CMPE 150 Fall 2005 Lecture 14

1
CMPE 150 Fall 2005Lecture 14

• Introduction to Computer Networks

2
Announcements

• Midterm moved to 11.04.
• In class, closed books/notes.
• We will have lab this week.
• Homework 1 is due today.
• Homework 2 will be up soon.

3
Today

• Finish Layer 2

4
Flow Error Control

• Stop and Wait.
• ACKs.
• ARQ.
• Noisy channels.
• ACKs.
• Sequence numbers.
• Timers.

5
Can we do better?

• Can we do better?
• Piggybacking.
• Bi-directional transmission.
• Wait for data packet and use that to piggyback
  the ACK.
• Use ACK field only a few additional bits in the
  header.
• But, how long should Layer 2 wait to send an ACK?
• ACK timers!

6
Can we do even better?

• In Stop and Wait, only 1 frame outstanding at any
  given point in time.
• Whats the problem with that?
• Loooong pipes.
• Fat pipes.

S
R
S
R
7
Sliding Window

• Window number of outstanding frames at any
  given point in time.
• So whats the window size of Stop and Wait?
• Every ACK received, window slides.

8
Sliding Window Example

• A sliding window of size 1, with a 3-bit sequence
  number.(a) Initially (b) After the first frame
  has been sent (c) After the first frame has been
  received(d) After the first acknowledgement has
  been received.

9
Sliding Window Basics

• Allows multiple frames to be in transit at the
  same time.
• Receiver allocates buffer space for n frames.
• Transmitter is allowed to send n (window size)
  frames without receiving ACK.
• Sequence number?

10
Sliding Window Receiver

• Receiver maintains window corresponding with
  frames it can receive.
• Receiver acks frame by including sequence number
  of next expected frame.
• Cumulative ACK acks multiple frames.
• Example if receiver receives frames 2,3, and 4,
  it sends an ACK with sequence number 5, which
  acks receipt of 2, 3, and 4.

11
Sliding Window Sender

• Sender maintains window corresponding to frames
  (sequence numbers) its allowed to send.
• Sequence numbers are bounded if frame reserves
  k-bit field for sequence numbers, then they can
  range from 0 2k -1.
• Transmission window shrinks each time frame is
  sent, and grows each time an ACK is received.

12
Example 3-bit sequence number and window size 7
A (Sender)
B (Receiver) 0 1 2 3 4 5 6 7 0 1 2
3 4 0 1 2 3 4 5 6 7 0 1
2 3 4
0
1
2
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
ACK 3
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
3
0 1 2 3 4 5 6 7 0 1 2 3 4
4
5
0 1 2 3 4 5 6 7 0 1 2 3 4
ACK 4
6
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
13
One-Bit Sliding Window Protocol
Two scenarios (a) Normal case. (b)
Abnormal case. Notation is (seq, ack, packet
number). An indicates where a network layer
accepts packet. ACK indicates last sequence
number received.
14
Bandwidth-Delay Product

• How large should the senders window be?
• Function of how fat and long the pipe is

BW
S
R
RTT
W BWRTT/data size
15
Pipelining
Receivers window size is 1 discard frames
after error with no ACK.
Go Back N
Receivers window size is large buffers all
frames until error recovered.
Selective Repeat

• Pipelining and error recovery. Effect on error
  when (a) Receivers window size is 1. (b)
  Receivers window size is large.

16
Medium Access Control

• MAC.
• Tanenbaum, Chapter 4.

17
Why MAC?

• Point-to-point versus shared-medium networks.
• Shared-medium networks use broadcast channels.
• A.k.a. multi-access or random access channels.
• MAC layer protocols regulate access to medium in
  shared-medium networks.

18
Where is the MAC Sub-Layer?
MAC
19
Where is the MAC Sub-Layer?
Application
Transport
Network
Link Control
DLL
MAC
PHY
20
MAC and LANs

• LANs typically use shared-medium.
• Examples?
• MAC layer critical!
• BTW, in wireless networks also!
• WANs typically use point-to-point connections.