Transcript ELEC6219-W2

ELEC6219:
Wireless and Mobile Networks
Session 6:
Local Area Networks - Case study:
Ethernet (Part II)
Adriana Wilde and Jeff Reeve
At the end of this session
you will be able to…
… evaluate CSMA/CD efficiency
 … appraise the Ethernet Retransmission algorithm
(exponential backoff )
 … differentiate between frames and packets
 … apply the backwards learning algorithm in Ethernet
network switches
The total time required to
What
transmit the message is:
1.
2.
3.
4.
5.
3*RTT
5*RTT
7*RTT
11*RTT
None of the above
100%
These answers refer to the exercise presented in
the OHP, from a past exam question
1
0%
0%
2
3
0%
4
0%
5
Where were we?
Yesterday we discussed the minimum frame
size requirement in Ethernet
We want to avoid sending frames that are
“too short”.
p. 302-303
Assumptions for Dynamic
Channel Allocation
Independent Traffic (station model)
Single Channel
Observable Collisions (collision assuption)
Continuous or Slotted Time
Carrier Sense or No Carrier Sense
p. 249-250 (T.4e)
p. 278-279 (T.5e)
The “small print”
 For a 10Mbit/s Ethernet, with a maximum length of 2500 m
and 4 repeaters, the RTT is set at about 50 µs (rounded up to
51.2), i.e. a minimum frame size of 512 bits.
The maximum round-trip delay of about 51.2 microseconds
also imposes a restriction on the physical size of the LAN:
D = RTT/2 * v (where ‘v’ is the signal propagation speed)
This results in a maximum value for D of 5120 m – if the
network was larger than this then it would be impossible to rely
on the RTT limit of about 51.2 µs (and in practice the cable
losses restrict the overall network size to be much smaller).
p. 277-278 (T.4e)
p. 302-303 (T.5e)
Talking point
CSMA/CD Efficiency (I)
CSMA/CD Efficiency (I)
CSMA/CD Efficiency (II)
CSMA/CD Efficiency (III)
Ethernet retransmission
algorithm (I)
This is the exponential back-off algorithm – it has been
highly successful:
When a collision is detected, both nodes wait and try again
(until successful)
 The basic concept is that the wait time is doubled at every
attempt (hence the term ‘exponential’)
 This would not work if the algorithm was deterministic: both
nodes would simply double their times in step and suffer
repeated collisions
Ethernet retransmission
algorithm (II)
Talking Point
Talking Point
Ethernet retransmission
algorithm (III)
LAN Terminology
Unit
Repeater
Function
a simple amplifier (collision aware)
(rarely used in modern systems)
Hub
essentially a multi-port repeater
Switch (or ‘Bridge’, or ‘Transparent Bridge’) S&F Frames
(‘S&F’ = ‘store and forward’)
LAN
NETWORK
Router
S&F Packets (typically IP packets)
(i.e. Network (L3) Level
Ethernet Switches (I)
Consider a four-port switch, connected as below:
A
Port 0
B
Port 1
C
Port 2
D
Port 3
When the switch receives a frame, it must decide what to
do with it.
p. 281-283 (T.4e)
p. 306-308 (T.5e)
Ethernet Switches (II)
• Is it a broadcast (is the destination address = FF…FF)?
If so, the frame is flooded (i.e. forwarded on every
port - this frame has to go everywhere, so there is no
choice)
• Is the destination address location known? (i.e. the
port that it is connected to)
– No: flood the frame (i.e. transmit on every port)
– Yes: filter the frame (i.e. transmit only on the port
connected to the destination address)
How do we know the location of the destination address?
p. 306-308
Ethernet Switches (III)
How do we know the location of the destination
address?
From an internal table: we just look it up.
However, this table is initially empty (e.g. on switch-on),
the switch needs to build it up by examining the traffic
through the switch. This is the backwards learning
algorithm: the switch inspects all the source addresses
and updates the internal table.
p. 306-308
Ethernet Switches (IV)
Consider an example: after reset, a frame is sent from A
to B: this is flooded onto ports 1,2 and 3
Next a frame is sent from D to A: this frame is filtered
(transmitted on port 0 only) because the switch is now
aware (from the first frame) of the location of A.
The ‘time’ field is to allow a timeout on every entry. This
allows the connections between the nodes and the switch
to be changed without reconfiguration: after a short
delay the existing entries will timeout and be removed,
allowing the new locations to be discovered and entered
into the table.
p. 306-308
At the end of this session
you should be able to…
 … note the limitations of CSMA/CD
 … apply the Ethernet Retransmission algorithm
(exponential backoff ), appraising Ethernet as a successful
LAN technology
 … differentiate between frames and packets
 … note the advantages of switches for network
management
Study
Enter strategies
question text...
1. I have a partner and/or
group study to revise
the material
2. I devote a couple of
hours every week to
revise the lecture
material
3. Both of the above
4. None of the above (yet)
37%
29%
21%
13%
1
2
3
4
What next?
This is the end of the first two layers of our
model.
We have just “loosely” covered the first 4 chapters in the Tanenbaum