Wide Area Networks

A Wide Area Network (WAN) is used to connect LANs. A WAN spans multiple locations and can span great geographic distances (see Figure 7.11). A WAN requires a connection between two LANs. For WANs that are located relatively close together, such as in the same city, a dedicated connection can be purchased to facilitate fast and confidential data communications. This type of WAN is easier to set up and maintain because the number of routers that data must pass through is limited. A WAN that spans a larger area, such as across states, still requires a connection between each LAN, but can be significantly harder to set up and maintain because of the larger distance that data must travel.

Figure 7.11: A WAN is Two or More LANs that are Connected over a High Speed Line(s)

The Internet's predecessor, ARPAnet, was the first WAN to be created. ARPAnet was formed in the late 1960s by the United States Defense Advanced Research Project Agency (ARPA). The University of California at Los Angeles (UCLA) and the Stanford Research Institute were the first two locations created to create ARPAnet. After that, many more universities and research centers were connected in this large WAN. Eventually ARPAnet grew to what is now the Internet. The Internet is a good example of a WAN and is the largest decentralized network in existence today.

A WAN is implemented using several different types of devices. A Channel Service Unit/Data Service Unit (CSU/DSU) is used to terminate a digital channel at the site of the LAN. The line then typically goes to the nearest Central Office (CO). The communications provider is responsible for the quality and reliability of the line from the CO to the CSU/DSU. The communications provider can help when troubleshooting a link failure by providing a loopback test that will check the line from the CO to the CSU/DSU.

A channel bank is used to combine several low-speed lines into a high-speed data access medium. A full T1 is merely 24 low speed 64K lines combines together using a channel bank, or multiplexer, to provide a high-speed transmission medium.

An organization can purchase a dedicated/leased line from a communications provider who will offer several options and speeds that can be used to connect LANs together.

Cabling

A T1 provides a connection speed of 1.536 Mbps. It uses a total of 24 single 64 Kbps channels combined together to provide a 1.536 Mbps link. There is a framing overhead that adds an extra .8 Mbps to the link to create a combined speed of 1.544 Mbps. For test purposes, you should remember that a T1 speed is 1.544 Mbps. The European version of a T1 is the E1. The E1 uses 32 different channels at 64kbps to provide a total of 2Mbps. A T1 and E1 can be interconnected for cross-compatibility.

A T3 is another common WAN link medium. It is faster then a T1 and can transmit at speeds of 44.736 Mbps. A T3, as well as a T1, travel through a telephone-switched network. The T3 has a European version as well, which is known as an E3. An E3 can provide transmission speeds of 34Mbps.

Note that T1s and T3s are often purchased by companies to allow the LAN to access the Internet. A WAN can be created by a virtual connection between a LAN using the same T1 or T3 or a dedicated line may be purchased to connect each LAN.

Frame relay is a technology that supports using T1 lines, which operate at 1.544 Mbps, and T3 lines, which operate at 45 Mbps. Frame relay is a packet-switching technology with variable length frames.

X.25

X.25 is a packet-switching technology that is a very popular protocol for WANs. The X.25 standard defines the point-to-point interaction between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE). A DTE is located on the organization's side of the WAN link. A DCE is the communications carrier's equipment. X.25 is similar to TCP/IP in that they are both packet-switched technologies. X.25 provides error checking from node to node. The protocol communicates on a virtual circuit and does not require a dedicated line between locations.

X.25 is not commonly used in North America anymore, except for in financial transaction systems, which still use the technology for redundancy. The exceptions are direct interaction payment systems, which facilitate direct payment transactions, which still use X.25 technology. Europe, however, still widely deploys and uses X.25.

Integrated Services Digital Network

An Integrated Services Digital Network (ISDN) is a standard for a service that provides a digital connection of phone lines. An ISDN connection can provide a speed of 64 Kbps and can be used to transmit voice or data. A Basic Rate ISDN (BRI) is one form of ISDN service that provides two channels that operate at 64 Kbps each and one channel that is dedicated to transmitting control information. Primary Rate ISDN (PRI) is another type of ISDN service that provides 23.64 Kbps channels and one channel to transmit flow-control information.

ISDN, when first deployed, was thought to be very popular because it could supply connections that allowed for speeds greater than the standard modem speeds of the time. Because of other technologies that followed ISDN that could provide even greater speeds and close to the same cost, ISDN never became as popular.

Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) is a technology that comes from ISDN that can provide very fast access speeds. ATM is a switching technology also. This technology is replacing typical frame relay technologies. ATM is sometimes referred to as cell relay so that it is not confused with frame relay. The first implementation of ATM functioned at 155 Mbps and later implementations functioned at 622 Mbps. ATM supports transmission over fiber-optic lines. It can transmit data, multimedia, and voice over the same lines. Each packet is set to 53 bytes. Because each byte is the same length, hardware error correction can be used to ensure data integrity.