148 Communication Networks
Copyright © 2005 PragSoft
line is idle and it starts transmitting its frame. To avoid collisions, it continues monitoring the echo bits and compares them against the transmitted bits. It suspends
transmission as soon as it detects a discrepancy between the two.
11.2.2. The Data Link Layer
Two data link layer protocols are provided. LAP-B see Chapter 3 is used for X.25 connections over B channels. LAP-D is similar to LAP-B and is used for D
channel connections. It is capable of supporting multiple network level connections. LAP-D is defined by CCITT recommendation I.441.
The LAP-D frame structure is very similar to the HDLC frame structure see Chapter 3 and similarly supports three types of information, supervisory, and
unnumbered frames via its control field. The only field where there are notable differences is the address field which consists of the following components:
•
A Terminal Endpoint Identifier TEI which uniquely identifies a user device.
•
A Service Access Point Identifier SAPI which uniquely identifies a network level LAP-D user entity. Four such entities have been identified: call control,
packet-mode call control, X.25 packet level, and management.
•
A CommandResponse CR bit which indicates whether the frame is a command or a response.
The LAP-D commands and responses are summarized in Figure 11.129. LAP- D provides two types of service: acknowledged and unacknowledged.
Figure 11.128 Primary rate frame structures. 23B+D Frame
30B+D Frame Bit
Description Bit
Description
1 Framing Bit
1-8 Framing channel
2-9 B1
9-16 B1
10-17 B2
17-24 B2
18-25 B3
25-32 B3
26-33 B4
33-40 B4
34-41 B5
41-48 B5
42-49 B6
49-56 B6
50-57 B7
57-64 B7
58-65 B8
65-72 B8
66-73 B9
73-80 B9
74-81 B10
81-88 B10
82-89 B11
89-96 B11
90-97 B12
97-104 B12
98-105 B13
105- 112
B13 106-113
B14 113-
120 B14
www.pragsoft.com Chapter 11: Integrated Services Digital Network
149
114-121 B15
121- 128
B15 122-129
B16 129-
136 D
130-137 B17
137- 144
B16 138-145
B18 145-
152 B17
146-153 B19
153- 160
B18 154-161
B20 161-
168 B19
162-169 B21
169- 176
B20 170-177
B22 177-
184 B21
178-185 B23
185- 192
B22 186-193
D 193-
200 B23
201- 208
B24 209-
216 B25
217- 224
B26 225-
232 B27
233- 240
B28 241-
248 B29
249- 256
B30
150 Communication Networks
Copyright © 2005 PragSoft
Figure 11.129 LAP-D command and response frames. Frame Type
Command Response Description
Information I
I Used for exchange of user data.
Supervisory RR
RR Receive Ready. Station is ready to receive frames
and acknowledged receipt of earlier frames. RNR
RNR Receive Not Ready. Station is unable to receive
frames, but acknowledged receipt of earlier frames.
REJ REJ
Reject. Rejects a frame according to the Go-Back- N scheme.
Unnumbered SABM
Set Asynchronous Balanced Mode. SABME
Set Asynchronous Balanced Mode Extended. UI
Unnumbered Information. Used for unacknowledged mode of information transfer.
UA Unnumbered Acknowledgment. Unsequenced
acknowledgment frame. DISC
Disconnect. Forces a slave station into disconnect mode.
DM Disconnect Mode. Used by a slave to indicate it is
in disconnect mode. XID
XID Exchange identification information.
FRMR Frame Reject of a protocol-violating frame.
The acknowledged service involves the exchanging of all three types of frames
between the network and a TE. First, a link is requested using the SABM or SABME command, which may be accepted by a UA response or rejected by a DM
response. Then, the two ends may exchange information frames using the I command. The supervisory frames are used during this phase for error control using
the Go-Back-N scheme and flow control using the sliding window protocol. Finally, the connection may be terminated by a DISC command. FRMR is used to
reject a frame which violates the protocol e.g., incorrect frame length or invalid sequence numbers. This results in the connection being aborted. The XID command
is used for exchanging identification information e.g., negotiating a new set of values for LAP-D parameters.
The unacknowledged service provides basic exchange of user information
using the UI command, which are received unacknowledged. No error control or flow control is provided, although CRC-based error detection is still performed and
incorrect frames are simply discarded.
Though LAP-D is based on LAP-B, some important differences exist. Firstly, unlike LAP-B which is designed for point-to-point connection, LAP-D can support
multiple DEs connected via a multidrop line. Secondly, LAP-D supports UI frames, while LAP-B only supports sequenced information frames. Lastly, LAP-D uses a
different addressing scheme.
