Transcript ppt
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 5.1-5.4 1 Timing Recovery Synchronous source sends periodic information blocks Network output not periodic Networ k 2 Impact of clock rate differences arriva l times Receiver too slow buffer overflow time send times playout times Tplayout time Receiver too fast buffer starvation time time Receiver speed just right Many late packets Tplayout time Tplayout time 3 Adaptive Clock Recovery Buffer for information blocks Error signal + Add t4 t3 t2 t1 Playout command Smoothing filter Adjust frequency - Recovered clock Timestamps Counter 4 Clock Recovery with Synchronous Network Receiver Transmitter f f fs Network fr fn Network clock 5 The Data Link Layer Network Layer NLPDU Network Layer “packet” DLSDU DLSAP DLSAP Data Link Layer DLSDU DLPDU Data Link Layer “frame” Physical Layer Physical Layer 6 HDLC Configurations Unbalanced Point-to-point link Commands Primary Secondary Responses Unbalanced Multipoint link Commands Primary Responses Secondary Secondary Secondary Balanced Point-to-point link between Combined Stations Primary Secondary Commands Responses Responses Secondary Commands Primary 7 HDLC Frame Format Flag Address Control Information FCS Flag 8 Control Field Format Information Frame 1 2-4 0 5 N(S) 6-8 P/F N(R) N(R) Supervisory Frame 1 0 S S P/F M M P/F Unnumbered Frame 1 1 M M M 9 Connection Establishment and Release SABM UA Data transfer DISC UA 10 Exchange of Frames -Normal Response Mode Secondaries B, C Primary A B, RR, 0, P X B, I, 0, 0 B, I, 1, 0 B, I, 2, 0,F 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 11 Exchange of Frames - Asynchronous Balanced Mode Combined Station A Combined Station B B, I, 0, 0 B, I, 1, 0 A, I, 0, 0 X A, I, 1, 1 B, I, 2, 1 A, I, 2, 1 B, I, 3, 2 B, REJ, 1 B, I, 4, 3 A, I, 3, 1 B, I, 1, 3 B, I, 2, 4 B, I, 3, 4 B, RR, 2 B, RR, 3 12 PPP Frame Format Flag Address 01111110 1111111 All stations are to accept the frame Control 00000011 Unnumbered frame Protocol Information CRC flag 01111110 Specifies what kind of packet is contained in the payload, e.g., LCP, NCP, IP, OSI CLNP, IPX 13 A Typical Scenario-- PPP Phase Diagram 1. Carrier Detected Dead 7. Carrier Dropped failed Establish Terminate 6. Done 2. Options Negotiated failed Authenticate 5. Open 4. NCP Configuration 3. Authentication Completed Network 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. 14