You are here: CSP Developer’s Guide: Common Channel Signaling > 2 Introduction to SS7 > Message Transfer Part (MTP)
Overview
This section includes information on the following MTP features:
• Preventive Cyclic Retransmission (PCR)
• Craft Alerting
• ANSI Activation Link Test
• Periodic Link Test
• National Variant Support for China
• National Variant Support for Japan TTC and Japan NTT/DDI See also MTP3 for Japanese Telecommunications and Configuring JT ISUP Variant Stack
• Link Set Configuration
Preventive Cyclic Retransmission (PCR)
Preventive Cyclic Retransmission (PCR) is an alternative method for SS7 signaling link error correction. PCR should be used in situations with large propagation delays, such as satellite circuits.
MSUs are stored by the transmitting terminal until a positive acknowledgment (ACK) is received. When no new MSUs are to be sent, unacknowledged MSUs are retransmitted cyclically until positively acknowledged.
PCR is enabled and configured with the PPL Config Bytes of the MTP2 Transmission Control (TXC) component (0x26.)
To implement PCR, you must modify the Config Bytes of the MTP2 TXC component (0x26) with the PPL Configure message as follows:
• Change the value of Config Byte 2 to 0x01 (PCR) using the PPL Configure message.
MTP PPL Config Byte values are listed in the SS7 PPL Information.
• Set the N2 Parameter Value (Config Bytes 4 and 5) to an appropriate value for the signaling link loop delay using the following formula:
N2 = Tloop (in microseconds)/125 microseconds +1
For example, the typical satellite loop delay is approximately 500 milliseconds, therefore:
N2 = (500000/125) +1 = 4001 bytes
The default value for Config Bytes 4 and 5 is 0xFFFF, which is for Basic mode error checking.
Important! To initiate configuration changes, take the link out of service and then bring it back in service with the Service State Configure message. Configuration changes will also take affect if the link fails and realigns.
Example
This example PPL Configure message shows the typical PPL Configuration required to implement PCR. The following configuration is performed:
• Config Byte 2 (Mode) is set to 0x01 (PCR)
• Config Bytes 4 and 5 (N2 Parameter Value) are set to 0x0FA1 (4,001 bytes)
Trace
H->X FE 00 15 00 D7 00 00 FF 00 01 09 02 03 00 26 01 03 02 01 04 0F 05 A1 CS
|
BYTE |
Field Description |
Value |
|---|---|---|
|
0 |
0xFE |
|
|
1 |
Length, MSB |
0x00 |
|
2 |
Length, LSB |
0x15 |
|
3 |
Message Type, MSB |
0x00 |
|
4 |
Message Type, LSB |
0xD7 |
|
5 |
Reserved |
0x00 |
|
6 |
Sequence Number |
0x00 |
|
7 |
Logical Node ID |
0xFF |
|
8 |
AIB (starting with Byte 0): |
0x00 (Single Entity) |
|
9 |
Number of Address Elements |
0x01 |
|
10 |
Address Element 1: Originating Channel Address Type |
0x09 |
|
11 |
Data Length |
0x02 |
|
12 |
Data[0] Stack ID |
0x00 |
|
13 |
Data[1] Link ID |
0x01 |
|
14 |
PPL Component ID, MSB |
0x00 |
|
15 |
PPL Component ID, LSB |
0x26 |
|
16 |
PPL Entity |
0x01 |
|
17 |
Configuration Data |
0x03 |
|
18 |
Location 1: Byte Number |
0x02 |
|
19 |
Location 1: Data |
0x01 (PCR) |
|
20 |
Location 2: Byte Number |
0x04 |
|
21 |
Location 2: Data |
0x0F |
|
22 |
Location 3: Byte Number |
0x05 |
|
23 |
Location 3: Data |
0xA1 |
|
24 |
Checksum |
0xCS |
If Craft Alerting is enabled, the host is sent a PPL Event Indication message with an event of Link Activation Failure (0x02) if the Excel platform experiences persistent problems while attempting to align SS7 signaling links.
By default, Craft Alerting is enabled for ANSI and disabled for ITU. Craft Alerting is enabled (0) or disabled (1) with PPL Config Byte 1 of the MTP3 LSAC component (0x002E.) The persistent timer (TA) is specified by PPL Timer 1 of the LSAC component. The default is 60 s.
Craft Alerting is not supported by the Japanese variants.
The testing of signaling links is provided through the LSAC (Signaling Link Activity Control) PPL component (0x002E). See ANSI T1.111 for more information about MTP3.
If the Activation Link Test is enabled, an SS7 link is not brought in service until the point codes on either end of the link are verified by the Excel platform. If the point codes are not verified, the host is sent a PPL Event Indication of Signaling Link Test Failure (0x03).
The Activation Link Test is enabled by default for all variants. It can be disabled with PPL Config Byte 2 of the MTP3 LSAC component.
The Activation Link Test cannot be disabled for the Japanese variants.
If the Periodic Link Test is enabled, the Excel platform periodically verifies the point codes and the Signaling Link Test on the link. If the point codes and the Signaling Link Code (SLC) do not verify, the link fails and the host is sent a PPL Event Indication of Signaling Link Test Failure (0x03).
The Periodic Link Test is enabled by default for all variants. It can be disabled with PPL Config Byte 3 of the MTP3 LSAC component. The default period of the test is 60 seconds. This is configured with PPL Timer 3 of the MTP3 LSAC component.
National Variant Support for China
To enable MTP support for the China variant, perform the following:
• Configure the signaling stack for the ITU variant
With the SS7 Signaling Stack Configure message, configure the Module Variant for ITU (0x01)
• Modify PPL Config Bytes with the PPL Configure message, as follows:
HMDT component (0x002B)
• Set Config Byte 1 (Variant) to 0x03 (China)
HMRT component (0x002C)
• Set Config Byte 1 (Variant) to 0x03 (China)
Important! MTP PPL configuration byte values are listed in the chapter, SS7 PPL Information. National variants may require additional User Part and protocol modifications.
"A" link sets must be configured with even numbered Link Set IDs (for example; 0, 2, 4.) "B" links must be configured with odd Link Set IDs (for example; 1, 3, 5.) An A/B link set pair configured to an adjacent signaling point must be configured with consecutive Link Set IDs (for example; 0/1, 2/3, 4/5.)