Protocol Layers and Messages

Observation Point 1 - LAPDm Over-the-Air Messages

The Link Access Procedure on the Dm channel (LAPDm) is the protocol for use by the data link layer on the radio interface. The frame structure and procedures for proper operation of the LAPDm are described in GSM 04.06.

Observation Point 2- RLC/MAC Over-the-Air Messages

This layer contains two functions: The Radio Link Control function provides a radio-solution-dependent reliable link. The Medium Access Control function controls the access signaling (request and grant) procedures for the radio channel, and the mapping of LLC frames onto the GSM physical channel. RLC/MAC is defined in GSM 04.60 for functions operating on the Packet Data Channel (PDCH), and in GSM 04.08 for functions operating on the CCCH (Common Control Channel).

Observation Point 3 - LLC Over-the-Air Messages

Logical Link Control (LLC): LLC was designed to be independent of the underlying radio interface protocols in order to allow introduction of alternative GPRS radio solutions with minimum changes to the NSS (Network Subsystem). The procedures for the LLC layer are described in GSM 04.64.

Observation Point 4 -GMM/SM Over-the-Air Messages

GPRS Mobility Management and Session Management (GMM/SM): This protocol supports mobility management functionality such as GPRS attach, GPRS detach (also applicable to the test application), security, routing area update, location update, PDP context activation, and PDP context deactivation. GMM/SM is defined in GSM 04.08

Observation Point 5- SMS Over-the-Air Messages

This observation point captures the messages exchanged between the LLC layer and the SMS sub-layers. See SMS Logging Control for more information.

Observation Point 6- GSM L3 Over-the-Air Messages

GSM Layer 3 (GSM L3) is defined in GSM 04.08. GSM L3 includes messages that are transmitted over the Common Control Channels. These are:

• BCCH information (System information messages)
• PAGCH messages (Immediate assignments)
• RACH accesses

CCCH messages are useful mainly for the purpose of verifying correct channel allocation through the logging of RACH accesses and the related assignments. Note that the System Information Messages will only be visible whenever changes are made to the base station parameters. They are no continuously logged, although they are continuously broadcast on the BCCH.

All references in this section are relative to GSM 04.08 or GSM 04.60.

Observation Point 7 - SNDCP Over-the-Air Messages

Subnetwork Dependent Convergence Protocol (SNDCP): This transmission functionality maps network-level characteristics onto the characteristics of the underlying network. SNDCP is specified in GSM 04.65.

Observation Point 8 - IP Messages

Unlike most of the other protocols, IP only has a single type of message: the datagram. For each datagram, the IP header fields will be displayed by name and the user data will be displayed as a hex dump.

For each datagram, the following fields will be displayed:

• Version
• Internet Header Length
• Type of Service
• Total Length
• Identification
• Flags
• Fragment Offset
• Time to Live
• Protocol
• Header Checksum
• Source Address - Only the IP address is displayed. No attempt is made to look up the host name
• Destination Address - Only the IP address is displayed. No attempt is made to look up the host name
• Data - This contains the user data and any IP options displayed in hex.

For a more detailed explanation of these fields, refer to the Internet Engineering Task Force (IETF) document RFC 791.

Observation Point 9 - RLP Over-the-Air Messages

The Radio Link Protocol (RLP) stack layer provides reliable transportation of data between the mobile station and the PPP layer by using a Frame Check Sequence (FCS) for error detection and providing for retransmission of corrupted or missing data. RLP is specified in GSM 04.22.

Observing the messages set and received by this layer is only useful if you are using GSM data channels in non-transparent mode. See GSM Circuit Switched Data (CSD) for more information.

Observation Point 10 - PPP Over-the-Air Messages

The Point-to-Point Protocol (PPP) is associated with data channels. It allows a mobile station to transfer IP packets to and from the IP layer. PPP is specified in the Internet Engineering Task Force (IETF) document RFC 1548.

For more information about data channel support in the test set, see Data Channel .

Observation Point 11 - PPP HDLC Over-the-Air Messages

The PPP High Level Data Link Control observation point provides access to the full PPP frame that is sent or received by the test set. Some header and footer octets are removed from the frame displayed in the PPP observation point. The additional octets viewable in the PPP HDLC layer are:

PPP Observation Point and PPP HDLC Observation Point Comparison

Observation Point 12 - RRLP Over-the-Air Messages

See RRLP Configuration for more information about the RRLP layer support in the test set.

Related Topics

Protocol Logging