Signal Unit Formats

SUs transfer information, which originates from higher layers (MTP3, ISUP, SCCP, TUP, and so on) in the form of messages, over the signaling link. MTP2 is similar to data network bit-oriented link protocols such as HDLC, SDLC, and LAPB. The primary difference with these protocols comes from the performance requirements in terms of lost and out-of-sequence messages and delay.

There are three types of SUs, each with its own format: the fill-in signal unit (FISU), the link status signal unit (LSSU), and the Message Signal Unit (MSU). An in-service signaling link carries a continuous SU stream in each direction.

FISUs and LSSUs are used only for MTP2 functions. MSUs also contain the same MTP2 fields, but they have two additional fields filled with information from MTP3 and Level 4 users that contain the real signaling content. This chapter describes the MTP2 fields and the functions they perform. It begins by presenting the three SU formats.

Fill-In Signal Units

FISUs are the most basic SU and carry only MTP2 information. They are sent when there are no LSSUs or MSUs to be sent, when the signaling link would otherwise be idle. Sending FISUs ensures 100 percent link occupancy by SUs at all times. A cyclic redundancy check (CRC) checksum is calculated for each FISU, allowing both signaling points at either end of the link to continuously check signaling link quality. This check allows faulty links to be identified quickly and taken out of service so that traffic can be shifted to alternative links, thereby helping meet the SS7/C7 network's high availability requirement. Because MTP2 is a point-to-point protocol, only the MTP2 level of adjacent signaling points exchanges FISUs.

The seven fields that comprise a FISU, shown in Figure 6-1, are also common to LSSUs and MSUs. MTP2 adds the fields at the originating signaling point and processes and removes them at the destination signaling point (an adjacent node).

Link Status Signal Units

LSSUs carry one or two octets of link status information between signaling points at either end of a link. The link status controls link alignment, indicates the link's status, and indicates a signaling point's status to the remote signaling point. The presence of LSSUs at any time other than during link alignment indicates a fault—such as a remote processor outage or an unacceptably high bit error rate affecting the ability to carry traffic.

The timers associated with a particular status indication govern the transmission interval. After the fault is cleared, the transmission of LSSUs ceases, and normal traffic flow can continue. As with FISUs, only MTP2 of adjacent signaling points exchanges LSSUs. LSSUs are identical to FISUs, except that they contain an additional field called the Status field (SF). Figure 6-2 shows the eight fields that comprise an LSSU.

Currently only a single-octet SF is used, even though the specifications allow for a two-octet SF. From the single octet, only the first 3 bits are defined. These bits provide the status indications shown in Table 6-1.

Table 6-1. Values in the Status Field

C	B	A	Status Indication	Acronym	Meaning
0	0	0	O: Out of Alignment	SIO	Link not aligned; attempting alignment
0	0	1	N: Normal Alignment	SIN	Link is aligned
0	1	0	E: Emergency Alignment	SIE	Link is aligned
0	1	1	OS: Out of Service	SIOS	Link out of service; alignment failure
1	0	0	PO: Processor Outage	SIPO	MTP2 cannot reach MTP3
1	0	1	B: Busy	SIB	MTP2 congestion

Message Signal Units

As shown in Figure 6-3, MSUs contain the common fields of the FISU and two additional fields: the Signaling Information Field (SIF) and the Service Information Octet (SIO). MSUs carry the signaling information (or messages) between both MTP3 and Level 4 users. The messages include all call control, database query, and response messages. In addition, MSUs carry MTP3 network management messages. All messages are placed in the SIF of the MSU.

Figure 6-3. MSU Format

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MTP2 Overhead

Figure 6-4 shows an MSU. The MTP2 overhead is exactly the same for both LSSUs and FISUs, except that an LSSU has an SF.

Field Descriptions

Table 6-2 details the fields that are found inside the signal units. MTP2 exclusively processes all fields except the SIO and the SIF.

Table 6-2. Field Descriptions

Field	Length in Bits	Description
Flag	8	A pattern of 011111110 to indicate the start and end of an SU.
BSN	7	Backward sequence number. Identifies the last correctly received SU.
BIB	1	Backward indicator bit. Toggled to indicate an error with the received SU.
FSN	7	Forward sequence number. Identifies each transmitted SU.
FIB	1	Forward indicator bit. Toggled to indicate the retransmission of an SU that was received in error by the remote SP.
LI	6	Length indicator. Indicates how many octets reside between itself and the CRC field. The LI field also implies the type of signal unit. LI = 0 for FISUs, LI = 1 or 2 for LSSUs, and LI >2 for MSUs.
SF	8 to 16	Status field. Provides status messages in the LSSU only.
CK	16	Check bits. Uses CRC-16 to detect transmission errors.
SIO	8	Service Information Octet. Specifies which MTP3 user has placed a message in the SIF.
SIF	16 to 2176	Signaling Information Field. Contains the "real" signaling content. The SIF is also related to call control, network management, or databases query/response.