Lecture 8 LinkLayer Protocol

1
Lecture 8 - Link-Layer Protocol

• Frame Synchronization
• Flow Control
• Error Control
• Addressing
• Link Management

2
Flow Control

• Ensuring the sending entity does not overwhelm
  the receiving entity
• Preventing buffer overflow
• Transmission time
• Time taken to emit all bits into medium
• Propagation time
• Time for a bit to traverse the link

3
Model of Frame Transmission
4
Stop and Wait

• Source transmits frame
• Destination receives frame and replies with
  acknowledgement
• Source waits for ACK before sending next frame
• Destination can stop flow by not sending (or
  delaying) ACK
• Works well for few large frames

5
Fragmentation

• Large block of data may be split into small
  frames
• Limited buffer size
• Errors detected sooner (when whole frame
  received)
• On error, retransmission of smaller frames is
  needed
• Prevents one station occupying medium for long
  periods
• Stop and wait becomes inadequate

6
Delays

• Propagation delay
• d length of physical link
• s propagation speed in medium (2x108 m/sec)
• propagation delay d/s

• Transmission delay
• Rlink bandwidth (bps)
• Lpacket length (bits)
• time to send bits into link L/R

Note s and R are very different quantitites!
7
Stop and Wait Protocol Analysis

• Sender utilization
• The fraction of the time the transmitter is busy
• U 1/(1 2a) where a tprop/ttrans
• tprop Distance/Velocity
• ttrans Packet length/data rate

8
Performance of Stop-and-wait

• Stop-and-wait works, but performance stinks
• example 1 Gbps link, 15 ms e-e prop. delay, 1KB
  packet
• Tprop 15ms. a Tprop/Ttransmit 0.008/15
• U 1/ (1 2a)

9
Sliding Windows Flow Control

• Allow multiple frames to be in transit
• Receiver has buffer W long
• Transmitter can send up to W frames without ACK
• Each frame is numbered
• ACK includes number of next frame expected
• Sequence number bounded by size of field (k)
• Frames are numbered modulo 2k

10
Sliding Window Protocol

• Pipelining sender allows multiple, in-flight,
  yet-to-be-acknowledged pkts
• range of sequence numbers must be increased
• buffering at sender and/or receiver

• Two generic forms of pipelined protocols
  go-Back-N, selective repeat

11
Sliding Window Protocol

• Sender
• k-bit seq in pkt header
• window of up to N, consecutive unacked pkts
  allowed

• ACK(n) ACKs all pkts up to, including seq n -
  cumulative ACK
• may deceive duplicate ACKs (see receiver)

12
Sliding Window Protocol Example
13
Example Sliding Window
14
Sliding Window Protocol Analysis

• Let N be the size of the window
• Let a tprop/ttrans where tprop is the
  propagation delay and ttrans is the transmission
  delay
• U 1 if N gt 1 2a
• U N/(1 2a) if N lt 1 2a
• By increasing N you can achieve higher utilization

15
Sliding Window Enhancements

• Receiver can acknowledge frames without
  permitting further transmission (Receive Not
  Ready)
• Must send a normal acknowledge to resume
• If duplex, use piggybacking
• If no data to send, use acknowledgement frame
• If data but no acknowledgement to send, send last
  acknowledgement number again, or have ACK valid
  flag (TCP)

16
Error Control

• Types of error
• Packets (data packets and acks) are corrupted
• Packets (data packets and acks ) are lost

17
Basic elements of ARQ
18
Stop and Wait - ARQ

• 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 ACK0 and ACK1

19
Stop-and-wait ARQ Example