1. CSCI 465 D ata Communications and Networks Lecture 19 Martin van Bommel CSCI 465 Data Communications & Networks 1

2. Local Area Network (LAN) Wide Area Network (WAN) – Cover large geographical area – Can be either public or private Local Area Network (LAN) – Cover single building or cluster of buildings – Privately owned – Network assets and network management typically controlled by user organization CSCI 465 Data Communications & Networks 2

3. LAN Key Elements Topology – Way in which end points (stations) interconnect Transmission medium Wiring Layout Protocol Medium Access Control CSCI 465 Data Communications & Networks 3

4. LAN Topologies CSCI 465 Data Communications & Networks 4

5. LAN Topology: Bus and Tree Bus: Multipoint medium Stations attach to linear medium (bus) via tap Full-duplex between station and tap Transmission from any stations travels entire medium (both directions) Terminators required at each end of bus CSCI 465 Data Communications & Networks 5 Tree: Generalization of bus Branching cable with no closed loops Cable(s) begin at headend, travel to branches which may have branches of their own Each transmission propagates through network, heard by all stations

6. Bus and Tree Issues One station could dominate medium – Frames - Transmit small block at a time Two stations transmitting at same time will lead to garbled messages – Have medium access control protocols – Stations monitor control information in frames All stations hear all transmissions – Frames include headers with address Frames removed by terminators at ends CSCI 465 Data Communications & Networks 6

7. LAN Topology: Ring Set of repeaters joined by point-to-point links in a closed loop 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 CSCI 465 Data Communications & Networks 7

8. LAN Topology: Star Stations connect to common central node – Usually via two point-to-point links – One for transmission and one for reception Central Node (Hub) – Operates in broadcast fashion – Physical star – logically a bus – Only one station can transmit at a time – Hub could act as a frame switch send only to recipient CSCI 465 Data Communications & Networks 8

9. Choosing a LAN Topology Factors to consider include reliability, flexibility/expandability, and performance Bus/tree is most flexible Tree topology easy to lay out Ring provides high throughput, but reliability problems – single failure cripples network Star can be high speed for short distances, but has limited expandability – natural layout CSCI 465 Data Communications & Networks 9

10. LAN Transmission Media Unshielded Twisted Pair (UTP) – Early LANs used voice-grade cable – Scaling up for higher data rates not practical Shielded Twisted Pair (STP) – Much higher performance Baseband coaxial cable – Digital signaling – used in original Ethernet Broadband coaxial cable – Used in cable TV systems – Expensive, difficult to install and maintain Optical fiber – used in high-speed backbones CSCI 465 Data Communications & Networks 10

11. Selecting Transmission Media Capacity – Can it support expected traffic? Reliability – Can it meet requirements for availability? Types of data supported – Is it well-suited to the applications involved? Environmental scope – Can it provide service in required environments? CSCI 465 Data Communications & Networks 11

12. LAN Protocol Architecture CSCI 465 Data Communications & Networks 12

13. IEEE 802 Layers Physical Layer – Encoding / decoding of signals – Preamble generation / removal - synchronization – Bit transmission / reception – Transmission medium and topology Considered below physical layer of OSI model Critical in LAN design CSCI 465 Data Communications & Networks 13

14. IEEE 802 LLC and MAC Logical Link Control Layer (LLC) – Provides interface to higher levels – Performs flow and error control Medium Access Control (MAC) – Assemble data into frame – address, error control – Disassemble frame address recognition and error detection – Govern access to transmission medium For same LLC, several MAC options provided CSCI 465 Data Communications & Networks 14

15. LAN Protocols in Context CSCI 465 Data Communications & Networks 15

16. Logical Link Control (LLC) Transmission of link level protocol data units (PDUs) between stations Must support multi-access, shared medium – Some details of link access performed by MAC layer Addressing – source and destination LLC users – Referred to as service access points (SAPs) CSCI 465 Data Communications & Networks 16

17. LLC Services Unacknowledged connectionless service – Datagram style service – Delivery of data is not guaranteed no flow or error control mechanisms Connection mode service – Logical connection set up between two users flow and error control provided Acknowledged connectionless service – Datagrams are to be acknowledged CSCI 465 Data Communications & Networks 17

18. Medium Access Control (MAC) Controls access to the transmission medium Key parameters – Where Centralized – greater control, single point of failure Distributed – more complex, but more redundant – How Synchronous – capacity dedicated to connection – Similar to circuit switching – not optimal in LANs Asynchronous – response to demand – Round robin, reservation, or contention CSCI 465 Data Communications & Networks 18

19. Asynchronous MAC Systems Round robin - e ach station is given turn to transmit data – Decline or transmit up to pre-defined limit – Waste time polling idle stations Reservation - divide time on medium into slots – Stations reserve future slots – Good for stream traffic – lengthy, continuous data Contention - all stations contend for time – Good for bursty traffic – short, sporadic data – Simple to implement and efficient under light load – Tend to collapse under heavy load CSCI 465 Data Communications & Networks 19

20. MAC Frame Handling MAC layer receives data from LLC layer PDU is referred to as a MAC frame with fields: – MAC control – destination MAC address – source MAC address – LLC – data for next higher layer – CRC - frame check sequence (FCS) MAC layer detects errors and discards frames LLC optionally retransmits unsuccessful frames CSCI 465 Data Communications & Networks 20