WAN CARRIER TYPES
There are several physical signaling or carrier methods for transporting data on WANs. Some of the most common include the following:
Point-to-point
T-carrier
SONET
ISDN
Wireless.

Point-to-point carrier communications through public dial-up lines and leased telephone lines represent the most basic WAN carrier communications. For example, a simple WAN is established every time you employ a modem to make a modem-to-modem connection over a dial-up line. The modem at the other end may be connected to a network or to a computer that is a few miles away or a few thousand miles away. The physical mode of communication is an analog circuit that goes through telephone switching stations to establish a connection that lasts only as long as the communications sessions.
Another form of point-to-point communication is a leased telephone line that is used exclusively between two points, such as between a state university and the state Government offices. It bypasses the need to dial and find a switched circuit for a connection each time a communication session is started. Leased lines sometimes come with the line conditioning to reduce noise and provide more reliable communication than dial-up lines.
  T-Carrier
A T-carrier line is a dedicated telephone line that can be used for data communications to connect two different locations for continuous point-to-point communication. For example, some universities use T-carrier lines to connect to one another for Internet communications. Some states use T-carrier lines to connect branch offices and colleges to the government headquarters in the state capital. These lines offer dependable service over very long distances. T-carriers operate in a logical topology as the smallest T-carrier service; T-1 offers 1.544 Mbps data communication that can be switched to create multiple data channels for high-speed communication, as shown in Table 7.1.

For example, switching T-1 to the next level of service (called T-2) creates four channels. T-3 has 28 channels, and T-4 has 168 channels. Because T-carrier service is expensive, telephone companies offer functional services that use a portion of the T-1 services utilizing sub-channels with 64 Kbps speeds. This is possible because each T-1 service consists of 24 sub-channels, each of 64 Kbps called digital signal at level 0 (DS-0) channels.
An alternative to T-carrier lines is switched synchronous 56 Kbps communications and switched asynchronous 57.6 Kbps communications. Both provide digital communi­cation through data compression techniques and by using circuit switching methods that combine to yield an actual throughput of up to four times the base rate. Organizations use switched 56 Kbps communications because the rates are less than for T-carrier service and for the backup line required; to be used when the main T-carrier service is down.
T-carriers use one of the two types of switching techniques for transmitting information. One is time division multiple access (that is, TDMA) and the other is a combination of TDMA and statistical multiple access, which is the fast packet technology. The fast packet switching enables T-carrier service to take into account different channel access needs for handling voice, video and data.
  SONET
Synchronous Optical Network (SONET) is a standard for transmitting data on optical fibres. It was originally created to allow easier connection between carriers that were using different venders' products for their optical networks. SONET has become the de facto standard for carrying voice and data traffic over an optical network and ANSI has written a standard for it.
SONET is a high-speed technology for WANs that uses single mode fibre optical cable and communications based on T-3. The base T-3 level of SONET is called Synchronous Transport Signal Level 1 (STS-1). STS-1 can be incremented to higher levels that reflect multiples of T-3. SONET converts an electrical-based STS-x signal to an optical based signal called an Optical Carrier (OC). STS-1 frames can be converted and sent at the same time in multiples, through a process that interleaves the frames with one another to achieve faster STS-x and OC-x speeds.
Table 7.2 shows the STS-x to OC-x speeds that are possible with SONET.

  ISDN
The Integrated Services Digital Network (ISDN) is less of a network and more of a set of standards than the name implies. The ISDN standards were developed by the ITU-T as a vision for the direction that the world's public telecommunications system should take. They believed that ISDN would eventually replaced leased and switched circuits as we knew them.
ISDN is a WAN technology for delivering voice, data, and video services over telephone lines. ISDN uses digital technology to achieve faster and more reliable communications than are possible through none-ISDN POTS (Plain Old Telephone Service) lines. An ISDN line is physically a POTS or T-1 (twisted pair or fibre-optics) line; but with ISDN equipment at telco and customer premises.
The benefits of ISDN are as follows:
• It provides efficient multiplexed access to the public network. • It has the capabilities to support integrated-voice and data.
• It has a robust signaling channel, which is important for network management. • It provides an open system that is internationally defined.
  Wireless
In wireless technologies, the carrier is a signal radiated from an antenna or dish. The amount of radiated power and the gain are governed by the communications laws and codes of individual countries. The specific frequencies authorized for wireless communications are also governed by national and international communications agreements and treaties.
Wireless networking is one of the hottest topics in communications. Users want the convenience of not being tethered to a telephone jack or other communication port, especially if they are using a laptop computer. Network managers like the flexibility that wireless technology gives them. For example, when offices are moved or rearranged, it is not necessary to rewire the office and pull new cables to the new office locations. However, security is a significant concern with wireless communication because it is accomplished using radio transmissions that are susceptible to being intercepted. However, security problems are being solved and wireless usage is growing by leaps and bounds.

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