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Wireless+Networking+in+the+Developing+World_Part3 - Chapter...

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the standard, implementers are free to use whatever means are available to build their protocol stack. This means that any given layer from manufacturer A can operate with the same layer from manufacturer B (assuming the rele- vant speci fi cations are implemented and interpreted correctly). Here is a brief outline of the seven-layer OSI networking model: Layer Name Description 7 Application The Application Layer is the layer that most net- work users are exposed to, and is the level at which human communication happens. HTTP, FTP, and SMTP are all application layer protocols. The human sits above this layer, interacting with the application. 6 Presentation The Presentation Layer deals with data representa- tion, before it reaches the application. This would include MIME encoding, data compression, format- ting checks, byte ordering, etc. 5 Session The Session Layer manages the logical communica- tions session between applications. NetBIOS and RPC are two examples of a layer fi ve protocol. 4 Transport The Transport Layer provides a method of reaching a particular service on a given network node. Exam- ples of protocols that operate at this layer are TCP and UDP. Some protocols at the transport layer (such as TCP) ensure that all of the data has arrived at the destination, and is reassembled and delivered to the next layer in the proper order. UDP is a "con- nectionless" protocol commonly used for video and audio streaming. 3 Network IP (the Internet Protocol) is the most common Net- work Layer protocol. This is the layer where routing occurs. Packets can leave the link local network and be retransmitted on other networks. Routers perform this function on a network by having at least two network interfaces, one on each of the networks to be interconnected. Nodes on the Internet are reached by their globally unique IP address. Another critical Network Layer protocol is ICMP, which is a special protocol which provides various management messages needed for correct operation of IP. This layer is also sometimes referred to as the Internet Layer . Chapter 3: Network Design 31
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Layer Name Description 2 Data Link Whenever two or more nodes share the same physi- cal medium (for example, several computers plugged into a hub, or a room full of wireless devices all using the same radio channel) they use the Data Link Layer to communicate. Common examples of data link protocols are Ethernet, Token Ring, ATM, and the wireless networking protocols (802.11a/b/g). Communication on this layer is said to be link-local, since all nodes connected at this layer communicate with each other directly. This layer is sometimes known as the Media Access Control ( MAC ) layer. On networks modeled after Ethernet, nodes are re- ferred to by their MAC address. This is a unique 48 bit number assigned to every networking device when it is manufactured.
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