 Make-up class for  2:30-4:00 on Thu, 4/15  HW#3  Due 2:30pm on Thu, 4/8  Turn in to Meeyoung before class  No late turn-ins  Chapter 2.1.~2.5.

2 Problems to Consider When Connecting Node Directly  Encoding  Framing  Error Detection  Reliable Delivery  Media Access Control

3 Clock-Based Framing  SONET  Used mostly for digital transmission over optical fiber on long haul links  Sender and receive clocks synchronized  Part of SDH (Synchronous Digital Hierarchy)  Basic rate of 51.84Mbps (STS-1)  OC-n: OC-3, -12, -48, OC-192 (STS-1 x n)

4 Error Detection  Q: What is prob of having a 1-bit error?  Depends on BER of the medium  Goal of Error Detection  Maximize prob of detecting errors using a small # of redundant bits  Well-known algorithms:  Two-dimensional parity  Checksum  CRC

5 Error Detection vs Correction  When to use error correction?  BER is high  cost of retransmission is very high

6 Reliable Transmission  Stop-and-wait  acknowledgement, timeouts, 1-bit seq no.  Sliding window  sender: window size, last ack’d rcv’d, last frame sent  receiver: window size, largest acceptable frame, last frame rcv’d

7 Next Class  Finish Chap. 2.