Ethernet
Savera Tanwir
Ethernet
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Ethernet Cabling
Manchester Encoding
The Ethernet MAC Sublayer Protocol
The Binary Exponential Backoff Algorithm
Ethernet Performance
Switched Ethernet
Fast Ethernet
Gigabit Ethernet
IEEE 802.2: Logical Link Control
Retrospective on Ethernet
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Ethernet Cabling
The most common kinds of Ethernet cabling.
Ethernet Cabling (2)
Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.
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Ethernet Cabling (3)
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d)
Segmented.
Ethernet Cabling (4)
(a) Binary encoding, (b) Manchester encoding,
(c) Differential Manchester encoding.
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Ethernet MAC Sublayer Protocol
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Preamble
Start of Frame
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= 7 bytes of 10101010
= 1 byte of 10101011
Alternating pattern of ones and zeros, ending with two consecutive 1-bits
indicating that the next bit is the left-most bit in the left-most byte of the
destination address
Dest
Source
Length
Data
Pad
Checksum
= 6 bytes of MAC address
= 6 bytes of MAC address
= number of bytes of data
= comes down from network layer
= ensures 64 bytes (from Dest to checksum)
= 4 bytes of CRC
Why we need minimum Packet
Size
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All frames must take more than 2t to send
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As soon as a station detects a collision, it sends a 32 bit jamming sequence
and then stops transmission
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A station will minimally transmit 12 Bytes in case of collision (8 Byte Preamble + 4
Byte Jamming Sequence)
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Binary Exponential Backoff Algorithm
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After a collision, time is divided into discrete slots
with length equal to the worst-case delay (2τ)
AFTER THE FIRST COLLISION, each station
waits for 0 or 1 time slot before trying again
If two stations pick up the same random number,
they collide again, P(collision) = 0.5
AFTER THE SECOND COLLISION, each station
now picks up either 0, 1, 2 or 3 slot times,
P(collision) = 0.25
Binary Exponential Backoff Algorithm
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IF THE THIRD COLLISION OCCURS, each
station picks the number of slots to wait at random,
from ‘0’ to 23-1
In general, after ‘i ’ collisions, the random number
between 0 to 2 i - 1 is chosen
However, after 10 collisions, the expansion process
is halted at a maximum of 1023 slots
After 16 collisions, the NIC reports a failure to the
computer
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Binary Exponential Backoff Algorithm
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If the randomization interval for all collisions was 1023 max,
the chances of collision would have been negligible
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But the average wait time be hundred of time slots, resulting into a
significant delay
On the contrary, if the stations delayed for either zero or one
time slot, for say 100 stations, the only possibility of success
would have been when 99 of the stations chose ‘1’ and one
station chose ‘0’ or vice versa
The randomization interval grows exponentially, offering
pros of the two extremes and avoiding their bottlenecks
Ethernet Performance
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If the mean frame takes t sec to transmit, when many
stations have frames to send, channel efficiency
η=
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t
t + Tw
We denote the frame length, F, the network bandwidth, B,
the cable length, L, and speed of signal propagation, c, for
the optimal case of e contention slots per frame. With t =
F/B, we have (infinite stations)
η=
F /B
1
=
F /B + 2e(L /c) 1+ 2BLe /cF
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Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot
times.
Switched Ethernet
A simple example of switched Ethernet.
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Switched Ethernet
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Switch uses a high speed backplane between the
connectors
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Each card forms its own collision domain
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Local (on-card) destinations are sent there directly
Off-card are sent to the backplane
Collisions among cards are impossible
Each port forms its own collision domain (Using
buffers)
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Collisions are impossible
Ethernet Improvements
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Four data rates are currently defined for operation
over optical fiber and twisted-pair cables
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10 Mbps
100 Mbps
1000 Mbps
10 Gbps
- Classical/Legacy Ethernet (IEEE 802.3)
- Fast Ethernet (IEEE 802.3u)
- Gigabit Ethernet (IEEE 802.3z)
- 10-G Ethernet (IEEE 802.3ae)
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Fast Ethernet
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Fully backward compatible with Legacy Ethernet
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Even minimum frame size is same (64 Bytes)
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Maximum collision diameter is 250 meters instead of 2500 meters
The original fast Ethernet cabling.
Gigabit Ethernet
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Supports two modes of operation
Full-Duplex (default)
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All lines are buffered
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Therefore, CSMA/CD is not used in Full-Duplex mode (Just like in Fast
Ethernet)
Half-Duplex
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As Hubs don’t buffer, so collisions are possible
Reducing the maximum collision diameter again was not attractive,
so the minimum frame size was increased
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Gigabit Ethernet Carrier Extension
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Non-data, variable extension field used to extend the frame to 512 bytes
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Gigabit Ethernet
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Frame Bursting extension
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Optionally allows a station to transmit a series of frames without
relinquishing control of the transmission medium
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A station operating in burst mode may continue to initiate transmission of
additional frames until it reaches a "burst limit" of 65,536 bits (8192
bytes)
The transmitting MAC fills each inter-frame gap with extension bits
Gigabit Ethernet
(a) A two-station Ethernet. (b) A multistation
Ethernet.
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Gigabit Ethernet (2)
Gigabit Ethernet cabling.
IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol
formats.
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LLC Working
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The network layer on the sending machine passes a packet to LLC
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The LLC sublayer then adds an LLC header, containing DSAP, SSAP and
a Control field which has sequence and acknowledgement numbers
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The first address indicates a Destination Service Access Point
and the second address a Source Service Access Point
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Control field support the three modes
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These identify the higher-layer protocol which is using the LLC service
Type 1 (Unacknowledged connectionless service)
Type 2 (Reliable Connection-oriented service)
Type 3 (Acknowledged Connectionless service)
The resulting structure is then inserted into the payload field of an 802
frame and transmitted
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