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Chapter 2

Protocol Attributes

is only concerned primarily with the transmission and reception of data.
Because Frame Relay does not worry about flow-and-error control, it does not
reserve any space within the frame for this information.

The second difference is the DLCI number. Ten bits of the two-byte

Address

field are used to define the DLCI. To a Frame Relay frame, the

DLCI is the most significant address in the header. Figure 2.35 depicts a
Frame Relay frame. As you can see it is very similar to the X.25 frame, except
for the flow/error control field.

F I G U R E 2 . 3 5

Frame Relay frame structure

Integrated Services Digital Network (ISDN)

Integrated Services Digital Network (ISDN) is a service that allows tele-
phone networks to carry data, voice, and other digital traffic. There are two
types of ISDN interfaces: Basic Rate Interface (BRI) and Primary Rate Inter-
face (PRI). BRI uses two B channels and one D channel. Each of the two B
channels operates at 64Kbps bidirectionally; the D channel operates at
16Kbps. The B channels are used for transmitting and receiving data. The D
channel is used for protocol communications and signaling.

In contrast, PRI uses 23 B channels and one D channel. All 23 B channels

are added to a rotary group, as well. The D channel runs at the same line
speed as the B channels--64Kbps. Because of the D channel's additional line
speed, PRI has the equivalent line speed of a T1 circuit (1.5Mbps). In
Europe, PRI offers 30 B channels and 1 D channel, making it the equivalent
of an E1 circuit.

Just as there are two types of ISDN interfaces, there are two terminal

equipment types. Type 1 (TE1) is equipment that was built specifically for
use on ISDN. Type 2 (TE2) is equipment that was made before the ISDN
specifications, and it requires a terminal adapter to actually interface with
ISDN. Terminal equipment is comparable to DTE, mentioned in the Frame
Relay section. It is equipment such as computers or routers.

In order for terminal equipment to work, there must be a network termi-

nation to connect to. Three types of ISDN network terminations exist. These

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