1 / 18

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

CS3502, Data and Computer Networks: the physical layer-4. Synchronization. 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 . asynchronous transmission

nantai
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


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

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

  3. synchronization • synchronous transmission • start, end of data marked by flag byte (01111110) • flag pattern must not appear inside frame; bit-stuffing takes care of this • encoding -> need self clocking codes • exercise: give a FSM for bit stuffing for the flag 01110, and to unstuff bits at the receiver • what is the overhead (efficiency)?

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

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

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

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

  8. multiplexing problem: a transmission line operates at 1.544 Mbps, but 1 connection needs only 64 Kbps; so rest is wasted.... since 1.544 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

  9. multiplexing : FDM • analog signals with high bandwidth • TV Cable channels; broadcast radio; voice trunks * • must have Wlink>wi i.e.,link capacity greater than sum of channels. • main carrier is a composite of many subcarriers. each subcarrier may be modulated with 1 channel • 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. • guard band necessary to avoid interference

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

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

  12. multiplexing : TDM • synchronous TDM • time slot to each input line • 1 slot for synchronization • unused time slots lost • slot size 1 bit or 1 byte, in general • physical layer; no error or flow control • Q: how much buffer space needed? • 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?

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

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

  15. stat TDM - buffer space summary • average input rate  must be less than link capacity ; but  may exceed temporarily. • buffer space stores temporary overflows • 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

  16. the voice channel and telephone system • basic telephone network designed to deliver quality voice service; • voice emits analog signal - sound waves - from 30 to 10,000 Hz. Human ears detect up to 20K Hz. • most energy in 200-3500 Hz range; Standard analog voice channel is 4000 Hz. This key number selected many years ago by phone company. • standard PCM digital voice channel is 64 Kbps. • most local telephone loops still analog • all long distance in US is digital; majority is fiber.

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

  18. video channels and the cable TV system • TV cable system established only recent decades • switching equipment designed for broadcast TV • standard TV - needs 6 MHz per channel • copper coaxial cables capable of ~500 MHz; carry many TV channels. • 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 • already becoming a reality . Will threaten existence of old telcos. (note pending merger of ATT, TCI)

More Related