CD The Carrier Sense Multiple Access/ Collision Detection protocol is used to detect a collision in the media access control (MAC) layer. Once the collision was detected, the CSMA CD immediately stopped the transmission by sending the signal so that the sender does not waste all the time to send the data packet. Suppose a collision is detected from each station while broadcasting the packets. In that case, the CSMA CD immediately sends a jam signal to stop transmission and waits for a random time context before transmitting another data packet. If the channel is found free, it immediately sends the data and returns it. Advantages of CSMA/CD: 1. It is used for collision detection on a shared channel within a very short time. 2. CSMA CD is better than CSMA for collision detection. 3. CSMA CD is used to avoid any form of waste transmission. 4. When necessary, it is used to use or share the same amount of bandwidth at each station. 5. It has lower CSMA CD overhead as compared to the CSMA CA. Disadvantage of CSMA/CD 1. It is not suitable for long-distance networks because as the distance increases, CSMA CD' efficiency decreases. 2. It can detect collision only up to 2500 meters, and beyond this range, it cannot detect collisions. 3. When multiple devices are added to a CSMA CD, collision detection performance is reduced. CSMA/CA CSMA stands for Carrier Sense Multiple Access with Collision Avoidance. It means that it is a network protocol that uses to avoid a collision rather than allowing it to occur, and it does not deal with the recovery of packets after a collision. It is similar to the CSMA CD protocol that operates in the media access control layer. In CSMA CA, whenever a station sends a data frame to a channel, it checks whether it is in use. If the shared channel is busy, the station waits until the channel enters idle mode. Hence, we can say that it reduces the chances of collisions and makes better use of the medium to send data packets more efficiently. Advantage of CSMA/CA 1. When the size of data packets is large, the chances of collision in CSMA CA is less. 2. It controls the data packets and sends the data when the receiver wants to send them. 3. It is used to prevent collision rather than collision detection on the shared channel. 4. CSMA CA avoids wasted transmission of data over the channel. 5. It is best suited for wireless transmission in a network. 6. It avoids unnecessary data traffic on the network with the help of the RTS/ CTS extension. 5 Disadvantage of CSMA/CA 1. Sometime CSMA/CA takes much waiting time as usual to transmit the data packet. 2. It consumes more bandwidth by each station. 3. Its efficiency is less than a CSMA CD. Controlled Media Access Techniques In controlled access, the stations seek information from one another to find which station has the right to send. It allows only one node to send at a time, to avoid collision of messages on shared medium. The three controlled-access methods are: 1. Reservation 2. Polling 3. Token Passing Reservation In the reservation method, a station needs to make a reservation before sending data. The time line has two kinds of periods:  Reservation interval of fixed time length  Data transmission period of variable frames. If there are M stations, the reservation interval is divided into M slots, and each station has one slot. Suppose if station 1 has a frame to send, it transmits 1 bit during the slot 1. No other station is allowed to transmit during this slot. In general, ith station may announce that it has a frame to send by inserting a 1 bit into ith slot. After all N slots have been checked, each station knows which stations wish to transmit. The stations which have reserved their slots transfer their frames in that order. After data transmission period, next reservation interval begins. Since everyone agrees on who goes next, there will never be any collisions. Polling Polling process is similar to the roll-call performed in class. Just like the teacher, a controller sends a message to each node in turn. In this, one acts as a primary station(controller) and the others are secondary stations. All data exchanges must be made through the controller. The message sent by the controller contains the address of the node being selected for granting access. Although all nodes receive the message but the addressed one responds to it and sends data, if any. If there is no data, usually a “poll reject”(NAK) message is sent back. Problems include high overhead of the polling messages and high dependence on the reliability of the controller. Token Passing In token passing scheme, the stations are connected logically to each other in form of ring and access to stations is governed by tokens. A token is a special bit pattern or a small message, which circulate from one station to the next in some predefined order. In Token ring, token is passed from one station to another adjacent station in the ring whereas in case of Token bus, each station uses the bus to send the token to the next station in some predefined order. In both cases, token represents permission to send. If a station has a frame queued for transmission when it 6 receives the token, it can send that frame before it passes the token to the next station. If it has no queued frame, it passes the token simply. After sending a frame, each station must wait for all N stations (including itself) to send the token to their neighbors and the other N – 1 stations to send a frame, if they have one. There exists problems like duplication of token or token is lost or insertion of new station, removal of a station, which need be tackled for correct and reliable operation of this scheme. 7