Lan Technology

Topics: OSI model, Ethernet, Virtual private network Pages: 11 (2876 words) Published: April 25, 2015

Discuss the two basic “media access control” methods for shared media used in the TCP/IP suite. ANSWER:
In the Open Systems Interconnection (OSI) model of communication, the Media Access Control layer is one of two sub layers of the Data Link Control layer and is concerned with sharing the physical connection to the network among several computers. Each computer has its own unique MAC address. Ethernet is an example of a protocol that works at the Media Access Control layer level. There are two basic media access control methods for shared media: 1. Controlled access method.

2. Contention based access method.
Controlled accessed method:
When using the controlled access method, network devices take turns, in sequence, to access the medium. This method is also known as scheduled access or deterministic. If a device does not need to access the medium, the opportunity to use the medium passes to the next device in line. When one device places a frame on the media, no other device can do so until the frame has arrived at the destination and has been processed by the destination. Although controlled access is well-ordered and provides predictable throughput, deterministic methods can be inefficient because a device has to wait for its turn before it can use the medium. Connection based access method:

Also referred to as non-deterministic, contention-based methods allow any device to try to access the medium whenever it has data to send. To prevent complete chaos on the media, these methods use a Carrier Sense Multiple Access (CSMA) process to first detect if the media is carrying a signal. If a carrier signal on the media from another node is detected, it means that another device is transmitting. When the device attempting to transmit sees that the media is busy, it will wait and try again after a short time period. If no carrier signal is detected, the device transmits its data. Ethernet and wireless networks use contention-based media access control.

Discuss the five data types used in the different layers of the OSI network model. Show the encapsulation and de-capsulation technique used among the different layers. ANSWER:

Application Layer 
This top layer defines the language and syntax that programs use to communicate with other Programs. The application layer represents the purpose of communicating in the first place. For example, a program in a client workstation uses commands to request data from a program in the server. Common functions at this layer are opening, closing, reading and writing files, transferring files and email messages, executing remote jobs and obtaining directory information about network resources.

Presentation Layer 
When data are transmitted between different types of computer systems, the presentation layer Negotiates and manages the way data are represented and encoded. For example, it provides a common denominator between ASCII and EBCDIC machines as well as between different floating point and binary formats. Two protocols used for this purpose. This layer is also used for encryption and decryption.

Session Layer
Provides coordination of the communications in an orderly manner. It determines one way or two way communications and manages the dialog between both parties for example, making sure that the previous request has been fulfilled before the next one is sent. It also marks significant parts of the transmitted data with checkpoints to allow for fast recovery in the event of a connection failure.

Transport Layer 
This layer is responsible for overall end-to-end validity and integrity of the transmission. The lower layers may drop packets, but the transport layer performs a sequence check on the data and ensures that if a 12MB file is sent, the full 12MB is received. OSI transport...
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