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Chapter 2

Protocol Attributes

Frame Structure

Now that you understand how the two end systems communicate, move a
little deeper into X.25 by looking at the frame structure. Refer to Figure 2.34
for the graphical representation of the X.25 frame. X.25 frames are very sim-
ilar to SDLC frames. Each end is bounded with a Flag field. Starting from
the right and moving to the left, we start with a Flag segment. After the Flag
field, the frame contains the frame field control and address field.

F I G U R E 2 . 3 4

X.25 frame structure

Next is the Data field that contains the Layer 3 X.25 packet, which in turn

holds the Layer 3 packet header and upper-layer data. Finally, the frame
ends with the FCS and Flag fields. The Layer 2 frame is simply a LAPB
frame.

Frame Relay

The Frame Relay technology is more advanced than X.25. Frame Relay was
developed as a digital packet-switching technology, whereas X.25 was an
analog technology. The technology used in Frame Relay allows it to multi-
plex several different data flows over the same physical media.

Frame Relay also uses permanent and switched virtual circuits between

data terminal equipment (DTE). These virtual circuits have unique identifi-
ers that allow the Frame Relay to keep track of each logical data flow. The
identifier is known as a DLCI (data link connection identifier). The DLCI
number is used to create a logical circuit within a physical circuit. Multiple
logical circuits can be created within one physical circuit. Look at the fol-
lowing router-configuration excerpt:

interface Serial1/5

description Physical Circuit

no ip address

no ip directed-broadcast

encapsulation frame-relay

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