1. 1 CP2022 - Lecture 3 Computer communications 2

2. 2 Information quality and reliability A reliable communication implies reliable information but.. Other factors influence its quality e.g. Timeliness of the delivered information Speed or frequency of the information provision Completeness of the information Selectivity of the information Relevance or specificity of the information

3. 3 Data compression Data compression is the reduction of unstructured redundancy in data Many schemes are used - For example: JPEG images are compressed Fax documents are compressed Internet Audio is compressed Compression ratios depend on scheme and redundancy

4. 4 Data compression 2 Two forms of compression Lossless and lossy Lossless Lossless Where the original data is recoverable Used for text, programs, and similar data Lossy Lossy Where some information is lost used for some picture formats, sound & video

5. 5 Communication systems For systems to be effective they need to fit the purpose of the communication be able to transfer the amount of data required Choice is not always available Equipment can determine limits E.g. modem access limits data rate to 56 kbps Compression + increasing system data rates multimedia communication

6. 6 Transmission Media Various types are used Twisted Pair, Coaxial and Fibre Twisted pair - can be unshielded or shielded UTP - STP

7. 7 Transmission Media (2) Central conductor Insulators Screen conductor Coaxial Cable

8. 8 Transmission Media (3) Optical conductor Optical cladding Protective plastic coating Optical fibre - Uses laser light

9. 9 Characteristics of communication systems Choice of system is determined by need Usually Capacity is over-riding concern Pre-emptive choice can save future expense E.g. using fibre now - allows later system upgrades Actual system determines its characteristics E.g. Typical system error rate determines the error control mechanism employed

10. 10 Typical systems PSTN The telephone system Designed for voice Data rate - up to 56 kbps ISDN the digital system Designed for Voice, data, fax etc. 2x64k bps channels =128Kbps minimum Can be aggregated for higher rates

11. 11 Systems Cellular networks Two main digital systems GSM &PCN Same technology - different frequencies used Now 3G also available Data rates Data can be used simply at 9.6k or 14.4k bps GPRS can be faster (up to115kbps) EDGE claims up to 384kbps 3G phones claims speeds of 2Mbps (asymmetric links – faster to handset than to server)

12. 12 Networks Classified by size, technology and protocol Local Area Networks Small area, High speed ring or bus, IEEE 802.X Wide Area Networks Large area, slow speed sparsely connected, Internet, OSI or SNA Metropolitan Area Networks City wide, fibre network, high speed technology.

13. 13 Local Area Networks Under the control of a single authority e.g. a company or university Typically a Ring, Star or Bus network Bus is commonest with IEEE 802.3 being most common 10 Mbps protocol Higher speed networks 100baseT is now popular - uses hub concentrators/repeaters Upgraded from older 10baseT networks Gigabit Ethernet also available (MI block)

14. 14 LAN topologies The three main topologies used in LANs

15. 15 LAN hub wiring Typical 10 or 100 baseT configuration Star-like BUS system!

16. 16 Wide Area Networks nodes Typically sparsely connected nodes e.g.

17. 17 High speed LANs Standards at 100 –1000 Mbps Mostly Bus systems can use similar cables as 10 Mbps but.. length and cable properties more critical Other options include desktop integration of ATM at 155+ Mbps (switched)

18. 18 Switching Switching allows full speed of network to individual computers So 10 Mbps switched is 10Mbps per user Minimum expected speed = Backbone speed Number of users E.g. For a 100 Mbps backbone with 24 users@10 Mbps Minimum speed = 4.1 Mbps (not 10 Mbps)

19. 19 LAN standards Local area network standards are made and published by IEEE (US organisation) IEEE 802 series Examples: 802.3 - Ethernet (CSMA/CD bus systems) 802.5 - Token Ring 802.11 - Wireless networks 802.16 - Broadband Wireless MAN

20. 20 WAN standards Most WANs now use Internet standards TCP/IP the underlying transport protocols Other network protocols have been used X.25 - Public switched networks SNA - IBM networks OSI - Government networks

21. 21 Narrow band restrictions WANs share data links Total capacity in each link is shared Speed determined by slowest link Routing used to determine best route for data Route choice needs to dynamically change to adapt to network change

22. 22 Broadband systems Increase in speed in WANs requires newer technology ATM seems the popular choice Becoming more widely used Allows 155+ Mbps switched links Simple technology - very high speed switches Connection-oriented service Full speed end-to-end if required Useful for data-intensive multimedia

23. 23 The Internet The Internet is the current network of choice for many users A network of networks Partially administered by the Internet Society (IAB, IETF) Becoming more regulated as its use increases Access via Internet Service Providers (ISPs)

24. 24 Summary There are many ways to communicate between computers Networks offer a mechanism for wider links LANs, WANs and the Internet combine to give universal communication based on Standard Standard protocols Standard Standard technologies Standard Standard topologies