Biblio5

The changeover and changeback procedures involve communication with other signaling point(s). For example, in the case of changeover from a failing signaling link, the two ends of the failing link exchange information (via an alternative path) that normally enables retrieval of messages that otherwise would have been lost on the failing link.

A signaling network has to have a signaling traffic capacity that is higher than the normal traffic offered. However, in overload conditions (e.g., due to network failures or extremely high traffic peaks) the signaling traffic management function takes flow control actions to minimize the problem. An example is the provision of an indication to the local user functions concerned that the MTP is unable to transport messages to a particular destination in the case of total breakdown of all signaling routes to that destination point. If such a situation occurs at an STP, a corresponding indication is given to the signaling route management function for further dissemination to other signaling points in the network.

13.4.1.2 Signaling Link Management. Signaling link management controls the locally connected signaling link sets. In the event of changes in the availability of a local link set, it initiates and controls actions with the objective of restoring the normal availability of that link set.

The signaling link management interacts with the signaling link function at level 2 by receipt of indications of the status of signaling links. It also initiates actions, also at level 2, such as initial alignment of an out-of-service link.

The signaling system can be applied in the method of provision of signaling links. Consider that a signaling link probably will consist of a terminal device and data link. It is also possible to employ an arrangement in which any switched connection to the far end may be used in combination with any local signaling terminal device. Here the signaling link management initiates and controls reconfigurations of terminal devices and signaling data links to the extent such reconfigurations are automatic. This implies some sort of switching function at layer 1.

13.4.1.3 Signaling Route Management. Signaling route management only relates to the quasiassociated mode of signaling (see Section 7.7). It transfers information about changes in availability of signaling routes in the signaling network to enable remote signaling points to take appropriate signaling traffic actions. For example, a signaling transfer point may send message indicating inaccessibility of a particular signaling point via that signal transfer point, thus enabling other signaling points to stop routing messages to an inoperative route.

13.5 SIGNALING DATA-LINK LAYER (LAYER 1)

A signaling data link is a bidirectional transmission path for signaling, comprising two data channels operating together in opposite directions at the same data rate. It constitutes the lowest layer (layer 1) in the SS No. 7 functionality hierarchy.

A digital signaling data link is made up of digital transmission channels and digital switches or their terminating equipment, providing an interface to SS No. 7 signaling terminals. The digital transmission channels may be derived from a digital multiplex signal at 1.544, 2.048, or 8.448 Mbps having a frame structure as defined in CCITT Rec. G.704 (see Chapter 6) or from digital multiplex bit streams having a frame structure specified for data circuits in CCITT Recs. X.50, X.51, X.50 bits, and X.51 bit.

The operational signaling data link is exclusively dedicated to the use of SS No. 7

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signaling between two signaling points. No other information may be carried by the same channels together with the signaling information.

Equipment such as echo suppressors, digital pads, or A/ m-law converters attached to the transmission link must be disabled in order to ensure full-duplex operation and bit count integrity of the transmitted data stream. In this situation, 64-kbps digital signaling channels are used which are switchable as semipermanent channels in the exchange.

The standard bit rate on a digital bearer is 64 kbps. The minimum signaling bit rate for telephone call control applications is 4.8 kbps. For other applications such as network management, bit rates lower than 4.8 kbps may also be used.

The following is applicable for a digital signaling data link derived from a 2.048Mbps digital path (i.e., E1). At the input/ output interface, the digital multiplex equipment or digital switch block will comply with CCITT Recs. G.703 for electrical characteristics and G.704 for the functional characteristics—in particular, the frame structure. The signaling bit rate is 64 kbps. The standard time slot for signaling is time slot 16. When time slot 16 is not available, any time slot available for 64-kbps user transmission rate may be used. No bit inversion is performed.

For a signaling data link derived from an 8.448-Mbps (E2) digital link, the following applies: At the multiplex input/ output interface, there should be compliance with CCITT Recs. G.703 for electrical characteristics and G.704 for functional characteristics—in particular, the frame structure. The signaling bit rate is 64 kbps. The standard time slots for use of a signaling data link are time slots 67–70 in descending order of priority. When these time slots are not available, any channel time slot available for 64-kbps user transmission rate may be used. No bit inversion is performed (Ref. 5).

For North American applications of SS No. 7, BOC Notes on the LEC Net- works—1994 (Ref. 6) states that data rates from 4.8 kbps to 64 kbps may be used.

13.6 SIGNALING LINK LAYER (LAYER 2)

This section deals with the transfer of signaling messages over one signaling link directly connecting two signaling points. Signaling messages delivered by upper hierarchical layers are transferred over the signaling link in variable-length signal units. The signal units include transfer control information for proper operation of the signaling link in addition to the signaling information. The signaling link (layer 2) functions include:

•Signaling unit delimitation;

•Signal unit alignment;

•Error detection;

•Error correction;

•Initial alignment;

•Signal link error monitoring; and

•Flow control.

All of these functions are coordinated by the link state control, as shown in Figure 13.4.

13.6.1 Signal Unit Delimitation and Alignment

The beginning and end of a signal unit are indicated by a unique 8-bit pattern, called the flag. Measures are taken to ensure that the pattern cannot be imitated elsewhere in

Figure 13.4 Interactions of functional specification blocks for signaling link control. Note: The MSUs, LSSUs, and SUs do not include error-control information. (From ITU-T Rec. Q.703, Figure 1/ Q.703, p. 2 [Ref. 7].)

the unit. Loss of alignment occurs when a bit pattern disallowed by the delimitation procedure (i.e., more than six consecutive 1s) is received, or when a certain maximum length of signal unit is exceeded. Loss of alignment will cause a change in the mode of operation of the signal unit error rate monitor.

13.6.2 Error Detection

The error detection function is performed by means of the 16 check bits provided at the end of each signal unit. The check bits are generated by the transmitting signaling link terminal by operating on the preceding bits of the signal unit following a specified algorithm. At the receiving signaling link terminal, the received check bits are operated by using specified rules which correspond to that algorithm. If consistency is not found between the received check bits and the preceding bits of the signal unit according to the algorithm, then the presence of errors is indicated and the signal unit is discarded.

13.6.3 Error Correction

Two forms of error correction are provided: the basic method and the preventive cyclic retransmission method. The basic method applies to (a) signaling links using nonintercontinental terrestrial transmission means and (b) intercontinental signaling links where one-way propagation is less than 15 ms.

The preventive cycle retransmission method applies to (a) intercontinental signaling links where the one-way delay is equal to or greater than 15 ms and (b) signaling links established via satellite.

In cases where one signaling link with an intercontinental link set is established via satellite, the preventive cycle retransmission method is used for all signaling links of that set.

The basic method is a noncompelled, positive/ negative acknowledgment, retransmission error correction system. A signal unit that has been transmitted is retained at the

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transmitting signaling link terminal until a positive acknowledgment for that signal unit is received. If a negative acknowledgment is received, then the transmission of new signal units is interrupted and those signal units which have been transmitted but not yet positively acknowledged (starting with that indicated by the negative acknowledgment) will be transmitted once, in the order in which they were first transmitted.

The preventive cyclic retransmission method is a noncompelled, positive acknowledgment, cyclic retransmission forward error correction system. A signal unit which has been transmitted is retained at the transmitting signaling unit terminal until a positive acknowledgment for that signaling unit is received. During the period when there are no new signal units to be transmitted, all signal units which have not been positively acknowledged are retransmitted cyclically.

The forced retransmission procedure is defined to ensure that forward error correction occurs in adverse conditions (e.g., degraded BER and/ or high-traffic loading). When a predetermined number of retained, unacknowledged signal units exist, the transmission of new signal units is retransmitted cyclically until the number of acknowledged signal units is reduced.

13.6.4 Flow Control

Flow control is initiated when congestion is detected at the receiving end of the signaling link. The congested receiving end of the link notifies the remote transmitting end of the condition by means of an appropriate link status signal and it withholds acknowledgments of all incoming message signal units. When congestion abates, acknowledgments of all incoming signal units are resumed. When congestion exists, the remote transmitting end is periodically notified of this condition. The remote transmitting end will indicate that the link has failed if the congestion continues too long.

13.6.5 Basic Signal Unit Format

Signaling and other information originating from a user part is transferred over the signaling link by means of signal units. There are three types of signal units used in SS No. 7:

1. Message signal unit (MSU);

2. Link status signal unit (LSSU); and

3. Fill-in signal unit (FISU).

These units are differentiated by means of the length indicator. MSUs are retransmitted in case of error; LSSUs and FISUs are not. The MSU carries signaling information; the LSSU provides link status information; and the FISU is used during the link idle state—it fills in.

The signaling information field is variable in length and carries the signaling information generated by the user part. All other fields are fixed in length. Figure 13.5 illustrates the basic formats of the three types of signal units. As shown in the figure, the message transfer control information encompasses eight fixed-length fields in the signal unit that contains information required for error control and message alignment. These eight fields are described in the following. In Figure 13.5 we start from right to left, which is the direction of transmission.

The opening flag indicates the start of a signal unit. The opening flag of one sig-

Figure 13.5 Signal unit formats. (From ITU-T Rec. Q.703, Figure 3/ Q.703, p. 5 [Ref. 7].)

nal unit is normally the closing flag of the previous signal unit. The flag bit pattern is 01111110. The forward sequence number (FSN) is the sequence number of the signal unit in which it is carried. The backward sequence number (BSN) is the sequence number of a signal unit being acknowledged. The value of the FSN is obtained by incrementing (modulo 128) the last assigned value by 1. The FSN value uniquely identifies a message signal unit until its delivery is accepted without errors and in correct sequence by the receiving terminal. The FSN of a signal unit other than an MSU assumes the value of the FSN of the last transmitted MSU. The maximum capacity of sequence

numbers is 127 message units before reset (modulo 128) (i.e., a 7-bit binary sequence, 27 c 128 − 1).

Positive acknowledgment is accomplished when a receiving terminal acknowledges the acceptance of one or more MSUs by assigning an FSN value of the latest accepted MSU to the BSN of the next signal unit sent in the opposite direction. The BSNs of subsequent signal units retain this value until a further MSU is acknowledged, which will cause a change in the BSN sent. The acknowledgment to an accepted MSU also represents an acknowledgment to all, if any, previously accepted, though not yet acknowledged, MSUs.

Negative acknowledgment is accomplished by inverting the backward indicator bit (BIB) value of the signal unit transmitted. The BIB value is maintained in subsequently sent signal units until a new negative acknowledgment is to be sent. The BSN assumes the value of the FSN of the last accepted signal unit.

As we can now discern, the forward indicator bit (FIB) and the backward indicator bit together with the FSN and BSN are used in the basic error-control method to perform signal unit sequence control and acknowledgment functions.

as a consequence, carry out a rerouting of messages through the network. The signaling network functions are broken down into two basic categories:

1. Signaling message handling; and

2. Signaling network management (see Section 13.4.1 for description).

13.7.2 Signaling Message-Handling Functions

The signaling message-handling function ensures that a signaling message originated by a particular user part at an originating signaling point is delivered to the same user part at the destination point as indicated by the sending user part. Depending on the particular circumstances, the delivery may be made through a signaling link directly interconnecting the originating and destination points or via one or more intermediate signaling transfer points (STPs).

The signaling message-handling functions are based on the label contained in the messages which explicitly identifies the destination and origination points. The label part used for signaling message handling by the MTP is called the routing label. As shown in Figure 13.3 (upper-left portion), the signaling message-handling is divided up into the following:

•The message routing function, used at each signaling point to determine the outgoing signaling link on which a message is to be sent toward its destination point;

•The message discrimination function, used at a signaling point to determine whether or not a received message is destined to that point itself. When the signaling point has the transfer capability, and a message is not destined for it, that message is transferred to the message routing function; and

•The message distribution function, used at each signaling point to deliver the received messages (destined to the point itself) to the appropriate user part.

13.7.2.1 Routing Label. The label contained in a signaling message and used by the relevant user part to identify a particular task to which the message refers (e.g., a telephone circuit) is also used by the message transfer part to route the message towards its destination point. The part of the message that is used for routing is called the routing label, and it contains the information necessary to deliver the message to its destination point. Normally the routing label is common to all services and applications in a given signaling network, national or international. (However, if this is not the case, the particular routing label of a message is determined by means of the service indicator.) The standard routing label should be used in the international signaling network and is applicable in national applications. The standard routing label is 32 bits long and is placed at the beginning of the signaling information field (SIO). Its structure is illustrated in Figure 13.6.

The destination point code (DPC) indicates the destination of the message. The originating point code (OPC) indicates the originating point of the message. The coding of these codes is pure binary. Within each fold, the least significant bit occupies the first position and is transmitted first.

A unique numbering scheme for the coding of the fields is used for the signaling points of the international network irrespective of the user parts connected to each signaling point. The signaling link selection (SLS) field is used, where appropriate, in performing load sharing. This field exists in all types of messages and always in the same

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Figure 13.6 Routing label structure. (Based on Figure 3/ Q.704, p. 5, CCITT Rec. Q.704 [Ref. 8].)

position. The only exception to this rule is some message transfer part layer 3 messages (e.g., changeover order) for which the message routing function in the signaling point of origin of the message is not dependent on the field. In this particular case the field does not exist as such, but is replaced by other information (e.g., in the case of the changeover order, the identity of the faulty link).

In the case of circuit-related messages of the TUP, the field contains the least significant bits of the circuit identification code [or the bearer identification code in the case of the data user part (DUP)], and these bits are not repeated elsewhere. In the case of all other user parts, the SLS is an independent field. In these cases it follows that the signaling link selection of messages generated by any user part will be used in the load-sharing mechanism. As a consequence, in the case of the user parts which are not specified (e.g., transfer of charging information) but for which there is a requirement to maintain order of transmission of messages, the field is coded with the same value for all messages belonging to the same transaction, sent in a given direction.

In the case of message transfer part layer 3 messages, the signaling link selection field exactly corresponds to the signaling link code (SLC) which indicates the signaling link between destination point and originating point to which the message refers.

13.8 SIGNALING NETWORK STRUCTURE

13.8.1 Introduction

In this section several aspects in the design of signaling networks are treated. These networks may be national or international. The national and international networks are considered to be structurally independent and, although a particular signaling point (SP) may belong to both networks, SPs are allocated signaling point codes according to the rules of each network.

Signaling links are basic components in a signaling network connecting signaling points. The signaling links encompass layer 2 functions that provide for message error control. In addition, provision for maintaining the correct message sequence is provided.

Signaling links connect signaling points at which signaling network functions such as message routing are provided at layer 3 and at which the user functions may be provided at layer 4 if it is also an originating or destination point. An SP that only transfers messages from one signaling link to another at level 3 serves as a signaling transfer point (STP). The signaling links, STPs, and signaling (originating or destination) points may be combined in many different ways to form a signaling network.

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If discrimination between international and national signaling point codes is necessary at a signaling point, the network indicator is used.

13.9 SIGNALING PERFORMANCE: MESSAGE TRANSFER PART

13.9.1 Basic Performance Parameters

ITU-T Rec. Q.706 (Ref. 10) breaks down SS No. 7 performance into three parameter groups:

1. Message delay;

2. Signaling traffic load; and

3. Error rate.

Consider the following parameters and values:

Availability. The unavailability of a signaling route set should not exceed 10 min per year.

Undetected Errors. Not more than 1 in 1010 of all signal unit errors will go undetected in the message transfer part.

Lost Messages. Not more than 1 in 107 messages will be lost due to failure of the message transfer part.

Messages out of Sequence. Not more than 1 in 1010 messages will be delivered out of sequence to the user part due to failure in the message transfer part. This includes message duplication.

13.9.2 Traffic Characteristics

Labeling Potential. There are 16,384 identifiable signaling points.

Loading Potential. Loading potential is restricted by the following four factors:

1. Queuing delay;

2. Security requirements (redundancy with changeover);

3. Capacity of sequence numbering (127 unacknowledged signal units); and 4. Signaling channels using bit rates under 64 kbps.

13.9.3 Transmission Parameters

The message transfer part operates satisfactorily with the following error performance:

•Long-term error rate on the signaling data links of less than 1 × 10−6; and

•Medium-term error rate of less than 1 × 10−4.

13.9.4 Signaling Link Delays over Terrestrial and Satellite Links

Data channel propagation time depends on data rate (i.e., this reduces transmission time, thus transmitting a data message at 64 kbps requires half the time compared to 32 kbps),