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The Network Layer is intended to provide the upper layers a high degree of freedom from specific network connection protocols such as voice analog modem protocols V.xx. The Network Layer is therefore involved in the setup and teardown of connections. Also, the Network Layer identifies the connection as connection oriented or connectionless oriented. Sometimes, it is also used in data transfer. The Network Layer provides routing and addressing information to its adjacent layers. It is the uppermost layer of the three chained layers. The Network Layer also provides for the orderly interconnecting of both similar and dissimilar networks.
The Network Layer, which includes a predefined set of route tables, will calculate the open-shortest-path-first route used to identify the route the data will take from source to destination. In order to minimize the number of blocked transmissions (and therefore maximize revenue), the open-shortest-path-first routine examines the current connections and determines an appropriate transmission path that is not currently in use. As the name states, the routine looks for the shortest path available. In many instances, particularly in LAN applications, there may be only one choice and the calculation really does not exist. However, in the public sector, there could be thousands of choices available. (Do you want the Russian operator or the Chinese operator to listen to your Washington, D.C.-Bombay conversation?)
This aspect of communications has extremely important implications for video applications. And ATM/ADSL are very much concerned with video applications. Video that is sent to a user destination, stored, then played back at the user’s leisure is not too worrisome. But live video must traverse the transmission path in the exact order sent and the transmission delay from source to destination must be negligible, else the quality of the picture suffers dramatically. While the shortest path available may be okay for voice and bursty data communications, it is not necessarily any good for live video.
Figure 4-5. OSI Transport Layer
Here is $10,000 worth of consulting advice. The Network Layer includes the Management Information Base (MIB). The MIB is a difficult to read (and understand) software database that includes all of the known information regarding the network node. If you want the network to provide you any particular information, reports, etc., concerning the status of individual connections or group of connections (by location, SVC, PVC, port, switch, etc.), ask someone (engineering? sometimes they do not understand the MIB) if it is in the MIB. If the answer is no, don’t waste your time trying to get network information that does not exist.
The Network Layer responsibilities include:
Some Network Layer specifications include:
The Transport Layer is responsible for the delivery of data between origination and destination within the bounds of established reliability levels. There are five defined levels of reliability. The reliability level is established by the type of service requested from the service provider. This layer is also responsible for data multiplexing and demultiplexing. ATM does not utilize the Transport Layer functionality. ADSL does.
The five reliability levels are simple, multiplexing, basic error recovery, error recovery and multiplexing, and error detection and recovery class. For simple reliability, flow control and connection release are provided by the underlying network layers. Multiplexing utilizes flow control but does not specifically utilize error control which is provided by the underlying layers. Basic error recovery does not utilize flow control but can detect errors and provide some error control functionality. Error recovery and multiplexing utilizes flow control, error detection, and correction. Error detection and recovery utilizes flow control, error detection and correction including retransmission, routing around network path failures, and detecting and reacting to link inactivity.
Figure 4-6. OSI Session Layer
The Transport Layer responsibilities include:
Some Transport Layer specifications include:
The Session Layer establishes and maintains the exchange of data between origination and destination. Also, the Session Layer must provide for an orderly recovery from failures caused by any number of predictable and unpredictable events. This layer is the lowest layer of an application-oriented communications software program.
The Session Layer is responsible for:
Some Session Layer specifications include:
Figure 4-8. OSI Application Layer
The Presentation Layer is responsible for manipulating the data such that the application host will understand it. As an example, this layer residing on a UNIX host will interpret DOS formatted data so that the UNIX machine will understand the data correctly. A common interpretation issue addressed by this layer is the different method of using the carriage return/line feed in files between UNIX- and DOS-based machines.
Some Presentation Layer responsibilities are:
Some Presentation Layer specifications are:
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