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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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A WAN consists of routers and links. As previously mentioned, routers receive a routable data packet, such as Internet Protocol (IP) or Internetwork Packet Exchange (IPX), review the destination address located in the packet header, and decide which direction to send the packet next in order to forward it closer to the final destination. The routers maintain routing tables that adapt, via a routing protocol, to changes in the network. Refer to a later section (“Routing Protocols”) for a description of common routing protocols.

WANs fall into two main classes: private and public. Figure 8.10 illustrates these two approaches.

Private Point-to-Point WAN

With private WANs, the user organization or company owns and manages most of the network equipment, such as routers and communications circuits. Traditionally, organizations have implemented private point-to-point WANs to support communications between remote terminals and centralized mainframe-based applications. The following identifies the ramifications of using a private WAN:

  More suitable for a WAN requiring a low degree of meshing (centralized topology)
  Economically feasible service fees for metropolitan areas
  Higher initial cost for a greater number of hardware interfaces and circuit installations


Figure 8.10  Private versus public WAN approaches.

  Lease fees sensitive to the distance between sites (costs increase as the distance increases)
  Potentially higher operating costs due to required in-house management
  Fixed bandwidth

A common link between sites of a private point-to-point LAN is T1. T1 Bell labs originally developed T1 to multiplex multiple phone calls into a composite signal, suitable for transmission through a digital communications circuit. A T1 signal consists of a serial transmission of T1 frames, as shown in figure 8.11. Each frame transports an 8-bit sample of 24 separate channels. You can lease from a telephone service carrier an entire T1 circuit (1.544 Mbps) or only single channels (64 Mbps each).


Figure 8.11  The T1 signal format.

Public Packet Switching WAN

A public WAN is owned and operated entirely by a service provider. With the development of distributed client-server applications, most organizations now require technologies suitable for highly meshed topologies. In other words, there is a need to support communications among the remote sites, not just in a centralized data center. Thus, you should seriously consider leasing the use of a public packet switching WAN to support today’s demand for distributed computing. The following identifies the implications of using a public WAN:

  More suitable for a WAN requiring a high degree of meshing (i.e., distributed topology)
  Lower initial cost
  Potentially lower operating cost
  Lease fees not sensitive to the distance between sites
  Variable bandwidth (bandwidth on demand)
  Potentially lower operating costs due to carrier-provided management
  Most economical fees for service outside the metropolitan area

The following briefly describes each of the technologies that support public packet switching WANs. You can lease these as services from carriers such as AT&T and Sprint within most metropolitan areas.

  X.25. X.25 was the first public packet switching technology, which was developed by the CCITT and offered as a service during the 1970s that is still available today. X.25 offers connection-oriented (virtual circuit) service and operates at 64 Kbps, which is too slow for some high-speed applications. Designers made X.25 very robust to accommodate the potential for transmission errors resulting from transport over the metallic cabling and analog systems used predominately in the 1970s. Thus, X.25 implements very good error control. Some companies have a significant investment in X.25 equipment and are still supporting the technology. However, you should consider other packet switching technologies, such as frame relay, for new implementations.
  Frame Relay. Frame relay is a packet switching interface that operates at data rates of 56 Kbps to 2 Mbps. Actually, frame relay is similar to X.25, minus the transmission error control overhead. Thus, frame relay assumes a higher layer, end-to-end, protocol to check for transmission errors. Carriers offer frame relay as a permanent connection-oriented (virtual circuit) service. In the future, frame relay will be available as a switched virtual circuit service. To interface with frame relay service, you need to purchase or lease a Frame Relay Attachment Device (FRAD) or router with a frame relay interface. The FRAD or router interfaces a LAN (typically ethernet) to the local frame relay service provider via a T1 circuit. Frame relay is currently the most feasible technology available for interconnecting geographically disparate sites, especially if these sites span several metropolitan areas and applications are distributed.
  Switched Multimegabit Data Service (SMDS). Switched Multimegabit Data Service (SMDS) is a packet switching interface that operates at data rates ranging from 1.5 Mbps to 45 Mbps. SMDS is similar to frame relay, except SMDS provides connectionless (datagram) service. You can access a local SMDS service provider via T1 or T3 (45 Mbps) circuits. SMDS is not available in all areas.
  Integrated Services Digital Network (ISDN). During the 1980s, the ISO developed a set of standards for the Integrated Services Digital Network (ISDN). The goal of the ISDN is to offer worldwide multimedia services via a single standard network connection, which means having one connection to television, radio, telephone, and computer networks. This goal has never been met, but many carriers offer data communications services via ISDN interfaces. Today, you can lease an ISDN circuit that operates at 64 Kbps using digital signals and optical fiber circuits. This supports telephone and data traffic requiring synchronous transmission, such as video conferencing.

Information Security

As explained in Chapter 1, “Introduction to Wireless Networking,” wireless networks are prone to breaches of information security. When sending data over a wireless network, leased lines, or a public data network, you run the risk of someone stealing the information. If you plan to send sensitive information over a WAN, consider the use of encryption and other mechanisms to counteract this problem. For example, you can use encryption devices on all links to scramble the data, making it meaningless. For connections to the Internet, place a firewall at the entry point to the Internet. A firewall filters out unauthorized access from others on the Internet to your company’s resources. A typical firewall configuration blocks all incoming traffic, but enables users from within to access Internet services.


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