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Encoding Schemes in OSI Layer Model

Details: Your manager has asked you to describe the frame format of a typical Ethernet packet. Prepare a written report. Be sure to discuss Manchester encoding, 4B/5B encoding, 8B/10B encoding, the cable grades required for different speeds of Ethernet, and Ethernet performance as more hosts are added to a network.

Objective: Discuss networking concepts using the OSI Model as a framework.
Describe protocols at the different layers of the OSI model and explain their functionality.

Solution Preview

1. Ethernet Frame Format
64 48 48 16 <1500B 32 8
Preamble Dest Addr Src Addr Type Payload CRC Postamble

Preamble: 64 bit sequence of alternating 1s and 0s for receiver synchronization with the signal. In other words, this field indicates the beginning of a frame.
Destination and Source address: Every Ethernet adapter attached to a host has a unique 6-Byte (48bit) address.
Type: 16 bit type field is used to determine how long the frame is
Payload: Actual Data 0-1500 Bytes
CRC: Cyclic redundancy check used for error checking
Postamble: Indicates the end of framat.

2. Encoding schemes
2.1 Manchester Encoding

Manchester encoding (first published in 1949) is a synchronous clock encoding technique used by the physical layer to encode the clock and data of a synchronous bit stream. In this technique, the actual binary data to be transmitted over the cable are not sent as a sequence of logic 1's and 0's (known technically as Non Return to Zero (NRZ)). Instead, the bits are translated into a slightly different format that has a number of advantages over using straight binary encoding (i.e. NRZ).

In the Manchester encoding, a logic 0 is indicated by a 0 to 1 transition at the centre of the bit and a logic 1 is indicated by a 1 to 0 transition at the centre of the bit. Note that signal transitions do not always occur at the ‘bit boundaries’ (the division between one bit and another), but that there is always a transition at the centre of each bit. The Manchester encoding rules are summarized below:
Original Data Value Sent
Logic 0 0 to 1 (upward transition at bit centre)
Logic 1 1 to 0 (downward transition at bit centre)

Note that in some cases you will see the encoding reversed, with 0 being represented as a 0 to 1 transition. The two definitions have co-existed for many years. The Ethernet Blue-Book and IEEE standards (10 Mbps) describe the method in which a Logic 0 is sent as 0 to 1 transition, and a Logic 1 as a one to zero transition (where a zero is represented by a less negative voltage on the cable). Note that because many physical layers employ an inverting line driver to convert the binary digits into an electrical signal, the signal on the wire is the exact opposite of that output by the encoder. Differential physical layer transmission, (e.g. 10BT) does not suffer this inversion.

A Manchester encoded signal ...

Solution Summary

Solution discusses Manchester encoding, 4B/5B encoding, 8B/10B encoding, the cable grades required for different speeds of Ethernet, and Ethernet performance as more hosts are added to a network.

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