Wide Area Networks (WAN) span across large geographical distances in order to provide connectivity for various parts of the network infrastructure. Unlike the Local Area Network (LAN) environment, not all WAN components are owned by the specific enterprise they serve. Instead, WAN equipment or connectivity can be rented or leased from service providers.
Most service providers are well trained in order to make sure they can properly support not just the traditional data traffic but also voice and video services (which are more delay sensitive) over large geographical distances.
Another interesting thing about WANs is that, unlike LANs, there is typically some initial fixed cost and some periodic recurring fees for the services. With wide area networking, not only do you not own the connection and some of the equipment but you will also have to regularly pay fees to the service providers. This is one of the reasons why you should avoid over-provisioning (i.e., buy only the bandwidth you think you will use). This leads to the need for implementing effective Quality of Service mechanisms to avoid buying additional WAN bandwidth. The high costs are usually associated with the recurring fees that might appear in the case of over-provisioning the bandwidth.
WAN technology design requirements are typically derived from the following:
- Application type
- Application availability
- Application reliability
- Costs associated with a particular WAN technology
- Usage levels for the application
WAN Categories
An essential concept in WAN categorisation is circuit-switched technology, the most relevant example of this technology being the Public Switched Telephone Network (PSTN). One of the technologies that fall into this category is ISDN. The way circuit-switched WAN connections function is by becoming established when needed and terminating when they are no longer required. Another example that reflects this circuit-switching behaviour is the old-fashioned dial-up connection (dial-up modem analog access over the PSTN).
| Note: Not too long ago, dial-up technology was the only way to access Internet resources, offering an average usable bandwidth of around 40Kbps. Nowadays, this technology is almost extinct. |
The opposite of the circuit-switched option is leased-line technology. This is a fully dedicated connection that is permanently up and is owned by the company. Examples of leased lines include Time Division Multiplexing (TDM)-based leased lines. These are usually very expensive because a single customer has full use of the connection.
Another popular category of WAN technology involves packet-switched networks. In a packet-switched infrastructure, shared bandwidth utilises virtual circuits. The customer can create a virtual path (similar to a leased line) through the service provider infrastructure cloud. This virtual circuit has a dedicated bandwidth, even though technically this is not a real leased line. Frame Relay is an example of this type of technology.
Some legacy WAN technologies include X.25, which is the predecessor of Frame Relay. This technology is still present in some implementations but it is very rare (Frame Relay is also pretty rare nowadays).
Another WAN category you may have heard about is cell-switched technology. This is often included in packet-switched technologies, as they are very similar. A cell-switched technology example is ATM (Asynchronous Transfer Mode, which is also pretty rare nowadays). This operates by using fixed-size cells, instead of using packets (as used in Frame Relay). Cell-switched technologies form a shared bandwidth environment so that the service provider can guarantee customers a certain level of bandwidth through their infrastructure.
Broadband is another growing WAN category and this includes technologies such as the following:
- DSL
- Cable
- Wireless
Broadband has the capability of taking a connection, like the old-fashioned coaxial cable that carries TV signals, and figuring out how to use different aspects of that bandwidth. For example, by using multiplexing an additional data signal could be transmitted along with the original TV signal.
Figure 41.1 – WAN Categories
As detailed in Figure 41.1 above, there are many options when discussing WAN categories and this is just a general introduction to them. All of these technologies can support the needs of modern networks that operate under the 20/80 design rule, meaning 80% of the network traffic uses some kind of WAN technology in order to access remote resources.