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2004.3.25. Make-up class for 2004.3.23.? 3/27 or 4/15 HW#2 Be careful when you submit; don’t broadcast your solution! Chapter 2.1.~2.4. 1 Problems to Consider When Connecting Node Directly Encoding Framing Error Detection Reliable Delivery Media Access Control 2 Point to Point Data Link Control one sender, one receiver, one link: easier than broadcast link: no Media Access Control no need for explicit MAC addressing e.g., dialup link, ISDN line popular point-to-point DLC protocols: PPP (Point-to-Point Protocol) HDLC (High level data link control) SONET (Synchronous Optical Network) 3 Encoding Many ways to “modulate” process of encoding source data onto a carrier signal with frequency f Digital data -> digital signal Digital data -> analog signal Analog data -> digital signal Analog data -> analog signal 4 Digital Data -> Digital Signal Encoding 5 Digital Data -> Digital Signal NRZ Manchester NRZI 4Bit/5Bit 6 Framing Definition? Task of recognizing where a frame begins and ends Byte-oriented: PPP Bit-oriented: HDLC Clock-based framing: SONET 7 PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fields Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc) 8 PPP Data Frame info: upper layer data being carried check: cyclic redundancy check for error detection 9 Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag? Sender: adds (“stuffs”) extra < 01111110> byte after each < 01111110> data byte Receiver: two 01111110 bytes in a row: discard first byte, continue data reception single 01111110: flag byte 10 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data 11 Bit-Oriented Protocols HDLC Similar to PPP Uses bit stuffing 12 Clock-Based Framing SONET Used mostly for digital transmission over optical fiber 13 Acknowledgements Slides ### copyright Ross 1996-2000, Jim Kurose and Keith 14