Data and Computer Communications

1. Data and Computer Communications Chapter 15 – Local Area Network Overview Ninth Edition by William Stallings Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011

2. What is a computer network? • most basic version is two computers that are connected by a cable • a number of independent computers linked together to share data and peripherals, such as hard disks and printers

3. Advantages of a computer network • Share information (or data) • Share hardware and software • Centralize administration and support

4. LAN Applications • LAN consists of a shared transmission medium and a set of stations (computers, printers, …) • Today, some LANs don’t use shared transmission medium anymore in order to improve the transmission performance • LANs usually are owned by the organization that is using the network to interconnect equipment. • There are different types of LANs, each with its own protocols. • Applications of LAN • Personal computer LANs • Share resources (e.g., printers) • Share information (e.g., files) • “Limited” data rate (10Mbps – 1000Mbps) • Backend networks • Interconnecting large systems (mainframes and large storage devices) • High data rate • Storage Area Networks • A separate network to handle storage needs • Hard disks, tape libraries, CD arrays • Detaches storage tasks from specific servers

5. Frame Transmissionon Bus LAN

6. Bus and Tree

7. Frame TransmissionRing LAN

8. Ring Topology • a closed loop of repeaters joined by point-to-point links • receive data on one link & retransmit on another • links unidirectional • stations attach to repeaters • data transmitted in frames • circulate past all stations • destination recognizes address and copies frame • frame circulates back to source where it is removed • Medium Access Control determines when a station can insert frame

9. Star Topology

10. Star Topology • each station connects to common central node • usually via two point-to-point link, one for transmission and one for reception

11. Star Topology • Each station is directly connected to a central node • Usually via two point-to-point links • Two types of central node • Simple one: operate in a broadcast fashion • Transmission of a frame from one station to the central node is retransmitted on all of the outgoing links • Only one station can transmit at a time • The central node is referred to as ahub • Complex one: act as frame-switching device • An incoming frame is buffered in the central node and then retransmitted on the outgoing link to the destination station • Intelligent and powerful • More than one stations can transmit at the same time • Buffers are required at the central node to resolve conflict (if more than one frames are destined to the same station at the same time) • The central node is referred to as a switch

12. Hubs • Central element of a star topology • Each station connects to hub by two lines • Transmit and receive • So a link consists of two unshielded twisted pairs (UTP) • Hub acts as a repeater • When one station transmits, hub repeats signal to each station • Physically star, logically bus • Limited to about 100 m • High data rate and poor transmission qualities of UTP • Optical fiber may be used for about 500 m • Transmission from any station received by all other stations • No privacy, security issues • If two stations transmit at the same time, collision

13. LAN Topologies

14. Bus LAN Transmission Media cont…

15. Bus LAN Transmission Media For bus topology, only baseband coaxial cable has achieved widespread use

16. Ring and Star Topologies

17. Media Available

18. Choice of Medium • constrained by LAN topology • capacity • to support the expected network traffic • reliability • to meet requirements for availability • types of data supported • tailored to the application

19. Choice of Topology • medium • wiring layout • access control

20. Choice of Topology • Transmission medium • Twisted pair: popularly used by today’s Ethernet • Baseband coaxial cable (digital signaling): was used by the original Ethernet • Broadband coaxial cable (analogsignaling): not popular due to the cost • Optical fiber: popularly used by Ethernet • Air: Wireless LAN becomes very popular today • Installation and maintenance • For bus and ring topology, installation also means removing some existing links, so it is costly • For ring topology, a failure of one link disable the entire network • For star topology, it can take advantage of the natural layout of wiring in a building, and installation/maintenance of one link does not affect other links • Star topology is the most popular one today

21. Section15.2

23. LAN Protocol Architecture Open Systems Interconnection (OSI) Logical link control (LLC) Medium access control (MAC) Physical

24. 802 Layers • Physical Layer • Encoding/decoding of signals • Preamble generation/removal • for synchronization • Bit transmission/reception • Transmission medium and topology • Medium Access Control • Manage access to a shared-access medium • Not found in traditional point-to-point layer 2 data link protocol • Logical Link Control • Provide interfaces (called “services”) to higher layers • Perform flow and error control

25. IEEE 802 Layers • Logical Link Control Layer (LLC) • provide interface to higher levels • perform flow and error control • Media Access Control • on transmit, assemble data into frame • on reception, disassemble frame, perform address recognition and error detection • govern access to transmission medium • for same LLC, may have several MAC options

26. LAN Protocols in Context

27. MAC addresses • Sometimes called Ethernet address • Uniquely identify each computer, printer, or device in a network • Stored in the network interface card (NIC) read-only memory, burned-in address by manufacturer • Interface is where 2 systems meet and interact • Used by • Ethernet • 802.11 wireless networks • Bluetooth

28. Logical Link Control • transmission of link level PDUs (Protocol data unit) between stations • must support multi-access, shared medium • relieved of some details of link access by the MAC layer • addressing involves specifying source and destination LLC users • referred to as service access points (SAPs)

29. LLC Services

30. LLC Service Alternatives

31. Bridge Function

32. Bridges • connects similar LANs with identical physical and link layer protocols • minimal processing • reasons for use: • reliability • performance • security • geography

33. Bridge Design Aspects • no modification to frame content or format • no encapsulation • exact bitwise copy of frame • buffering to meet peak demand • contains routing and address intelligence • may connect more than two LANs • bridging is transparent to stations

34. Connection of Two LANs

35. Why Not One Large LAN? • There are several reasons for the use of multiple LANs connected by bridges • Reliability • For a single large LAN, a fault on the network may disable communication for all devices. • By using bridges, the network can be partitioned into self-contained units • Performance • Performance on a LAN declines with an increase in the number of devices or the length of the wire. • A number of smaller LANs will often give improved performance if devices can be clustered so that intranetwork traffic exceeds internetwork traffic. • Security • Multiple LANs may improve security of communications. • Geography • A single LAN is always limited by its diameter. • Two separate LANs are needed to support devices clustered in two geographically distant locations. • In summary • Bridges provides an extension to the LAN that requires no modification to the communications software in the stations attached to the LANs. • It appears to all stations on the two (or more) LANs that there is a single LAN.

36. Bridges andLANs withAlternativeRoutes

37. Fixed Routing • simplest and most common • suitable for internets that are stable • a fixed route is selected for each pair of LANs • usually least hop route • only changed when topology changes • widely used but limited flexibility