Message Transfer Part (MTP)

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.

• Set the N2 Parameter Value (Config Bytes 4 and 5) to an appropriate value for the signaling link loop delay using the following formula:

For example, the typical satellite loop delay is approximately 500 milliseconds, therefore:

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.

This example PPL Configure message shows the typical PPL Configuration required to implement PCR. The following configuration is performed:

BYTE	Field Description	Value
0	Frame	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): Address Method	0x00 (Single Entity)
9	Number of Address Elements	0x01
10	Address Element 1: Originating Channel Address Type	0x09 (SS7 Signaling Link)
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 (SS7 MTP2 TXC)
16	PPL Entity	0x01 (PPL Config Bytes)
17	Configuration Data Number of Data Locations	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 (with byte 4: 4,001)
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 CSP 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.

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 CSP. 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.

If the Periodic Link Test is enabled, the CSP 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.

With the SS7 Signaling Stack Configure message, configure the Module Variant for ITU (0x01)

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.)

See Message Transfer Part.

American National Standards Institute. ANSI administers and coordinates the U.S. voluntary standardization and conformity assessment system for the
telecommunications industry. More details on these standards may be
viewed at www.ansi.org.

Link that connects to the same node

See Signaling System Number Seven.

Positive acknowledgment to a message

See Programmable Protocol Language.

Message Transfer Part level 2 - data link level

The condition of a component at a given instant. The current "context" of a PPL object, such as Idle, Seizing, Answered. In PPL files, a State is represented by a circle.

A group of data bits in a specific format, with a flag at each end to indicate the beginning and the end of the frame.

Most Significant Bit

Least Significant Bit

See Address Information Block.

The information in an AIB that follows the header, down to the data field. An AIB can have one or more Address Elements. AEs correspond to Span, Channel, and SS7 Link.

A programmable software component that uses PPL state machines that can be modified with the PPL Tool.

A condition that, when met, drives a state machine from one state to another. A condition that drives a Component out of its current state by invoking an associated primitive.

In PPL, a condition that, when met, drives a state machine (PPL Component) from one state to another, by invoking an associated Primitive. In PPL files, an Event is marked by an arrow and text.

International Telecommunications Union (UN)

Message Transfer Part level 3 - network level

Digital communications link - voice, data, and video at a 1.544 Mbps rate (24 64 Kbps channels)