1 ELEN602 Lecture 10 Review of last lecture –M/G/1 and M/D/1 Results Multi-access Networks Medium Access Control –ALOHA –Slotted ALOHA

M Shared Multiple Access Medium  Multiple Access Communications

3 Medium Sharing Techniques Static Channelization Dynamic Medium Access Control Scheduling Random Access Approaches to Sharing

4 Satellite Channel = f in = f out Satellite Communications

5 Multidrop telephone lines Inbound line Outbound line Multidrop Telephone line

6 Ring networks Multitapped Bus Ring Networks and Buses

7 Wireless LAN

8 A transmits at t = 0 Distance d meters t prop = d / seconds AB B transmits before t = t prop and detects collision shortly thereafter AB A B A detects collision at t = 2 t prop Impact of Bandwidth-Delay Product

9 Transfer Delay Load E[T]/E[X]   ma x 1 1 Typical Frame Transfer Delay versus Load

10 Transfer Delay Load E[T]/E[X]   ma x 1 1 a a a > a BW-Delay Product and Performance

11 (a) RAM ROM Ethernet Processor (b) Typical LAN

12 Data Link Layer CSMA-CD Token Ring Logical Link Control Physical Layer MAC LLC Wireless LAN Network Layer Physical Layer OSI IEEE 802 Various Physical Layers Other LANs

13 PHY MAC PHY MAC PHY MAC Unreliable Datagram Service MAC Sublayer

14 PHY MAC PHY MAC PHY MAC Reliable Packet Service LLC A C A C LLC Layer

15 Destination SAP Address Source SAP Address Information 1 byte 1 Control 1 or 2 Destination SAP Address Source SAP Address I/G 7 bits1 C/R 7 bits 1 I/G = Individual or group addressC/R = Command or response frame LLC PDU Structure

16 LLC Header IP Data MAC Header FCS LLC PDU IP Packet LLC PDU and MAC Frame

17 t t0t0 t 0 -X t 0 +X t 0 +X+2t prop t 0 +X+2t prop  Vulnerable period Time-out Backoff period Retransmission if necessary First transmission Retransmission ALOHA Random Access Scheme

18 Ge -G Ge -2G G S Throughput for ALOHA and Slotted ALOHA

19 t (k+1)X kX t 0 +X+2t prop  t 0 +X+2t prop Vulnerable period Time-out Backoff period Retransmission if necessary Slotted ALOHA

20 A Station A begins transmission at t=0 A Station A captures channel at t=t prop CSMA Random Access Scheme

21 sensing P-Persistent CSMA Scheme

22 Non-Persistent CSMA S G Throughput of CSMA

23 1-Persistent CSMA S G Throughput of 1-persistent CSMA