CS3502, Data and Computer Networks: the physical layer-4

Synchronization u to transport bits from X to R, R must know when X is transmitting, in order to correctly interpret the signals; 2 standard ways are synchronous and asynchronous. u asynchronous transmission u small groups of bits (5-10 bits) u each small group synchronized separately u simple signaling (NRZ) u short distances only; eg, PC to printer u start and stop bits mark the bit group u how much overhead? how efficient is this?

synchronization u synchronous transmission u start, end of data marked by flag byte ( ) u flag pattern must not appear inside frame; bit-stuffing takes care of this u encoding -> need self clocking codes u exercise: give a FSM for bit stuffing for the flag 01110, and to unstuff bits at the receiver u what is the overhead (efficiency)?

interfacing u this means translating from 1 physical protocol to another u digital devices usually have a very limited data transmission/reception capability - not able to transmit onto a network directly u examples: u digital to analog (modems) u digital to digital (PC to LAN) u 4 parts of standard interface: u mechanical u electrical u functional u procedural

interfacing : EIA -232 standard u terminology u DTE - data terminal equipment -the device which we wish to connect to the network u generic term for data source, data terminal (sink), or both u examples: PC, computer terminal, workstation u DCE - data circuit terminating equipment - the device which interfaces with the network u creates, maintains and terminates connection with network u signal conversion and coding u example: modem

interfacing : EIA -232 standard u 25 pin connector; most apps. don’t use all u signal/line types: data, control, timing, ground. (note Table 5.1 list) u 15 meters max distance u +3 to +25 volts for 0; -3 to -25 V for 1. u unbalanced/asymmetric connection (circuit completed by ground). u 1 data line each way, so full duplex possible u more details in text;Tanenbaum p114.

interfacing : ISDN physical connector u standard for ISDN connections (Integrated Services Digital Network) u ISDN basic data rate: 144 Kbps u symmetric - this gives better electrical properties u more logic, less circuits: 8 pins u 2 data pins each way = 4 data pins u date circuits carry both data and control information u other pins for power sources

multiplexing problem: a transmission line operates at Mbps, but 1 connection needs only 64 Kbps; so rest is wasted.... since Mbps costs about $2K/ month. solution : share the link among many users, each paying only their part. purpose : to utilize as much of the line as possible 3 techniques: FDM, synch TDM, statistical TDM

multiplexing : FDM u analog signals with high bandwidth u TV Cable channels; broadcast radio; voice trunks *  must have W link >  w i i.e.,link capacity greater than sum of channels. u main carrier is a composite of many subcarriers. each subcarrier may be modulated with 1 channel u example: a carrier has a total bandwidth of 240 MHz, from 54 to 294 MHz. Subcarriers are centered every 6 MHz; each forming 1 channel. u guard band necessary to avoid interference

multiplexing : FDM u FDM problems u crosstalk - can occur between neighboring channels, if overlap too close u intermodulation noise - possible on high capacity links over distance u noise, clarity - over distance, analog signals more vulnerable than digital; gradually being replaced in most areas. u switching - not as efficient with analog signals

multiplexing : TDM u two types: synchronous and statistical u synchronous TDM u digital data u signal - usually digital; can be analog signal coded digitally u data rate of link must be greater than sum of inputs u similar to timesharing computers u example: T1 multiplexer u standards: DS0, DS1 (T1), DS3 (T3); OCn; EC1

multiplexing : TDM u synchronous TDM u time slot to each input line u 1 slot for synchronization u unused time slots lost u slot size 1 bit or 1 byte, in general u physical layer; no error or flow control u Q: how much buffer space needed? u Q: what capacity needed for 24 voice channels? how many voice channels possible on a T3 line? OC3? OC12? how many T1 lines on an OC12? OC48?

statistical TDM u another way of assigning time slots u if input rates irregular, varied, synch TDM could be wasteful; stat. TDM could be more efficient u slots are assigned dynamically, as needed; u requires more intelligence; more of a data link layer function u frames must have more control information; u show fields of a possible frame u more overhead than synch. TDM; closer to a MAC type protocol

comparison: stat and synch TDM u synch TDM u fixed number slots per round u can waste slots u timing simpler, fixed u format simpler u stat TDM u variable number slots per frame u doesn’t waste slots u more overhead, complexity; similar to data link function u Q: how much buffer space needed for stat TDM?

stat TDM - buffer space summary  average input rate must be less than link capacity  ; but may exceed  temporarily. u buffer space stores temporary overflows u buffer size must be estimated based on expected input rates and their arrival distribution. Given these we can calculate buffer size (queue length); but in reality never can be completely sure.  link utilization is given by  a standard queueing formula  as  approaches 1, queue (buffer) size becomes very large, quickly; approaching infinity as  reaches or exceeds 1  utilization  of no more that 0.8 is good rule of thumb

the voice channel and telephone system u basic telephone network designed to deliver quality voice service; u voice emits analog signal - sound waves - from 30 to 10,000 Hz. Human ears detect up to 20K Hz. u most energy in Hz range; Standard analog voice channel is 4000 Hz. This key number selected many years ago by phone company. u standard PCM digital voice channel is 64 Kbps. u most local telephone loops still analog u all long distance in US is digital; majority is fiber.

the voice channel and telephone system u voice not very sensitive to most noise and distortion; for this and other reasons, local telco loops not well suited to modern data networks u However, the local telco networks are one of few comm. links between homes, businesses and rest of the world u Structure of U S Telephone networks /companies u local loops “last mile” and telcos u long distance networks and companies u network equipment

video channels and the cable TV system u TV cable system established only recent decades u switching equipment designed for broadcast TV u standard TV - needs 6 MHz per channel u copper coaxial cables capable of ~500 MHz; carry many TV channels. u these cables have capacity to carry thousands of voice channels and/or high speed data -- but need appropriate switching equipment at home office, and in homes u already becoming a reality. Will threaten existence of old telcos. (note pending merger of ATT, TCI)