2.14. Identifying the Basic Characteristics of Various WAN Technologies
Generally speaking, a wide area network (WAN) is the result of connecting multiple local area networks (LANs). Another characteristic of a WAN is the fact that the organization rarely owns all of the lines of communication on which it is sending data. In addition, WAN technologies are typically much slower than LAN technologies. WAN technologies that are comparable in speed to LAN technologies are typically very expensive.
2.14.1. Critical Information
Since WAN communication uses completely different types of communication lines and equipment than LAN communication, it only makes sense that the technologies used would also be different. WAN technologies have evolved over time to provide remote office and remote access users with faster and more reliable communications methods and communications media. In this section, we will examine each of these types of communication technologies.
2.14.1.1. Packet Switching
Packet switching is a technology whereby each packet of a data communication can take a separate route to its destination. It has been used in the past with X.25 technology to ensure communication through a questionable network of communications. If packets should arrive out of order, special devices called packet assemblers and dissemblers are used to put the packets back into the correct order before presenting them to the computer. In today's more reliable networks, packet switching is used with frame relay to speed up communications by creating more available bandwidth on multiple permanent virtual circuits (PVCs).
2.14.1.2. Circuit Switching
Circuit switching is a technology whereby a single communication channel is opened at the start of a session and that single channel is used throughout the communication. All packets are sent over the same channel and the communication channel is closed when the session has ended. Normal telephone lines use a circuit switching method of communication.
2.14.1.3. Integrated Services Digital Network (ISDN)
Integrated Services Digital Network (ISDN) is an international standard for sending voice, data, and video over digital telephone lines or normal telephone wires. There are two types of ISDN, Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI offers only 128Kbps bandwidth using two 64Kbps B (bearer) channels and one 16Kbps D (delta) channel. Primary Rate Interface (PRI) offers tremendously more bandwidth, using twenty three 64Kbps B channels and one 64Kbps D channel in the United States and thirty 64Kbps B channels with one 64Kbps D channel in Europe. Originally, ISDN used baseband transmission on copper wire, but the new form of ISDN (B-ISDN) offers a broadband signal using a fiber-optic cable. ISDN is often used as a backup line in organizations that also have higher bandwidth and more expensive main connection lines.
2.14.1.4. Fiber Distributed Data Interface (FDDI)
Fiber Distributed Data Interface (FDDI) is a set of protocols used for sending digital data over a fiber optic cable. FDDI networks use a token-passing system and a dualring topology. They can support data rates up to 100Mbps using the new FDDI-2 extension. FDDI networks are typically used as a backbone for a WAN.
2.14.1.5. T Carrier Level 1 (T1)
A T Carrier level 1 (T1) is a United States–based dedicated line that can carry 1.544Mbps in 24 separate 64Kbps channels. Organizations can lease an entire T1 or only a part of a T1, referred to as Fractional T1. Each 64Kbps channel can be configured to carry voice or data transmissions. T1s are a popular choice for businesses connecting to the Internet and for Internet service providers (ISPs). They can also be used to directly connect multiple geographical locations of the same organization. Similar lines are available outside of the United States, such as the E1 in Europe and the J1 in Japan.
2.14.1.6. T Carrier Level 3 (T3)
A T Carrier level 3 (T3) is a United States–based dedicated line that can carry about 43Mbps on 672 separate 64Kbps channels. Each channel can be configured for voice or data transmissions. T3s are primarily used by ISPs that are connecting directly to the backbone of the Internet. T3s are also used to form the backbone of the Internet itself. Similar lines are available outside of the United States, such as the E3 in Europe and the J3 in Japan.
2.14.1.7. Optical Carrier (OCx)
Optical Carrier (OC) is a term used to describe the speed of networks that conform to the Synchronous Optical Network (SONET) standard, which allows data streams at different rates to be multiplexed. This adds greatly to the flexibility of data networks throughout the world. The term OCx covers a range of speeds, including the following:
OC-1 = 51.85Mbps
OC-3 = 155.52Mbps
OC-24 = 1.244Gbps
OC-48 = 2.488Gbps
OC-192 = 9.952Gbps
OC-255 = 13.21Gbps
2.14.1.7.1. X.25
X.25 was an original standard used for packet-switching networks. It allowed for reliable communication through a very unreliable medium at that time, namely the telephone companies' standard lines with analog switches. X.25 was very slow and has since been replaced by Frame Relay, which is much faster.
2.14.2. Exam Essentials
Be able to define the differences between packet switching and circuit switching. Packet-switching networks use multiple paths and might send each successive packet down a different path, depending on the traffic and the health of the network. In contrast, circuit-switching networks open a channel of communication and send all of the packets down that channel. Packet-switching networks might stay connected when no data is being sent, whereas circuit-switching networks terminate the connection when the communication in finished.
Be familiar with ISDN terminology. There are two basic types of ISDN: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Each ISDN channel can carry a maximum of 64Kbps. BRI uses two 64Kbps and one 16Kbps D channel, whereas PRI uses twentythree 64Kbps D channels and one 64Kbps D channels. In both cases, only the B channels are counted for bandwidth.
Know the characteristics of FDDI. FDDI uses a fiber-optic cable and a token-passing method of access with a dualring topology. The newest FDDI-2 networks are capable of speeds up to 200Mbps. FDDI is typically used for the backbone of an organization's WAN.
Be familiar with T carrier levels. T1 lines can carry 1.544Mbps on twentyfour 64Kbps channels of communication. This type of line is usually used by an organized as a leased line to connect offices or to connect directly to the Internet without using an ISP. In contrast, T3 lines can carry about 43Mbps on 672 separate 64Kbps channels. This type of line is generally used by very large organizations or by ISPs that connect directly to the backbone of the Internet. These types of lines might also be used to build the backbone of the Internet itself.
Be able to describe OCx. OCx is a speed designation used for fiber-optic lines that conform to the SONET standard. Be familiar with the various speeds of fiber-optic communication.
Know the characteristics of the X.25 technology. X.25 was one of the original technologies used on packet-switching networks. X.25 offered very slow but reliable communication through the unreliable telephone lines in the 1980s. It has all but been replaced by Frame Relay.