Figure 3.1: A packet’s journey from a host to a WAN device. The WAN transmission is continuous and does not have to wait for acknowledgement or permission.

Let’s take a look at how this process would work in a T1 line. T1 has 24 slots in each frame; each slot is 8 bits, and there is 1 framing bit:

24 slots x 8 bits + 1 framing bit = 193 bits

T1 frames are transmitted 8,000 frames per second, or one frame every 125 microseconds:

193 bits x 8,000 = 1,544,000 bits per second (bps)

When you have a higher bandwidth, the frame is bigger and contains more slots (for example, E1 has 32 slots). As you can see, this is a great increase in the effective use of the bandwidth.

Another asynchronous serial transmission method is Asynchronous Transfer Mode (ATM). ATM is a cell−based switching technology. It has a fixed size of 53 octets: 5 octets of overhead and 48 octets of payload. Bandwidth in ATM is available on demand. It is even more efficient relative to the serial transmission method because it does not have to wait for assigned slots in the frame. One Ethernet frame can consist of multiple consecutive cells. ATM also enables Quality of Service (QoS). Cells can be assigned different levels of priority. If there is any point of congestion, cells with higher priority will have preference to the bandwidth. ATM is the most widely used WAN serial transmission method.

Note ATM is covered in more detail in Chapter 8.

WAN Transmission Media

The physical transmission media that carry the signals in WAN are divided into two kinds: narrowband and broadband. A narrowband transmission consists of a single channel carried by a single medium. A broadband transmission consists of multiple channels in different frequencies carried on a single medium.

The most common narrowband transmission types are T1, E1, and J1. See Table 3.1 for the differences among the transmission types and where each is used. The time slots specify how much bandwidth (bit rate) the narrowband transmissions have.

Table 3.1: Narrowband transmission types.

Narrowband is most commonly used by businesses as their WAN medium because of its low cost. If more bandwidth is needed than narrowband can provide, most businesses use multiple narrowband connections.

55

The capability of broadband to carry multiple signals enables it to have a higher transmission speed. Table 3.2 displays the various broadband transmissions, which require more expensive and specialized transmitters and receivers.

Table 3.2: The different broadband transmission types and their bandwidth.

Digital signal 2 (DS2), E2, E3, and DS3 describe digital transmission across copper or fiber cables. OC/STS resides almost exclusively on fiber−optic cables. The OC designator specifies an optical transmission, whereas the STS designator specifies the characteristics of the transmission (except the optical interface). There are two types of fiber−optic media:

∙Single−mode fiber—Has a core of 8.3 microns and a cladding of 125 microns. A single light wave powered by a laser is used to generate the transmission. Single−mode can be used for distances up to 45 kilometers; it has no known speed limitation. Figure 3.2 shows an example of a single−mode fiber.

Figure 3.2: Single mode fiber.

∙Multimode fiber—Has a core of 62.5 microns and a cladding of 125 microns. Multiple light waves powered by a light−emitting diode (LED) are used to power the transmission. Multimode has a distance limit of two kilometers; it has a maximum data transfer rate of 155Mbps in WAN applications. (It has recently been approved for use for Gigabit Ethernet.) Figure 3.3 shows an example of a multimode fiber. The core and cladding boundary work as a mirror to reflect the light waves down the fiber.

Figure 3.3: Multimode fiber.

Synchronous Transport Signal (STS)

Synchronous transport signal (STS) is the basic building block of the Synchronous Optical Network (SONET). It defines the framing structure of the signal. It consist of two parts: STS overhead and STS payload. In STS−1, the frame is 9 rows of 90 octets. Each row has 3 octets of overhead and 87 octets of payload, resulting in 6,489 bits per frame. A frame occurs every 125 microseconds, yielding 51.84Mbps.

56