Ethernet Technology

Receiving a Frame
Reveiving/sensing
the next frame
CSMA/CD
(cont’d)
Back-off Algorithm - Collision Recovery
N
Error frame
More bits
received ?
Y
Framing error
Y
N
N
Receiving
completed ?
Frame too short
(collision) ?
Address matched ?
Decapsulating header/trailer
Received
successfully
Ethernet Technology 17
Y
Y
N
CRC
correct ?
Y
Length (if)*
correct ?
N
Length error
Y
N
Submit to
higher layer
• Collision back-off and retransmission in Ethernets
~ Truncated Binary Exponential Back-off Algorithm (BEBA)
Let n = number of collisions experienced
k = Min(n,10) Truncation
then Retry in the r th slot-time , 1 r 2 k
~ Back-off time
RAND(0,2
min(n,10)
( n 16)
• Disadvantage of BEBA unfair
Last-in-First-out effect:
- Stations with no or few collisions will have a better chance
to transmit before stations that have waited longer.
Ethernet Technology 18
)
15 retries, 16 attampts in total
Waiting for # of slot-time
• Waiting for: 0 ~ (2 k -1) slot-time
Binary back-off
Backoff (cont’d)
Example: 10Base5 Medium Specification
• Waiting for: 0 ~ (ak + b)-1 slot-time Linear back-off
• Probability of successful retransmission ?
collision
Collide:
A ~ the first time
B ~ the second time
slot time = 512 bit-time = 2
reattempt
Randomly choose
From the 2 k time slots
Ethernet Technology 19
• 10Mbps, Manchester encoding
• Use a 50-ohm(D=10mm) coaxial cable (characteristic
impedance = 50 ohm)
(Impedance is a measure of how much voltage must be
applied to the cable to achieve a certain signal strength)
• The maximum length of a segment is 500m
• The maximum length of a 10Base5 network is 2500m
• The distance between any two adjacent taps being a multiple
of 2.5m (Tap); A maximum of 100 taps is allowed
• Slot time = 51.2 us; IFG = 9.6us
• Packet size: 64 bytes (Min) ~ 1518 bytes (Max)
Ethernet Technology 20