Updated Lecture 3 (Network) PDF

Transmission Medium & LAN Technologies Lecture 3 CCN.1 Lecture Objective  To discuss about LAN wiring and identify the major hardware components used in various wiring schemes  To discuss on how the medium is shared among devices  To discuss the oper...

Transmission Medium & LAN Technologies Lecture 3 CCN.1 Lecture Objective  To discuss about LAN wiring and identify the major hardware components used in various wiring schemes  To discuss on how the medium is shared among devices  To discuss the operations of LAN protocols CCN.2 Transmission Medium (Guided or wired connections) CCN.3 Twisted Pair A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair acts as a single communication link. The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable. CCN.4 Twisted Pair - Transmission Characteristics  Analog needs amplifiers every 5km to 6km  Digital can use either analog or digital signals needs a repeater every 2-3km  Limited distance  Limited bandwidth for a point to point analog signaling(1MHz)  For a long-distance digital point to point signaling data rates up to a few Mbps are possible.  For very short distances, data rates of up to 10 Gbps have been achieved in commercially available products.  Susceptible to interference and noise CCN.5 Advantages and Disadvantages of TP Advantages: The most common guided transmission medium for both analog and digital signals. Most commonly used medium in the telephone network. Less expensive. Easier to work with. Disadvantages:  Sensitive to EMI (Electro Magnetic Interference) and eavesdropping, especially unshielded.  Unsuitable for very high-speed data transmission.  Some data networking standards for TP are new and not entirely stable. CCN.6 Unshielded TP (UTP) & 5 Categories  UTP cable is rated according to its data-carrying capacity. Category 1: For analog and digital voice (telephone) and low speed data applications. Category 2: For Integrated Services and Digital Network (ISDN) and medium-speed data applications. Category 3: For High-speed data and LAN traffic up to 10 Mbps. Category 4: For LAN traffic up to 16 Mbps. Category 5: For LAN technologies such as 100 Mbps Ethernet. Category 6: For LAN technologies such as 10 Gbps Ethernet. CCN.7 Coaxial Cable A coaxial cable CCN.8 Coaxial Cable - Transmission Characteristics  superior frequency characteristics than TP  performance limited by attenuation & noise  analog signals amplifiers every few km closer if higher frequency up to 500MHz  digital signals repeater every 1km closer for higher data rates CCN.9 Coaxial Cable-Advantages and Disadvantages Advantages:  Technology and standards are mature, which promotes compatibility and interoperability of different vendors’ equipment.  Resists EMI better than bandwidths than twisted pair.  Supports higher bandwidths than twisted pair.  Heavier coax cable is relatively sturdy and resists rough treatment. Disadvantages:  Coax cable, like TP, is susceptible to EMI and eavesdropping.  Some coax cables are heavy, bulky, or expensive. CCN.10 Optical Fiber CCN.11 Optical Fiber - Benefits  greater capacity data rates of hundreds of Gbps over tens of km.  smaller size & weight  lower attenuation  electromagnetic isolation  greater repeater spacing 10s of km at least CCN.12 Optical Fiber - Transmission Characteristics  uses total internal reflection to transmit light effectively acts as wave guide for 1014 to 1015 Hz this covers portions of the infrared and visible spectra  can use several different light sources Light Emitting Diode (LED)  cheaper, wider operating temp range, lasts longer Injection Laser Diode (ILD)  more efficient, has greater data rate  relation of wavelength, type & data rate CCN.13 Optical Fiber Transmission Modes CCN.14 Optical Fiber Transmission Modes  Light from a source enters the cylindrical glass or plastic core. Rays at shallow angles are reflected and propagated along the fiber; other rays are absorbed by the surrounding material.  This form of propagation is called step-index multimode, referring to the variety of angles that will reflect.  With multimode transmission, multiple propagation paths exist, each with a different path length and time to traverse the fiber.  This causes signal elements (light pulses) to spread out in time, which limits the rate at which data can be accurately received.  This type of fiber is best suited for transmission over very short distances. CCN.15 Advantages and Disadvantages Advantages:  Immune to interference or detection outside the cable, so fiber optic cable is an extremely reliable and secure transmission media.  Supports very high bandwidths. Disadvantages:  Network interfaces and cables are relatively expensive.  Connections require high- precision manufacturing and careful handling.  Relatively complex to configure and install. CCN.16 LAN Technologies CCN.17 What’s a protocol? Human protocols: Network protocols:  “what’s the time?”  computers (devices) rather than  “I have a question” humans  introductions  all communication activity in Internet governed by protocols … specific messages sent … specific actions taken when message Protocols define the format, order received, or other of messages sent and received events among network entities, and actions taken on msg transmission, receipt CCN.18 What’s a protocol? A human protocol and a computer network protocol: Hi TCP connection request Hi TCP connection response Got the time? GET http://gaia.cs.umass.edu/kurose_ross 2:00 time Q: other human protocols? CCN.19 LAN (Bus Network: Ethernet)  Ethernet is a well-known and widely used network technology that employs bus topology  Invented at Xerox Corporation's in the early 1970s.  Digital Equipment Corporation (DEC), Intel Corporation, and Xerox later cooperated to devise a production standard. CCN.20 The Ethernet standard  IEEE now controls Ethernet standards, and develops  In original version, an Ethernet consisted of a single coaxial cable  A given Ethernet segment is limited to 500 meters in length, and requires a minimum separation of 3 meters each pair  Ethernet versions: Original Ethernet operated at 10 Megabits per second A later version known as Fast Ethernet operates at 100 Mbps Gigabit Ethernet operates at 1000 Mbps or 1 Gigabit per second Carrier Sense Multiple Access/Collision Detection(CSMA/CD) CCN.21 Sharing on an Ethernet A signal propagates from the sending computer to both ends of the shared cable. Sharing in LAN does not mean that multiple frames are being sent at the same time Instead, the sending computer has exclusive use of the entire cable during the transmission of a given frame other computers must wait CCN.22 Identifying Packet (Frame) Contents  The address does not specify what the packet contains  Many data items use the same representation a receiver cannot use data in the packet to determine what the packet contains  To inform the receiver about its contents each frame contains additional information that specifies the type of the contents CCN.23 Frame Headers and Frame Format  Each LAN technology defines the exact frame format  Most LAN technologies define a frame to consist of two parts a frame/packet header  that contains information such as the source and destination addresses a “payload” or “data area” that contains the information being sent CCN.24 Ethernet Frame Format CCN.25 Example – Ethernet Frame Format ( refer Fig 9.3 in the previous slide)  An Ethernet frame begins with a header that contains three fields  The 64-bits preamble alternating 1s and 0s that allow the receiver's hardware to synchronize with the incoming signal  The first two fields of the header contain HWA,  Destination Address and Source Address The third field of the header consists of  a 16-bits Ethernet “frame type”  Ethernet uses a 48-bit static addressing scheme  Addresses: 6 bytes if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol. otherwise, adapter discards frame  Type: indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)  CRC: checked at receiver, if error is detected, the frame is simply dropped CCN.26 Access networks: cable-based access cable headend … cable splitter modem C O V V V V V V N I I I I I I D D T D D D D D D A A R E E E E E E T T O O O O O O O A A L 1 2 3 4 5 6 7 8 9 Channels frequency division multiplexing (FDM): different channels transmitted in different frequency bands CCN.27 LAN Standards Ethernet (IEEE 802.3) Token Bus (IEEE 802.4) Token Ring (IEEE 802.5) DQDB (IEEE 802.6) Falls into one of two categories: Contention (802.3, Ethernet, for example) Controlled access (802.5, Token Ring, for example) FDDI, another form of controlled access, is an ANSI/ITU-T standard CCN.28 Random access protocols  when node has packet to send transmit at full channel data rate R. no a priori coordination among nodes  two or more transmitting nodes: “collision”  random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions)  examples of random-access MAC protocols: CSMA, CSMA/CD, CSMA/CA CCN.29 CSMA (carrier sense multiple access) simple CSMA: listen before transmit: if channel sensed idle: transmit entire frame if channel sensed busy: defer transmission  human analogy: don’t interrupt others! CSMA/CD: CSMA with collision detection collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection easy in wired, difficult with wireless  human analogy: the polite conversationalist CCN.30 CSMA: collisions  collisions can still occur with carrier sensing: spatial layout of nodes propagation delay means two nodes may not hear each other’s just-started transmission  collision: entire packet transmission time wasted distance & propagation delay play role in in determining collision probability CCN.31 Carrier Sense On Multi-Access (CSMA) algorithm  An Ethernet does not have a centralized controller. Check whether the cable is currently being used (a computer can check for a carrier)  If no carrier is present (if idle), the computer can transmit a frame  If a carrier is present (if busy), the computer must wait for the sender to finish before proceeding  All computers attached to an Ethernet participate in a distributed coordination scheme is called Carrier Sense Multiple Access (CSMA)  Checking for a carrier wave is called “carrier sense”. After aborting, NIC enters binary (exponential) backoff: after mth collision, NIC chooses K at random from {0,1,2, …, 2m- 1}. NIC waits K·512 bit times, returns to Step 2 more collisions: longer backoff interval CCN.32 Collision Detection (CD)  Collision is the interference/ mixture of two signals  Monitoring a cable during transmission is known as (CD)  Collision produces a garbled transmission prevents either of the two frames from being received correctly  CSMA/CD reduces the amount of time wasted in collisions  Whenever a collision is detected, a sending station immediately stops transmitting CCN.33 Binary Exponential Backoff with CSMA/CD  To avoid multiple collisions, each computer to delay after a collision before attempting to retransmit A maximum delay, d , and forces each computer to choose a random delay less than d  To avoid a sequence of collisions, stations double the range after each collision  A random delay from 0 to d after 1st collision  A random delay between 0 and 2d after 2nd collision,  A random delay between 0 and 4d after 3rd , and so on After a few collisions, the range from which a random value is chosen becomes large CCN.34 Binary Exponential Backoff  stations repeatedly resend when collide on first 10 attempts, mean random delay doubled value then remains same for 6 further attempts after 16 unsuccessful attempts, station gives up and reports error  1-persistent algorithm with binary exponential backoff efficient over wide range of loads CCN.35 Why does CSMA/CD use a random delay?  Randomization is used to avoid having multiple stations transmit simultaneously as soon as the cable is idle.  That is, the standard specifies a maximum delay, d, and requires each station to choose a random delay less than d after a collision occurs.  In most cases, when two stations each choose a random value, the station that chooses the smallest delay will proceed to send a packet and the network will return to normal operation. CCN.36 Token Bus (IEEE 802.4)  A token ring operates as a single, shared medium To send, it must wait for permission before it can access network Once it obtains permission, it has complete control of the ring As it transmits a frame,  the bits pass from the sender to the next computer,  then to the next computer and so on,  until the bits pass completely around the ring  and arrive back at the sender  If a frame is destined for a given computer It makes a copy of the frame as the bits pass around the ring CCN.37 CCN.38 Token Passing in Token Ring (802.5)  Whenever the network is unoccupied, it circulates a simple three-byte token. A Station with data to send, wait for the token.  If the token is free, the station may then send a data frame. It keeps the token and sets a bit inside its NIC as a remainder that it has done so, then sends its one data frame.  This data frame proceeds around the ring, being regenerated by each station.  The intended recipient recognizes its own address, copies the message, checks for errors, changes 4 bits in the last byte of the frame to indicate address recognized and frame copied.  The full packet then continues around the ring until it returns to the station that sent it.  The sender receives the frame and recognizes itself in the source address field.  It then examine the address-recognized bits, if they are set, it knows the frame was received.  Then the sender discards the used data frame and releases the token back to ring. CCN.39 Priority and Reservation:  Once a token has been released, the next station on the ring with data to send has the right to take charge of the ring.  Another option is possible – The busy token can be reserved by a station waiting to transmit, regardless of that station’s location on the ring.  Each station has a priority code.  A station waiting to transmit may reserve the next open token by entering its priority code in the access control (AC) field of the token or data frame.  A station with a higher priority may remove a lower priority reservation and replaces it with its own.  Among stations of equal priority, the process is first come, first served. CCN.40 Reference  Chapter – 4 Computer Networks and Internets, Douglas E. Comer, Prentice Hall. 3rd edition.  Chapters 8 and 9. Computer Networks and Internets, Douglas E. Comer, Prentice Hall. 4th Edition.  Chapters 16. Data and Computer Communications 8 th Edition by William Stallings CCN.41