1 ELEN 602 Lecture 6 Review of last lecture –Generator polynomials –Stop-and-wait, Go-back-N ARQ Today’s lecture –Selective Repeat ARQ –Analysis of all ARQ schemes –Synchronous service –HDLC and PPP Reading: Chaps

2 Networ k Synchronous source sends periodic information blocks Network output not periodic Timing Recovery

3 send times playout times arriva l times T playout time Receiver too slow buffer overflow T playout time Receiver too fast buffer starvation Many late packets T playout time Receiver speed just right Impact of clock rate differences

4 t4t4 t3t3 t2t2 t1t1 Timestamps Add Smoothing filter Adjust frequency Recovered clock Counter + - Buffer for information blocks Error signal Playout command Adaptive Clock Recovery

5 Network clock fnfn Transmitter Network Receiver fsfs frfr  f Clock Recovery with Synchronous Network

6 Physical Layer Data Link Layer Data Link Layer Network Layer DLSDU Network Layer Physical Layer DLPDU NLPDU “packet” “frame” DLSAP The Data Link Layer

7 PrimarySecondary Commands Responses Unbalanced Point-to-point link Primary Commands Responses Secondary Unbalanced Multipoint link Primary Secondary Commands Responses Balanced Point-to-point link between Combined Stations Primary Secondary Commands Responses HDLC Configurations

8 Flag Address ControlInformation FCS HDLC Frame Format

9 0N(S) N(R) P/F Information Frame N(R) P/F Supervisory Frame Unnumbered Frame 1 0 SS P/F 1 1 MMM M M Control Field Format

10 SABM UA DISC Data transfer Connection Establishment and Release

11 Primary A Secondaries B, C B, RR, 0, P B, I, 0, 0 B, I, 1, 0 B, I, 2, 0,F X B, SREJ, 1 C, RR, 0, P C, RR, 0, F B, SREJ, 1,P B, I, 1, 0 B, I, 3, 0 B, I, 4, 0, F B, I, 0, 5 Exchange of Frames -Normal Response Mode

12 Combined Station A Combined Station B B, I, 0, 0 A, I, 0, 0 B, I, 1, 0 B, I, 2, 1 A, I, 1, 1 A, I, 2, 1 X B, REJ, 1 B, I, 3, 2 B, I, 4, 3 B, I, 1, 3 B, I, 2, 4 B, I, 3, 4 A, I, 3, 1 B, RR, 2 B, RR, 3 Exchange of Frames - Asynchronous Balanced Mode

13 Flag flag Address Control Information CRC Protocol Unnumbered frame Specifies what kind of packet is contained in the payload, e.g., LCP, NCP, IP, OSI CLNP, IPX All stations are to accept the frame PPP Frame Format

14 A Typical Scenario-- PPP Phase Diagram Home PC to Internet Service Provider 1. PC calls router via modem. 2. PC and router exchange LCP packets to negotiate PPP parameters. 3. Check on identities. 4. NCP packets exchanged to configure the network layer, e.g., TCP/IP ( requires IP address assignment). 5. Data transport, e.g. send/receive IP packets. 6. NCP used to tear down the network layer connection (free up IP address); LCP used to shut down data link layer connection. 7. Modem hangs up. Dead Establish Authenticate Network Terminate Open failed 1. Carrier Detected 2. Options Negotiated 3. Authentication Completed 4. NCP Configuration 6. Done 7. Carrier Dropped 5.