As shown in Figure 3−7, ISDN provides a physical transport for upper−layer protocols. On an ISDN data circuit, you will have a layer 2 datalink encapsulation such as PPP, MLPPP, HDLC, or frame relay.
Encapsulated in the layer 2 datalink frame will be a layer 3 network layer protocol such as IP, IPX, or AppleTalk.
Figure 3−7: ISDN transport Layer 1
As shown in Figure 3−8, ISDN is defined as a three−layer protocol stack. Layer 1 of the ISDN stack is responsible for the physical transmission of the data on the ISDN circuit.
Figure 3−8: ISDN protocol stack
An ISDN BRI U interface circuit is carried over a single twisted pair utilizing 2B1Q data encoding. Data is first framed in a 240−bit frame consisting of 216 data bits and 24 bits of overhead. Eight of these 240 bit frames are then combined into a 1,920−bit superframe.
An ISDN PRI circuit is carried on a T1 circuit, using the same layer 1 Extended Super Frame (ESF) framing and Bipolar Eight Zero Substitution (B8ZS) line coding as a standard T1.
Layer 2
All traffic that flows on the D channel of a BRI or a PRI is encapsulated in an LAPD frame. The LAPD frame, shown in Figure 3−9, is very similar in structure to an HDLC frame. The LAPD signaling is formally
specified in the Q.921 specification. These LAPD frames can contain:
Figure 3−9: ISDN LAPD frame
Call setup information
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Link establishment, status, and maintenance information
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X.25 packet data
As shown in Figure 3−9, an LAPD frame contains the following fields:
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Flag — Every LAPD frame starts and stops with an 8−bit flag character, which is a 7E.
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Address — This 2−byte field is used to identify if the frame is carrying call control, management overhead, or X.25 traffic. It also indicates if the frame is a command frame or a response to a command.
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Control — The Control field is used to indicate if the LAPD frame is an information frame, a supervisory frame, or an unnumbered frame.
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Information — This field can carry up to 260 bytes of call control or address messages.
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Layer 2 Link Layer Establishment
When an ISDN device is first connected to the network, a synchronization process is initiated by the user equipment to synchronize to the network. As seen in Figure 3−10, six packets are exchanged between the router and the ISDN switch to establish the link as active and send an acceptable SPID (in the case of a BRI) to the switch. A trace of each of these packets is shown below.
Figure 3−10: ISDN layer 2 establishment ID Request
The router sends an ID Request message to the switch.
Port A DTE ID=72, 03/02/99, 09:37:53.655500 Length=10, Good FCS
LAPD − LINK ACCESS PROCEDURE D CHANNEL MOD 128:
Addr 64767 SAPI Layer 2 Management 63 Cmd/Resp 0 Ext Bit More Addr Octet 0 TEI 127 Ext Bit Final Addr Octet 1 Ctrl UI 03h P 0 TEI Management Entity ID 15 Reference number B34Fh Message Type ID REQUEST 01h Action Indicator 7Fh Ext Bit 1 FCS Good EE0Dh
ID Assigned
The switch responds with an ID Assigned message.
Port A DCE ID=84, 03/02/99, 09:38:12.393575 Length=10, Good FCS
LAPD − LINK ACCESS PROCEDURE D CHANNEL MOD 128:
Addr 65279 SAPI Layer 2 Management 63 Cmd/Resp 1 Ext Bit More Addr Octet 0 TEI 127 Ext Bit Final Addr Octet 1 Ctrl UI 03h P 0 TEI Management Entity ID 15 Reference number A237h Message Type ID ASSIGNED 02h Action Indicator 40h Ext Bit 1 FCS Good 102Fh
SABME
The router responds to the switch with a SABME message.
Port A DTE ID=85, 03/02/99, 09:38:12.400850 Length=5, Good FCS
LAPD − LINK ACCESS PROCEDURE D CHANNEL MOD 128:
Addr 129 SAPI Call Control Procedures 0 Cmd/Resp 0 Ext Bit More Addr Octet 0 TEI Auto Assign User Eqip 64 Ext Bit Final Addr Octet 1 Ctrl SABME 7Fh P 1 FCS Good A8D8h
UA
The switch responds with a UA message.
Port A DCE ID=86, 03/02/99, 09:38:12.429075 Length=5, Good FCS
LAPD − LINK ACCESS PROCEDURE D CHANNEL MOD 128:
Addr 129 SAPI Call Control Procedures 0 Cmd/Resp 0 Ext Bit More Addr Octet 0 TEI Auto Assign User Eqip 64 Ext Bit Final Addr Octet 1 Ctrl UA 73h F 1 FCS Good C412h
SPID
The router can now send its SPID to the ISDN switch. This SPID must match exactly with the SPID that is entered into the database of the switch.
Port A DTE ID=87, 03/02/99, 09:38:12.447850 Length=22, Good FCS
LAPD − LINK ACCESS PROCEDURE D CHANNEL MOD 128:
Addr 129 SAPI Call Control Procedures 0 Cmd/Resp 0 Ext Bit More Addr Octet 0 TEI Auto Assign User Eqip 64 Ext Bit Final Addr Octet 1 Ctrl I 0000h NS 0 NR 0 P 0 National ISDN Protocol Discriminator Call Control 08h Reference Flag Msg from Origination 0 Call Reference Value [len=0] 0 Message Type INFORMATION 7Bh INFO ELEMENT Service Profile ID [Code=58] [Len=11]
INFO ELEMENT Service Profile ID 89953010101 á The SPID is sent to the ISDN switch FCS Good CAF5h
Information
The switch responds with an Information frame to the router after the SPID has been successfully received.
Port A DCE ID=89, 03/02/99, 09:38:12.506075 Length=13, Good FCS
LAPD − LINK ACCESS PROCEDURE D CHANNEL MOD 128:
Addr 641 SAPI Call Control Procedures 0 Cmd/Resp 1 Ext Bit More Addr Octet 0 TEI Auto Assign User Eqip 64 Ext Bit Final Addr Octet 1 Ctrl I 0002h NS 0 NR 1 P 0 National ISDN Protocol Discriminator Call Control 08h Reference Flag Msg from Origination 0 Call Reference Value [len=0] 0 Message Type INFORMATION 7Bh INFO ELEMENT End Point ID [Code=59] [Len=2]
EXT 1 User service identifier 37 EXT 1 Interpreter 0 Interpreter terminal identifier 1 FCS Good 22D0h
Layer 2 Link Layer Status Checks
Every 10 seconds, the ISDN switch will send an RR packet (Receiver Ready) to the router and will expect an immediate reply. The following two packets show what this RR exchange looks like.
RR Sent from the ISDN Switch to the Router
The ISDN switch sends an RR (Receiver Ready) frame to the router.
Port A DCE ID=4, 03/01/99, 10:41:29.238050
Length=6, Good FCS
LAPD − LINK ACCESS PROCEDURE D CHANNEL MOD 128:
Addr 641 SAPI Call Control Procedures 0 Cmd/Resp 1 Ext Bit More Addr Octet 0 TEI Auto Assign User Eqip 64 Ext Bit Final Addr Octet 1 Ctrl RR 0103h NR 1 PF 1 FCS Good DBB8h
RR Reply Sent from the Router to the ISDN Switch
The router immediately responds with an RR (Receiver Ready) frame to the ISDN switch.
Port A DTE ID=5, 03/01/99, 10:41:29.246300 Length=6, Good FCS
LAPD − LINK ACCESS PROCEDURE D CHANNEL MOD 128:
Addr 641 SAPI Call Control Procedures 0 Cmd/Resp 1 Ext Bit More Addr Octet 0 TEI Auto Assign User Eqip 64 Ext Bit Final Addr Octet 1 Ctrl RR 0103h NR 1 PF 1 FCS Good DBB8h