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.
Transmission Type Number of Slots Bit Rate Region
T1 24 1.544Mbps North America
E1 32 2.048Mbps Africa, Asia (not including
Japan), Europe, Australia,
South America
J1 32 2.048Mbps Japan
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.
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