Computer Networks - Ethernet (Physical Properties, Repeater, Extended LAN, Transmitter Algorithm)

1. Ethernet IEEE 802.3

2. Ethernet
IEEE 802.3 – Ethernet
Uses CSMA/CD technology. So Ethernet is a multiple access network. i.e.,
set of nodes send and receive frames over a shared link.
 CSMA/CD (Carrier sense Multiple Access / Collision Detect),
 Carrier sense – all nodes can distinguish between an idle and a busy line.
 Collision Detect – node listens as it transmit and can therefore detect when a
frame it is transmitting has collided with a frame transmitted by another node.
Uses ALOHA as the root protocol.
Digital Equipment Corporation and Intel joined Xerox to define 10Mbps
Ethernet standard in 1978.
This standard formed the basis for IEEE standard 802.3

3. Ethernet
Physical Mediums,
It includes 100Mbps fast Ethernet & 1000Mbps or 1Gbps
Gigabit Ethernet.
Others are 10Base5, 10Base2, 10BaseT, 100BaseT,
1Base5, 10Broad36.
Fast Ethernet
100 Base T4
–4 pairs of UTP
100 Base X
100 Base Tx
–2 pairs of UTP or STP
100 Base Tx
–2 optical fibers

4. Ethernet
Physical Properties,
 Initially Ethernet was implemented on a coaxial cable up to 500m.
 This cable is similar to the type used for cableTV except that it typically has an
impedance of 50 ohms instead of cable TV’s 75 ohms.
 Hosts connect to an Ethernet segment by tapping into it.
Transceiver,
 Small device directly attached to the tap that detects when the line is idle
and drives signal when the host is transmitting.
 It also receives incoming signal.
 The transceiver is connected to an Ethernet adaptor which is plugged into
the host.
 Protocol is implemented on the adaptor.

5. Ethernet

6. Ethernet
Repeater,
 Multiple Ethernet segments can be joined together by a repeater.
 It forwards a digital signal.
 No more than four repeaters may be positioned between any pair of hosts.
 Ethernet has a total reach of only 2500m.
 Any signal placed on the Ethernet by a host is broadcast over the entire network.
 Signal is propagated in both directions.
 Repeaters forward the signal on all outgoing segments.
 Terminators attached to the end of each segment absorb the signals.
 Ethernet uses Manchester encoding scheme.

7. Ethernet
In 10 Base 2 (thin Ethernet), the cable is thin, so T-joint spliced into the
cable instead of tap the cable. Therefore 10 Base 2 is used in daisy chain
configuration.
With 10 Base T, the common configuration is to have several point
segments coming out of a multiway repeater called Hub.

8. Ethernet
Access Protocol,
The algorithm is commonly called Ethernet’s Media Access Control (MAC).
 MAC is implemented on the network adaptor.

9. Ethernet
Preamble – allows the receiver to synchronize with signal (sequence of
alternating 0s & 1s)
Src & dest addr – unique address belongs to the adaptor in the source
and destination host.
Type – packet type acts as demux key to identify the higher level
protocol.
Data – original data up to 1500 bytes.
Minimally a frame must contain at least 46 bytes of data, because frame
must be long enough to detect collision.
CRC – error detection code.

10. Ethernet
Ethernet Addresses
 Every Ethernet host in the world has a unique Ethernet addresses.
 This address belongs to the adaptor on the host.
 It is usually burnt in to the ROM.
 Ethernet addresses are typically printed in a human readable format.
 Sequence of six numbers separated by colons.
 1 byte of the 6 bytes address is given by a pair of hexadecimal digits, one for each of
the 4 bit nibbles in the byte.
e.g,
8:0:2b:e4:b1:2 – 00001000 00000000 00101011 11100100 10110001 00000010
 To ensure that every adaptor gets a unique address, each manufacturer of Ethernet
device is allocated a different prefix that must be prepended to the address every
adaptor they build.
e.g,
AMD has assigned the 24bit prefix 8:0:20

11. Ethernet
EthernetTransmitting Algorithm
When the adaptor has a frame to send & line is idle, it transmits the frame
immediately.
The upper bound of 1500bytes in the message means that the
adaptor can occupy the line for a fixed length of time.
When the adaptor has a frame to send and the line is busy, it waits for the
line to go idle then transmits immediately.
Ethernet is said to be 1 – persistent protocol, because an adaptor with the
frame to send transmits with probability 1 whenever a busy line goes idle.

12. Ethernet
Since there is no centralized control, it is possible for two or more
adaptors to begin transmitting at the same time.
Either because both found the line to be idle.
or, both been waiting for a busy line to become idle.
When this happens, the two or more frames are said to be collide on the
network.

13. Ethernet
Collision Detection
Since Ethernet supports collision detection, so each sender is able to
determine that a collision in progress.
At the moment an adaptor detects that, its frame is colliding with another,
Transmitter will send a 32 bit jamming sequence with 64-bit
preamble (totally 96 bits)
This 96 bit frame is called as runt frame.

14. Ethernet
Worst cast scenario, (two nodes are at opposite ends of the Ethernet)
To know, the frame it just sent did not collide with another frame, the
transmitter may need to send as many as 512 bits.
512 bits (64 bytes) -> 14 bytes of header + 46 bytes of data + 4 bytes of
CRC
Why 512 bits & why its length is limited to 2500m?
The farther apart two nodes are, the longer it takes for a frame sent by
one to reach the other and the network is vulnerable to collision during
this time.

15. Ethernet
Fig. (a) A sends a frame at time t; (b) A’s frame arrives at B
at time t + d; (c) B begins transmitting at time t + d and
collides with A’s frame; (d) B’s runt (32-bit) frame arrives at
A at time t + 2d.

16. Ethernet
‘A’ begins transmitting frame at time ‘t’
 The first bit of A’s frame arrives at ‘B’ at time t + d, where d is one link
latency.
 Suppose an instant before host A’s frame arrives, host B begins to
transmit its own frame.
B’s frame will immediately collide with A’s frame and this frame collision
will be detected by host B.

17. Ethernet
Host B will send the 32 bit jamming sequence.
 Host A will not know that the collision occurred until B’s frame reaches it,
which will happen at t + (2*d)
Host A must continue to transmit until this time in order to detect
collision.
Determined delay in the maximally configured Ethernet (2500m long) is
51.2 s. i.e., delay on 10Mbps Ethernet corresponds to 512 bits.
By limiting the maximum length of the Ethernet to 2500m we can limit the
Ethernet’s maximum latency to 51.2 s (fairly small value). So the access
algorithm works efficiently.

18. Ethernet
Exponential Back off,
Once an adaptor has detected a collision & stopped its transmission, it
waits a certain amount of time and tries again.
Each time the adaptor tries to transmit but fails, it doubles the amount of
time it waits before trying again.
This strategy of doubling the delay interval between each
retransmission attempt is known as exponential back off.

19. Ethernet
The adaptor first delays either 0 or 51.2 s selected at random.
If this effort fails, it then waits 0, 51.2s, 102.4s, 153.6s… before trying
again.
i.e., k * 51.2s, k= 0,1,2,3.
After the third collision, it waits k * 51.2s, k=0… 23 – 1
In general, the algorithm randomly selects a ‘k’ between 0 and 2n – 1 &
waits for k * 51.2s where ‘n’ is the number of collisions experienced so
far.

20. Ethernet
Advantages
 Ethernets are easy to administer & maintain.
 There are no switches that can fail & no routing and configuration tables that have to be
kept up to date.
 Easy to add a new host to the network.
 In expensive (i.e., cable is cheap)
Disadvantages
 Ethernets work best under lightly loaded conditions.
 Most Ethernets are used in conservative way.
 Most Ethernets are far shorter than 2500m with a round trip delay of closer to 5s than
51.2s.