In the previous section we briefly mentioned some of the considerations necessary for selecting the correct WAN technology. There are many decisions that must be made before being able to select the appropriate WAN type.
Potentially the biggest is the availability of any given WAN type. While it is nice to have a business in a well-developed urban region, remote sites (especially international sites) may severely restrict the possibilities. In general, asynchronous connections are almost always available. It would be wise to find out what types are available before considering any of the other factors.
Bandwidth limitations may also be a factor. If the requirements for a WAN are for small amounts of data or short connection times, then a circuit-switched network would be ideal. But if more data, such as video, voice, or large numbers of file downloads, will be transiting the WAN, then circuit-switched may not be good enough. In this case, packet switching such as Frame Relay becomes a better option. If the amount of bandwidth consumed is fairly constant and used over a short distance, then dedicated lines may make more sense.
Closely related to the bandwidth is the cost of the WAN link. While asynchronous connections and even ISDN are fairly cheap, the cost of a dedicated line may be too much for a business to absorb. Also, long distance charges may make these types of links too expensive. Unfortunately, this is an often-overlooked cost during initial designs. A good middle ground is a packet-switched network. Keep in mind that if the distance is short and the bandwidth is constant, dedicated lines may actually be cheaper than Frame Relay. On the other hand, many remote sites would be very expensive to set up each with a dedicated line. This is an ideal situation for Frame Relay, if the cost can be justified.
Ease of management can be an often-overlooked consideration of selecting the correct WAN type. The management features of a WAN link consist of the initial configuration as well as maintaining the link during normal operations. Dedicated lines tend to be easier to configure and maintain than the other types. However, with any of these link types, if connectivity to the remote site is lost, management of that site and that link becomes difficult, often requiring support staff on both ends (or a lot of travel for the network engineer).
Quality of Service (QoS), in this discussion, deals with the importance of a link to a remote site. Is the data being sent across the link so valuable that delay or loss of data will cost the business money, or the business itself could not weather a loss or delay? If this is the case, then a backup link may be required. Asynchronous connections tend to be unreliable in this sense but may act as the backup link to a Frame Relay or dedicated connection.
Closely matched with QoS is the reliability of the link in reference to the hardware. If the link is extremely important, then it might be prudent to carry spare parts, such as CSU/DSUs, modems, and even routers. There may also be an instance where two links are established so that no delay occurs if a single link fails.
The type of application traffic that may be needed to traverse the WAN link can dictate the type of WAN link required. Are the packets being delivered small, bursty traffic such as email, or are they predominantly sustained larger packets such as file transfers? This works in conjunction with the bandwidth requirements, but can also define the upper-layer protocols that may be needed. IPX/SPX and TCP/IP may both be necessary, and thus SLIP would not be a suitable protocol. Although HDLC might work in this instance, are both routers Cisco routers? If not, then PPP becomes the best solution.
Security is becoming a major concern with our networks as more people are connected to the Internet. Most people think about security with regard to hackers and crackers, but in truth security should also be concerned with inadvertent access by employees causing unintentional damage. Routers can be set up with access lists or filters to prevent access, but what about requiring authentication from your users across those WAN links? It may be necessary to implement an authentication server such as Radius or TACACS+. Preventing unauthorized viewing of data between sites or between users and sites can be accomplished with technologies such as virtual private networks (VPNs). Security concerns should always be addressed when deciding on the type of WAN link. Dedicated lines tend to be inherently more secure from prying eyes than using modems for accessing the Internet.