MTP Level 2 (MTP2) provides the data link layer functionality within the OSI model. The MTP2 layer in the SS7 protocol stack defines the functions and procedures of the signaling system for the reliable transfer of signaling messages over an SS7 signaling link. It performs functions such as message sequence validation, error checking and flow control.
The MTP2 layer is a service provider to the MTP3 link layer, and at the same time it is a service user of the underlying trunk. Two conceptual illustrations of the MTP2 layer are provided to the right. The first graphic shows how MTP2 fits into the OSI model. The second shows how it is implemented by TelcoBridges and identifies the most important parameters when configuring an MTP2 layer.
The MTP2 layer is used to create a flow control and buffering mechanism over the MTP1 layer. The flow control works both ways (toward upper layer and toward remote side) to avoid losing packets because of a lack of buffer, because of a temporary resource outage on the local/remote side (i.e., CPU too busy to process packets) or because of poor MTP Layer 1 transport quality (i.e., physical link errors). Because the MTP2 layer contains the buffering scheme, it allows upper layers to retrieve buffers that were not sent in the event of a MTP1 link failure. The MTP2 layer is also responsible to establish the connection with the peer MTP2 layer upon physical link activation.
MTP Level 2 provides the following basic features:
- Establish and release data link connections
- Transfer data
- Normal alignment procedure (ITU and ANSI only)
- Emergency alignment procedure
- Signal unit error rate monitor
- Alignment error rate monitor
- Basic error correction method
- Preventive cyclic retransmission error correction method (ITU and ANSI only)
- Congestion control and congestion abatement
- Inform MTP Level 3 on detection of error
- Message retrieval for changeover
- Processor outage procedure (ITU and ANSI only)
- Realignment on link failure
- Multiple variants, including ITU 1988 and 1992, ANSI 1988 and 1992
- Regulates data flow when system's resource utilization reaches configurable thresholds
MTP Level 2 does not directly provide the following features:
- Signal unit delimitation, alignment, and error detection
- Retransmission of FISU and LSSUs (ITU and ANSI only)
MTP Level 2 assumes these features are provided by MTP Layer 1, but also expects errors to be reported by layer 1 so that it can take any appropriate action. The MTP Layer 1 functions are implemented by the TMedia hardware platforms.
TelcoBridges and MTP2
TelcoBridges supports MTP2 as part of its SS7 signaling stack. It also provides the following functionality:
- 56, 64, n x 64 kbps low-speed links (LSL). Up to 64 links can be configured per TMedia hardware unit
- 1.5 and 2 Mbps high-speed links (HSL). Up to 2 HSL links can be configured per TMedia hardware unit
- Supports dynamic link configuration including the creation and removal of links during live system operation
Supported MTP2 variants
- ITU-T, Q.701, Q.703
- ANSI T1.111.3