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WAN Design

2.3 WAN Design 
2.3.1 WAN Communication
WANS are considered to be a set of data links connecting routers on LANs. User end stations and servers on LANs exchange data. Routers pass data between networks across the data links.
Because of cost and legal reasons, a communications provider or a common carrier normally owns the data links that make up a WAN. The links are made available to subscribers for a fee and are used to interconnect LANs or connect to remote networks. WAN data transfer speed (bandwidth) is considerably slower than the 100 Mbps that is common on a LAN. The charges for link provision are the major cost element of a WAN and the design must aim to provide maximum bandwidth at acceptable cost. With user pressure to provide more service access at higher speeds and management pressure to contain cost, determining the optimal WAN configuration is not an easy task.
WANs carry a variety of traffic types such as data, voice, and video. The design selected must provide adequate capacity and transit times to meet the requirements of the enterprise. Among other specifications, the design must consider the topology of the connections between the various sites, the nature of those connections, and bandwidth capacity.
Older WANs often consisted of data links directly connecting remote mainframe computers. Today’s WANs, though, connect geographically separated LANs. End-user stations, servers, and routers communicate across LANs, and the WAN data links terminate at local routers. By exchanging Layer 3 address information about directly connected LANs, routers determine the most appropriate path through the network for the required data streams. Routers can also provide quality of service (QoS) management, which allots priorities to the different traffic streams.
Because the WAN is merely a set of interconnections between LAN based routers, there are no services on the WAN. WAN technologies function at the lower three layers of the OSI reference model. Routers determine the destination of the data from the network layer headers and transfer the packets to the appropriate data link connection for delivery on the physical connection.

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