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Encoding Schemes in OSI Layer Model

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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.

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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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You and the other members of the IT team for Surebank need to get up to speed on the OSI model.


You are currently working for Surebank, a mid-size financial institution. The organization has approximately 80 people working in their main location where your office is located. The bank also has two branch locations. You have been very successful in your career as a Network Operation Specialist, serving at the help desk, assisting many users with routine problems with their desktop PC's and the company software. Your supervisor, the Network Administrator, was recently fired because of poor attendance. As you are the senior NOS and have performed very capably, you have been appointed as the Interim Network Administrator. If you do well, the interim position could become permanent for you.

Windows 2000 operating system and the Microsoft Office 2000 Suite is on all of the desktop computers (each staff member has a PC). These are the two primary applications you maintain on the network. The staff also uses a text-based interface to the financial system that operates on a mini-main frame similar to a UNIX machine. This interface is supported by consultants. You only need to coordinate the support calls for this software.

Each branch office has only two computers. Those computers are connected to the main office over a Frame Relay network that is supported by your local Internet Service Provider.

When you moved into your new office, you discovered that there is no network inventory, no network diagram, and very little information on how to maintain the network. You believe that the former Network Administrator took all of this information with him in retaliation for being terminated. You are not allowed to contact him, so you are on your own to learn and recreate the information you will need to be successful in this new position.

You will be able to use Internet search engines as well as have access to vendor information and technical resources to assist you in the work.

The main office computers are connected via an Ethernet network and a Token Ring network. Your predecessor was working on a presentation to your boss about having the Token Ring network replaced and converted to Ethernet. You have not been able to find the presentation so you will need to redo his work. In the coming weeks, you will need to both maintain current projects and systems, as well as take on any projects management asks of you.

You and the other members of the IT team for Surebank need to get up to speed on the OSI model. Describe the seven layers of the OSI model. Describe and explain each function of the seven layers, the protocols used, and the network components involved. State your findings as a short presentation.

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