Computer Networks (CS425)

Instructor: Dr. Dheeraj Sanghi

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IEEE 802.3 and Ethernet

Ethernet Physical Layer

A Comparison of Various Ethernet and IEEE 802.3 Physical-Layer Specifications

Characteristic Ethernet Value IEEE 802.3 Values
10Base5 10Base2 10BaseT 10BaseF 10 Base -TX 100BaseT4

Data rate (Mbps)

10

10

10

10

10

100

100

Signaling method

Baseband

Baseband

Baseband

Baseband

Baseband

Baseband

Baseband

Maximum segment length (m)

500

500

185

100

2,000

100

100

Media

50-ohm coax (thick)

50-ohm coax (thick)

50-ohm coax (thin)

Unshielded twisted-pair cable

Fiber-optic

Cat 5 UTP

Unshielded twisted-pair cable

Nodes/segment 100 100 30 1024 1024

Topology

Bus

Bus

Bus

Star

Point-to-point

Bus

Bus

10Base5 means it operates at 10 Mbps, uses baseband signaling and can support segments of up to 500 meters. The 10Base5 cabling is popularly called the Thick Ethernet. Vampire taps are used for their connections where a pin is carefully forced halfway into the co-axial cable's core as shown in the figure below. The 10Base2 or Thin Ethernet bends easily and is connected using standard BNC connectors to form T junctions (shown in the figure below). In the 10Base-T scheme a different kind of wiring pattern is followed in which all stations have a twisted-pair cable running to a central hub (see below). The difference between the different physical connections is shown below:


(a) 10Base5 (b)10Base2 (c)10Base-T

All 802.3 baseband systems use Manchester encoding , which is a way for receivers to unambiguously determine the start, end or middle of each bit without reference to an external clock. There is a restriction on the minimum node spacing (segment length between two nodes) in 10Base5 and 10Base2 and that is 2.5 meter and 0.5 meter respectively. The reason is that if two nodes are closer than the specified limit then there will be very high current which may cause trouble in detection of signal at the receiver end. Connections from station to cable of 10Base5 (i.e. Thick Ethernet) are generally made using vampire taps and to 10Base2 (i.e. Thin Ethernet) are made using industry standard BNC connectors to form T junctions. To allow larger networks, multiple segments can be connected by repeaters as shown. A repeater is a physical layer device. It receives, amplifies and retransmits signals in either direction.

Note: To connect multiple segments, amplifier is not used because amplifier also amplifies the noise in the signal, whereas repeater regenerates signal after removing the noise.

IEEE 802.3 Frame Structure

Preamble
(7 bytes)
Start of Frame Delimiter
(1 byte)
Dest. Address
(2/6 bytes)
Source Address
(2/6 bytes)
Length
(2 bytes)
802.2 Header+Data
(46-1500 bytes)
Frame Checksum
(4 bytes)

A brief description of each of the fields

Ethernet Frame Structure

Preamble
(8 bytes)
Dest. Address
(2/6 bytes)
Source Address
(2/6 bytes)
Type
(2 bytes)
Data
(46-1500 bytes)
Frame Checksum
(4 bytes)

A brief description of the fields which differ from IEEE 802.3

Truncated Binary Exponential Back off

In case of collision the node transmitting backs off by a random number of slots , each slot time being equal to transmission time of 512 bits (64 Byte- minimum size of a packet) in the following fashion:

No of Collision                            Random No of slots

1st                                                0-1

2nd                                               0-3

3rd                                                0-7

|                                                        |

|                                                        |

10th                                                0-1023

---------------------------------------------

11th                                                0-1023

12th                                                0-1023

|                                                        |

16th                                                0-1023

In general after i collisions a random number between 0-2^i-1 is chosen , and that number of slots is skipped. However, after 10 collisions have been reached the randomization interval is frozen at maximum of 1023 slots. After 16 collisions the controller reports failure back to the computer.

5-4-3 Rule

Each version of 802.3 has a maximum cable length per segment because long propagation time leads to difficulty in collision detection. To compensate for this the transmission time has to be increased which can be achieved by slowing down the transmission rate or increasing the packet size, neither of which is desirable. Hence to allow for large networks, multiple cables are connected via repeaters. Between any two nodes on an Ethernet network, there can be at most five segments, four repeaters and three populated segments (non-populated segments are those which do not have any machine connected between the two repeaters). This is known as the 5-4-3 Rule.

Image References:


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