WAN DESIGN AND MULTICAST CONSIDERATIONS
Connecting two LANs through a WAN for long distance voice, video, and data transmission involves matching the WAN to the speed and application requirements of the connected LANs. Do the following when you select a service:
- Match the WAN bandwidth to that required by the LAN and the software applications.
- Select a WAN service that is compatible with voice, video, data, conferencing, and multimedia applications.
- Find a service provider that can give you SLA (Service Level Agreement) to guarantee that the WAN matches the needs of your organization.
- For critical WAN communications, use a WAN service that can offer QoS.
The WAN services that are most compatible with these criteria include ATM, broadband, ISDN, and SONET; all high-speed WAN services capable of exceeding 100 Mbps and compatible with multimedia. ATM also incorporates QoS, which means it allocates bandwidth specifically for different application needs, such as for multimedia. Physical WAN lines that can provide high-speed fibre-optic communications are circuit switched technologies that include switched 56 Kbps, switched T-1, T-3 and B-ISDN. Frame relay connected to T-3 is another WAN option, although it only provides bandwidth up to 45 Mbps.
Individual users or small organizations that connect to a LAN through a WAN for multimedia access currently have options that include ISDN and cable modems. Other options are emerging in telecommunications and satellite technologies, for example low-orbit satellites that are intended to provide access at speeds greater than 50 Mbps. The first step is to carefully research the WAN technologies available in your area and to make service and cost comparison. Costs can vary not only in terms of the service but in terms of the types of circuits used for the service, QoS, and SLA. Also, in some cases, you may need to use more than one type of service, particularly for backup services for critical WAN communications. For example, the main service might be over a T-1 line and the backup might be switched 56-Kbps or microwave communications.
One way to reduce costs and improve the utilization of WAN connections is to schedule WAN usage around application use. High demand applications such as video conferencing are generally used during the high consumption dynamic hours, but other applications, for example video server applications used for training or classes, can be scheduled for off-hours. Another option is to deploy multiple video servers at different sites throughout the LAN/WAN as shown in Figure.
Users access through local video server during the day for multimedia applications. At night when there is less traffic on the WAN, the video servers can receive updates of new video contents, so that when a new course or training unit is installed on one server, it is copied to the other servers over the WAN. Microsoft Windows NT server, for example, has software that automatically replicates files and folders from one server to another.
Providing integrated voice, video, and data on the same network presents some unique design issues. Most important are the issues of designing networks with enough bandwidth for the heavy traffic created by such multimedia applications. Existing LANs can be redesigned by adding switches and routers. Many network administrators now turn to switched Fast Ethernet for relatively expansive bandwidth and immediate solutions. Others, particularly administrators who must design f
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