Link Access Protocol D-Channel (LAPD)

Through a low-latency, high-speed interface, Dialogic has exposed the Link Access Protocol D-Channel (LAPD) directly to the host application, bypassing Layer 3 and Layer 4. For direct communication with the ISDN Series 3 card, the host connects to the CCS I/O card with 10-BaseT Ethernet. A new CCS I/O Series 3 card or "packet engine" has been introduced to terminate Layer 2 (Q.921) only—the card does not have Layer 3 or Call Control capability. Packets are not sent to the host using the CSP Matrix Series 3 Card. Instead, they are sent from the CCS I/O Series 3 (using the local area network) to the LAPD host. Configuration and alarms are sent only to the matrix host.

Important! LAPD requires a product license. For details, refer to Downloading License Keys to the CSP in the Licensing Overview chapter in the Developer’s Guide: Overview and the Product License Download message (0x0079) in the API Reference.

To enable direct communication with the host through the CCS I/O card, changes were made to the Layer 2 interface, Layer 2 management, configuration, and internal communications. All pre-existing Layer 2 functionality is still supported. ISDN Layer 2 resides on the ISDN Series 3 card.

Support for Transmission Control Protocol (TCP) over IP has been added to CCS cards, with a two-byte sequence number. TCP messaging is used to optimize response time to and from the local host. A message must first be received from the host so that the port number can be stored for use in subsequent indications. Dialogic has also simplified the Subrate point-to-multi-point connect and disconnect messages.

Dialogic’s LAPD design allows for a maximum of 65,536 sequence numbers on the CCS I/O Series 3 card. The sequence numbers indirectly have an impact on the retry mechanism for switch-based (service card) messages. The retry mechanism allows the service card to resend any unacknowledged messages to the host on a regular interval for a defined number of retry times. With the use of the direct TCP/IP link between the service card and the host, a high volume of messages can be processed in a given time period, which in turn will often result in a large number of outstanding unacknowledged messages.

The host designates which E1 or T1 spans are assigned D channels (up to 128), and which D channels have what Service Access Point Identification (SAPI) and Terminal Endpoint Identification (TEI) values. The host can configure both the network side and the user-side Global System Mobile (GSM 8.56) Terminal Endpoint Identifiers (TEIs). Each D channel supports eight TEIs, and the Matrix system management logic has been modified so that 128 D channels can be configured on a single CSP node.

The LAPD entities serve as the network side, and communicates with the user side’s wireless base stations and transceivers. The signaling information is transmitted as LAPD packets over DS0 channels of E1 links. The LAPD can also serve as the user side.

LAPD Redundancy

For information on LAPD redundancy See ISDN Redundancy.

LAPD Limitations

• Management of card-level congestion for overload conditions is not supported

• Removing the CCS I/O card causes the ISDN Series 3 standby card to reset. Redundancy is dropped and the active card becomes single.

• Removing the CCS I/O of the ISDN Series 3 Active card causes a switchover to the ISDN Series 3 secondary card.

• You must remove both cards and re-insert if you are performing maintenance on either card.