Copyright 1999, S.D. Personick. All Rights Reserved. Telecommunications Networking II Lectures Local Area Networks [Ref: “Computer Networks” A.S. Tanenbaum]

Copyright 1999, S.D. Personick. All Rights Reserved. Local Area Networks LAN Overview Link Layer- packet error control and media access (coordination) protocols Network Layer: addressing and routing protocols Higher layer protocols Examples of LAN standards: Ethernet, token ring

Copyright 1999, S.D. Personick. All Rights Reserved. What are we trying to do? We are trying to create a local communications infrastructure which will allow computing appliances to exchange data in the form of packet data units (packets) We want this local communications infrastructure to be low in cost, and easy to implement and maintain

Copyright 1999, S.D. Personick. All Rights Reserved. A typical packet Payload: 64-to-2048 bytes Header Header contains: source address, destination address, special handling requirements, packet sequence number, information about packet history, error control bytes,...

Copyright 1999, S.D. Personick. All Rights Reserved. What are we trying to do? (cont’d) We would like our local communication infrastructure to employ “open” interfaces at appropriate points, so that portions of it can be provided by different suppliers; and so that we can accommodate multiple types of physical layer transmission media: wires, coaxial cable, fiber, wireless We would like our local communication infrastructure to support a wide range of applications

Copyright 1999, S.D. Personick. All Rights Reserved. Issues we need to address When packet data units (PDU’s) are transmitted across a physical medium (e.g., wire pairs), errors may occur We need to coordinate the actions of multiple computing devices that may wish to send PDU’s over the same medium We need a way to identify the intended recipient(s) of a PDU

Copyright 1999, S.D. Personick. All Rights Reserved. Issues We Need to Address We need to deal with the possibility that PDU’s may be inadvertently lost, duplicated, or delivered out of the order in which they were sent We need to deal with the possibility that some PDU’s may require expedited delivery We need to manage congestion when more PDU’s are being sent than the network can accommodate

Copyright 1999, S.D. Personick. All Rights Reserved. “Higher Layer Issues” We need a methodology that enables computing appliances to inform each other of their desire to communicate (establish a “session”), and to accept or decline such communication We may need to provide mechanisms for “secure” communication We may need to provide mechanisms which enable different higher layer protocols to inter- operate (e.g., converting text to speech)

Copyright 1999, S.D. Personick. All Rights Reserved. The Protocol Stack Application Layer (e.g., or WWW) Presentation Layer (e.g., Microsoft Word, plain text) Session Layer (e.g., accepting or denying access) Transport Layer (e.g., asking for retransmission of lost packets; re-ordering packets that are out of order) Network Layer (reading addresses and directing packets toward their intended destinations) Link Layer (coordinating access to the physical medium, and trying to move packets across the physical medium without making errors) Physical layer (e.g., wire pairs or fiber or wireless)

Copyright 1999, S.D. Personick. All Rights Reserved. Separation of Concerns Each layer in the protocol stack should be able to use the services of the layer(s) below it, without having to be concerned with the details of how those services are implemented. For example, the transport layer should not be concerned with, or dependent upon the details of how the network layer manages to get the right packets to the right destinations

Copyright 1999, S.D. Personick. All Rights Reserved. What each layer does The engineers who design physical layer (e.g. wireless) solutions focus on the problems of moving bits (1’s and 0’s) across their physical layer... with a specified probability of error, a specified data rate capability, and specified delay characteristics. E.g., wireless physical layer designers worry about the stuff we discussed in previous lectures

Copyright 1999, S.D. Personick. All Rights Reserved. What each layer does (cont’d) The engineers who design (and agree on standards for) link layer protocols are concerned with: - how to detect errors that occur when PDU’s are communicated across a physical layer; and how such detected errors might also be corrected (error detecting codes, error correcting codes, retransmission) -how to coordinate access to the physical layer by multiple computing appliances which are using the same physical layer medium at the same time (“media access control”)

Copyright 1999, S.D. Personick. All Rights Reserved. What each layer does (cont’d) Engineers who design (and agree on standards for) network layer protocols are concerned with: -how destination addresses (and possibly specific routing requests) are incorporated into PDU’s -how routers (network packet switches) coordinate to establish routes over which packets can be moved toward their intended destinations -how requests to reserve network capacity are specified and accepted (or denied)

Copyright 1999, S.D. Personick. All Rights Reserved. What each layer does (cont’d) Engineers who design (and agree on standards for) transport layer protocols are concerned with -how to ensure end-to-end PDU transport integrity by: detecting lost or duplicated packets; requesting retransmissions of lost packets; re-ordering packets that are received out-of-order; adjusting the rate at which packets are sent across the network to accommodate congestion and other problems; storing packets for retransmission or initial transmission, if required.

Copyright 1999, S.D. Personick. All Rights Reserved. The protocol “hour glass” , WWW, Telephony,... Wire pairs, Fiber, Coax, Wireless,... Fewer is better: TCP, UDP IP Token passing CSMA-CD CSMA-CA HDLC

Copyright 1999, S.D. Personick. All Rights Reserved. HDLC (see Tanenbaum pp ) | Address | Control | Data | Checksum | > or =

Copyright 1999, S.D. Personick. All Rights Reserved. HDLC Each packet data unit (frame) includes a “cyclic redundancy check” (CDC) sequence that allows for the detection of most single or multiple bit errors. Frames with detected errors are discarded. The header of each frame also includes a “frame sequence number” that is typically between 0-7, but could be between 0 and (2**n)-1, where n is between 3 and 7 Frames are acknowledged after they are correctly received The maximum number of outstanding, unacknowledged frames is: (2**n) -1 (e.g., 7 ) Retransmission is used to replace unacknowledged or rejected frames

Copyright 1999, S.D. Personick. All Rights Reserved. HDLC Frames (includes retransmissions) Acknowledgements 210 NR=3* *NR=3 indicates that the next expected frame sequence number = 3

Copyright 1999, S.D. Personick. All Rights Reserved. Cyclic Redundancy Check Examples of error detecting methods: Parity bit = sum of the binary digits in the part of the packet you want to protect ( modulo 2). A single bit error causes the parity bit not to match the parity calculated at the receiver CRC-16 or CRC-32: Divide the data field by a binary polynomial with either 2**16 or 2**32 as its most significant bit. The CRC is the remainder

Copyright 1999, S.D. Personick. All Rights Reserved. Cyclic Redundancy Check (cont’d) Example: At the transmitter…. Data sequence to be protected is: (4 byte example)… = DS Polynomial is: CRC- CCITT (16 bit CRC): = P Divide DS by P; the remainder is the CRC

Copyright 1999, S.D. Personick. All Rights Reserved. CRC (binary long division) | ________________ (First remainder + next digit) _________________ (Second remainder + next digit)

Copyright 1999, S.D. Personick. All Rights Reserved. Cyclic Redundancy Check (cont’d) Append CRC (remainder) to data being protected: [DS CRC] ……. At the receiver... Divide [DS CRC] by P; The result should be a zero remainder

Copyright 1999, S.D. Personick. All Rights Reserved. Media Access Protocols Sharing a physical link among multiple (simultaneous) users Objectives are: to minimize waiting time to obtain access to a link; to get a high level of achievable throughput on the link; and to provide a “fair” sharing of the link capacity among users. These objectives are sometimes at odds with each other

Copyright 1999, S.D. Personick. All Rights Reserved. Aloha* Protocol Transmit your packet, if it gets through…fine. If it collides with another packet, then try again. Used to share links with long end-to-end delay (vs. packet duration) *Early radio network in the Hawiian Islands

Copyright 1999, S.D. Personick. All Rights Reserved. Aloha Protocol (with fixed length packets) N users; each user sending k packets per second; fixed packet length = T (seconds). Channel bit rate = B (bits/sec) The probability that any transmitter packet does NOT experience a collision= exp [-2(N-1)kT]; I.e. none of the other N-1 users transmits a packet within T seconds (before or after) of the time I start sending my packet. Throughput = BT x Nk x exp [-2(N-1)kT] For N>>1, maximum throughput = B/(2e)

Copyright 1999, S.D. Personick. All Rights Reserved. Carrier Sense Multiple Access A variation on Ahola, where you listen to see if the “channel” is being used (e.g., listen for a “carrier”) before transmitting. If the end-to-end delays are short compared to the length of a transmitted packet, this can result in high utilization of the channel If the point-to-point (through the medium) delay between users is large compared to the duration of a transmitted packet, then this becomes the same as Aloha

Copyright 1999, S.D. Personick. All Rights Reserved. CSMA/CD CSMA with collision detection If a collision is detected, the colliding transmitters “back off” for a randomly selected amount of time, in order to try to prevent another collision when the re- transmit

Copyright 1999, S.D. Personick. All Rights Reserved. Token Passing Each transmitter has an opportunity (a “turn”) to send packets... defined as holding the “token”. After a predefined length of time or number of transmitted packets, it must pass the token to another transmitter.

Copyright 1999, S.D. Personick. All Rights Reserved. Internetworking Protocol (IP) Networking Layer Issues are: Addressing Creating routing tables (in routers) Segmentation and re-assembly of IP packets Domain name service

Copyright 1999, S.D. Personick. All Rights Reserved. IP Addressing (IPv4) 0-255, 0-255, 0-255, bytes of address: e.g., Class A: 1 byte network/ 3 bytes subnet Class B: 2 bytes network/ 2 bytes subnet Class C: 3 bytes network/ 1 byte subnet

Copyright 1999, S.D. Personick. All Rights Reserved. Routing Source Host (computer) Destination Host (computer) local router backbone router Drexel.edu Bellatlantic.net

Copyright 1999, S.D. Personick. All Rights Reserved. Setting Up Routing Tables Each router (packet switch) communicates with its neighbors, using a complex protocol to determine where it should route packets to Routing tables are continuously updated When an incoming packet arrives, the router compares the destination address of the packet to the listings in its routing table to determine which output to route the packet to

Copyright 1999, S.D. Personick. All Rights Reserved. Domain Name Service The domain name service complements IP routing by providing a mechanism for translating domain “names” into IP addresses ece.drexel.edu =>

Copyright 1999, S.D. Personick. All Rights Reserved. TCP (Transport Layer) Keeps track of incoming packets to detect lost packets Stores transmitted packets for possible retransmission Acknowledges (or NACKS) receipt of packets Adjusts window size to accommodate end- to-end packet loss and delay conditions

Copyright 1999, S.D. Personick. All Rights Reserved. Ethernet Standards (802.3) see Tanenbaum pp is an IEEE standard for the use of CSMD/CD on a shared medium (e.g., coaxial cable, wire pairs cable, fiber optic cable, wireless) “Ethernet” is the commonly used name for LANs; but it really refers to a specific product that was developed at Xerox PARC

Copyright 1999, S.D. Personick. All Rights Reserved Layer is specified for various types of cable: -10Base5: Thick coax, 500m per segment* -10Base2: Thin coax, 200m per segment* -10BaseT: Twisted pair, 100m per segment -10BaseF: Fiber optics, 2000m per segment -higher speed versions of *electronic repeaters, bridges or hubs allow concatenation of segments

Copyright 1999, S.D. Personick. All Rights Reserved Layer 1 (cont’d) Manchester bipolar coding is used to avoid ambiguities between 0 volts and a logical “0”, and also to produce “d.c. balance”, which is helpful for designing the transceivers that interface to LANs Binary “1” => high-low (+.85v, -.85v) Binary “0” => low-high (-.85v, +.85v)

Copyright 1999, S.D. Personick. All Rights Reserved Layer 2 Layer 2 frames consist of: -7 bytes of preamble to allow for transceiver synchronization ( x 7) -1 byte “start of frame” ( ) -6 byte (48 bit) destination (global) address -6 byte (48 bit) source (global) address -2 byte “length of frame” - Between 0 and 1500 bytes of data -Up to 46 bytes of “pad” (if needed) - 4 byte (16 bit) checksum (CRC)

Copyright 1999, S.D. Personick. All Rights Reserved Layer 2 (cont’d) When a station detects a collision, it sends out a 48 bit “noise burst” to warn all other stations As per CSMD-CD definition, stations listen before they transmit. In the event of a detected collision, stations “back off” and wait a random amount of time before sending again. Frames must be longer than 2T, where T is the maximum 1-way delay. Minimum frame duration = 51,200 ns

Copyright 1999, S.D. Personick. All Rights Reserved. Token Ring Standards (802.5) see Tanenbaum pp Physical layer is formed by point-to-point links between adjacent stations, connected to form a closed ring -Each received bit is buffered, examined, possibly changed, and re-transmitted by each station The token consists of a 3-byte pattern. Changing one bit of this pattern converts it to the first 3 bytes of a normal data frame

Copyright 1999, S.D. Personick. All Rights Reserved (continued) In “listen mode” each node on the ring inserts 1 bit of buffer delay as part of the total delay around the ring In “transmit mode” a node inserts its frames into the ring, and removes its inserted bits as they come back around the ring. When finished transmitting, a transmitter reinserts the token on to the ring

Copyright 1999, S.D. Personick. All Rights Reserved Physical layer 1, 4 or 16 Mbps on shielded twisted pairs Differential Manchester bipolar coding: +/ volts “Bypass center” (central hub) can be used to bypass failed nodes (star-shaped ring)

Copyright 1999, S.D. Personick. All Rights Reserved Layer 2 Token holding time is 10 msec (100, Mbps) The data field of a frame has no byte number limit (except as limited by the allowed token holding time) Frames are acknowledged by the receiving station by changing a bit in the “frame status” byte (after the CRC)