Ethernet and Wireless LAN

1

• Ethernet and Wireless LAN

2
Last time Reliable Transmission

• Goal Create a mechanism at the data-link layer
  that is capable of recognizing when an error has
  occurred on a point-to-point link (corrupted
  frame or lost frame) and take measures to correct
  it.
• Approach Say the burden of detecting the error
  lies on the receivers side. When it detects a
  corrupt frame, it requests the sender to
  retransmit. When it detects that frame has been
  lost, it requests the sender to retransmit. Does
  this suffice?

3
Stop-and-wait 1-bit SeqNo
Every frame and ACK is tagged with a sequence
number. This allows for the detection of
duplicated frames and ACKs.
Shortcoming
Example 1.5 Mbps link, RTT 45ms. Frame size
1KB. How many bits per time unit can this
protocol achieve?
1024 8 / 0.045 182 Kbps
Question How can this rate be improved?
4
Sliding Window (SW)
Assumption SeqNum n-bit sequence
number. Protocol variables Send window size
(SWS), Last ACK received (LAR), Last frame sent
(LFS). Invariant (at sender)
5
A Few Notes on Sliding Window

• When a timeout occurs, the amount of data in
  transit decreases.
• If a frame N arrives before (N-1), the receiver
  could send NAK for (N-1). The sender will
  timeout, though
• A receiver can use selective acknowledgments to
  indicate exactly what frames have arrived, this
  way the sender can keep the pipe full.
• The window sizes determine how many frame buffers
  there will be on the sender and on the receiver
  sides.
• Common settings are RWS1 and RWSSWS. RWSgtSWS is
  not a smart choice the receiver will have more
  buffers that frames that may arrive out of order.
• Sequence numbers in practice, you can only use a
  range of values that means you have to use mod
  arithmetic.

6
Frame Order and Flow Control

• Question Does the data-link layer have the
  responsibility to send frames in order up to the
  network layer? Is there any benefit to passing up
  frames as soon as they receive?
• Question Does the sliding window protocol serve
  to control the flow of packets from sender to
  receiver?

7
Ethernet
8
Ethernet (IEEE 802.3)
Created in 1978 by DEC, Intel, and Xerox. Later
standardized by the IEEE. Versions 10Mbps,
100Mbps (fast Ethernet), 1000Mbps (Gigabit
Ethernet).
Cable Coax, up to 500m. Taps At least 2.5m
apart. Transceiver Detect idle line, drive
signal on TX, receive signal on RX.
9
Physical Properties
Repeater Amplifies and forwards electric
signals. No more than 4 between any two hosts
determines a maximum reach of 2500m. Terminators
Placed at the end of a segment to avoid signals
from bouncing back. Medium Broadcast. Maximum
number of hosts 1024.




Host
Repeater
Since the medium is shared, all hosts compete for
the same link, what creates the possibility of
collisions.
10
Frames and Addresses
Ethernet frame
Encoding Manchester. Framing Bit-oriented. Addre
sses Unique to each adaptor, 48-bit long
assigned by manufacturer. Written as
802be4b12 (MAC address). First 24 bits
indicate manufacturer.
When a frame is put on the wire, it is received
by all hosts. A host picks up only the frames
addressed to itself unless it is in promiscuous
mode. Broadcast address all bits 1. Multicast
address first bit 1, but not all. The adaptor
only passes to hosts the frames it accepts.
11
Transmitter Algorithm

• The algorithm on the receivers end is simple
  the complexity of medium access control (MAC)
  lies on the senders side.

Sender has frame to transmit. Sender listens to
the medium. If line is busy, wait until
free. Transmit frame immediately.
Sender has frame to transmit. Sender listens to
the medium. If line is busy, wait until
free. Transmit frame immediately with probability
p, defer to another host with probability q1-p.
1-persistent protocol
p-persistent protocol
Question What happens when more than one host
decides to transmit at the same time?
12
Carrier Sense Multiple Access with Collision
Detection (CSMA/CD)
Host A listens and finds the bus idle.
Host A starts TX.
Host B starts TX.
Host A detects collision.
Host B completes TX.
Host A backs off.
COLLISION!
Host B backs off.
time
Host B listens and finds the bus idle.
Host B detects collision.
Host B listens and finds the bus idle.
Host B starts TX.
13
Frame Collisions

• When a host detects a collision, it first
  transmits a 32-bit jamming sequence (plus the
  64-bit preamble) and then stops the transmission.
  The shortest transmission in the event of a
  collision is a 96-bit runt frame 64 bits of
  preamble plus 32 bits. This only happens if the
  two hosts are close together. When they are
  farther apart as many as 512 bits may be
  necessary. Ethernets shortest frame size is
  chosen to be 512 bits

14
Ethernet Efficiency

• The efficiency of an Ethernet can be determined
  using a relatively simple formula

where
t end-to-end propagation time
T transmission time of a packet
e.g. cable is 300 meters long, so t is
300/(300106) 1 us Packet is 1000 bytes long,
bandwidth 10 Mbps, a 0.00125 ?
1/(16.440.00125) 0.99, 100Mbps 0.93, 1Gbps
0.55
15
Wireless LAN (IEEE 802.11)
16
Concepts in Radio Communications
Coverage f(PowerTX)
D
A
E
B

• Interference

C
Multipath
Noise
17
Types of Wireless Networks
Fixed Infrastructure Access Points
wired backbone
AP
AP
AP
Ad Hoc

• Easy to deploy.
• Good in changing environments.
• Allows for node mobility.
• Can be designed for self-configurability.
• Can be designed for scalability.

18
Remember Ethernet?When Problems Get Worse

• Assume that all wireless devices use the same
  channel. Arbitration of access to the medium
  (Medium Access Control, or MAC, a protocol in the
  Data Link layer) is similar to Ethernets
  CSMA/CD.
• Most radios in wireless networking cant transmit
  and receive at the same time, so we cant detect
  collisions. Instead, well do CSMA/CA (collision
  avoidance).
• Collisions are bad because they reduce the
  effective bandwidth and also because they cause
  waste of power.
• Even when two transmissions do not collide, they
  may still interfere with each other causing bit
  error rates to rise.

19
The Hidden Node Problem

• Station B can sense stations A and C.
• Stations A and C cant sense each other.
• Problem coordinate transmissions from A and C so
  as to avoid collisions.

A
B
C
20
The Hidden Node Problem

• Station C can sense stations A and B.
• Stations A and C cant sense each other.
• Problem coordinate transmissions from A and C so
  as to avoid collisions.

A
B
C
Solution RTS/CTS/DATA/ACK handshake A sends
RTS (request-to-send) to B, B sends CTS
(clear-to-send) to A, C hears CTS and stays
quiet, A sends DATA to B, B replies to A with an
ACK.
21
The Exposed Node Problem
A
B
C
D
An exposed node is one that is in range of the
transmitter, but outside range of the receiver. B
sensing the traffic between C and D, so falsely
not sending to A.
Problem Exposed nodes reduce bandwidth.
22
Data Link Layer Medium Access Control
(Coordinated access to a shared resource)

• Power is a scarce resource so is the RF
  spectrum.
• Collisions lead to wasted power and wasted
  spectrum.
• Need to impose some kind of access discipline so
  as to avoid collisions.

23
The MAC Layer Challenge

• Maximize throughput
• Minimize collisions.
• Avoid exposed nodes.
• An interesting option schedule medium access.
• Related challenges
• Clock synchronization.
• Distributed coordination for determining
  schedule.

24
IEEE 802.11 DCF (CSMA/CA)
start
NAV starts with the Duration field value of the
last transmission sensed on the medium and counts
down to zero.
NAV0
NO
YES
Sense Medium
Medium Idle
Random Backoff Time
NO
YES
Transmit Frame
Collision?
YES
NO
25
802.11 Frame Format