Data and Computer Communications

1. Data and Computer Communications Updated: 2/9/2009

2. Data Link Control Protocols • They are required to provide a layer of logic to manage exchange of data over a link • frame synchronization • flow control • error control • addressing • control and data • link management

3. Flow Control • Objectives • Sender does not flood the receiver - prevents buffer overflow • Maximize throughput • Basic Idea • Sender throttled until receiver grants permission • Impacted by • Transmission time • time taken to emit all bits into medium • Propagation time • time for a bit to traverse the link • Two basic types are discussed • Stop-and-Wait FC • Sliding Window FC

4. A virtual time sequence diagram Illustrating the sender-receiver relationship Space Time Diagrams

5. Stop and Wait Flow Control • Basic Idea • Source transmits frame • Destination receives frame and replies with acknowledgement (ACK) • Source waits for ACK before sending next • Destination can stop flow by not send ACK

6. Performance • Works well for a few large frames • Stop and wait becomes inadequate if large block of data is split into small frames

7. Efficiency of Stop&Wait We define parameter a: Utilization (or efficiency) can be calculated as: What happens if a changes? Examples of signal speed: Light in vacuum: 300 m/ms Light in fiber: 200 m/ms Electricity: 250 m/ms The textbook uses B for tprop and L for tframe

8. Performance Examples How high is the satellite? Assume data rate = 56 Kbps: Find tfrme, a, and Utilization Assume frame size is 500 bytesFind tfrme, a, and Utilization

9. Performance Examples

10. Maximum Utilization Utilization Ideal Plot the efficiency plot For the problems using Matlab Actual a

11. Sliding Windows Flow Control • Allows multiple numbered frames to be transmitted • Receiver has buffer W long • Transmitter sends up to W frames without ACK • Receiver sends an ACK • ACK includes number of next frame expected • Sequence number is bounded by size of field (k) • frames are numbered modulo 2k • giving max window size of up to 2k - 1 • Receiver can ack frames without permitting further transmission (Receive Not Ready) • Must send a normal acknowledge to resume • if have full-duplex link, can piggyback ACks

12. Sliding Window Diagram W=5, 3 bits are required, ACK: 0,1,2,3,4,0,1,

13. Sliding Window Example Why W is not 8? Window shrinking until ACK is received Window expands as soon as ACK is sent Window expands as soon as ACK is received W=7, 3 bits are required, ACK: 0,1,2,3,4,6,7,0,1,

14. Efficiency of Sliding Window Protocol

15. Error Control • Detection and correction of errors such as: • Lost frames • Damaged frames • Common techniques use: • Error detection • Positive acknowledgment • Retransmission after timeout • Negative acknowledgement & retransmission

16. Automatic Repeat Request (ARQ) • Error control mechanisms used in • stop and wait • go back N • selective reject (selective retransmission)

17. ARQ in Stop and Wait ACK0 / ACK1 • Source transmits single frame • Wait for ACK • If received frame damaged, discard it • transmitter has timeout • if no ACK within timeout, retransmit • if ACK damaged,transmitter will not recognize it • transmitter will retransmit • receive gets two copies of frame • use alternate numbering and ACK0 / ACK1 • pros and cons • simple • inefficient Remember: Special case where W=1

18. ARQ in Go Back N • Based on sliding window • If no error, ACK (RR) as usual • Use window to control number of outstanding frames • If error, reply with rejection (REJ) • Discard that frame and all future frames until error frame received correctly • Transmitter must go back and retransmit that frame and all subsequent frames Frames 5 & 6 are resent Lost ACK, P=1 Nothing sent until ACK is received Lost ACK, P=1

19. Go Back N Scenarios which are handled • Damaged Frame • Frame received with error • Frame lost • Last frame lost • Damaged Ack • One ack lost, next one makes it • All acks lost • Damaged Nack • Maximum Window = 2n -1 with n-bit sequence numbers

20. Selective Reject • Also called selective retransmission • Only rejected frames are retransmitted • Subsequent frames are accepted by the receiver and buffered • Cons and pros • Minimizes retransmission • Receiver must maintain large enough buffer • More complex logic in transmitter • Applications • Less widely used • Useful for satellite links with long propagation delays Resend Only Corrupted one Only frame 4 is resent P=1; Resent ACK Nothing sent until ACK is received Applet: http://media.pearsoncmg.com/aw/aw_kurose_network_3/applets/SelectRepeat/SR.html

21. Performance Comparison P is the probability that a single frame is in error; Assume that ACK and NAK are never in error.

22. High Level Data Link Control (HDLC) • An important data link control protocol • station types: • Primary – issue commands • Secondary - issue Responses • Combined - issues commands and responses • link configurations • Unbalanced - 1 primary, multiple secondary • Balanced - 2 combined stations

23. HDLC Transfer Modes • Normal Response Mode (NRM) • Response from secondary • Asynchronous Balanced Mode (ABM) • Combined Station, either station initiates transmission, has no polling overhead, widely used • Asynchronous Response Mode (ARM) • unbalanced config, secondary may initiate transmit without permission from primary, rarely used

24. HDLC Frame Structure • synchronous transmission of frames • single frame format used for all data types Data Header Trailer Frame Check Sequence = FCS

25. Flag Fields and Bit Stuffing From Jain

26. Address Field • Identifies secondary station that sends or receives frame • Usually 8 bits long • May be extended to multiples of 7 bits • LSB indicates if is the last octet (1) or not (0) • all ones address 11111111 is broadcast Frames arte multiples of 8-bit (octets) Last octet

27. Control Field • Different for different frame type (I, S, U) • Information - data transmitted to user (next layer up) • Flow and error control piggybacked on information frames (includes N(s) and N(R)) • Supervisory - ARQ when piggyback not used • Unnumbered - supplementary link control • First 1-2 bits of control field identify frame type • I=0, S=10, U=11

28. Control Field • Use of Poll/Final bit depends on context • In command frame is P bit set to 1 to solicit (poll) response from peer • In response frame is F bit set to 1 to indicate response to soliciting command • Sequence number usually 3 bits • can extend to 7 bits as shown below S=Supervisory function bit http://www.erg.abdn.ac.uk/users/gorry/course/dl-pages/hdlc-control.html

29. Information & FCS Fields • Information Field • Exists in information and some unnumbered frames • It must contain integral number of octets • Variable length • Frame Check Sequence Field (FCS) • Used for error detection • Either 16 bit CRC or 32 bit CRC

30. HDLC Operation • consists of exchange of information, supervisory and unnumbered frames • Consists of three phases • Initialization • by either side, set mode & sequence • data transfer • with flow and error control • using both I & S-frames (RR, RNR, REJ, SREJ) • disconnect • when ready or fault noted

31. HDLC Operation Example Set normal response/responses Set mode accepted Full Duplex Refer to HDLC Commands HDLC Commands

32. HDLC Operation Example