Tài liệu Tín hiệu trong các mạng viễn thông P6 pptx

A signaling terminal identifies each outgoing block by a block completed number BC bits 18-20.. The BN in a message is also used by signal transfer-points STP to route the message to its

6 SIGNALING SYSTEM NO 6

channel signaling system There are two versions of this system, both of which were first deployed in the mid-1970s CCITT No.6 [l, 23 is still in use in the

System, has been used in the U.S toll network [3,4], but has now been replaced

by the North American version of signaling system No.7

chapter, we use the acronym SS6 when discussing matters that are common to both versions

systems In SS6, a signaling data link usually consists of a pair of these analog voiceband channels However, SS6 signaling data links can also be implemented

as pairs of digital (PCM) transmission channels

SS6 signaling can be used for FDM (analog) and PCM (digital) trunks, which

1.6.4) However, the signaling data links have to be carried on transmission channels without CME

SS6 was designed originally for call control applications only Around 1980, the Bell System expanded CCIS to include query-response transactions between exchanges and centralized network databases

126

Copyright  1998 John Wiley & Sons, Inc ISBNs: 0-471-57377-9 (Hardback); 0-471-22415-4 (Electronic)

Figure 6.1-1 shows a SS6 signaling link in which the data link consists of a pair of analog voiceband (300-3400 Hz) transmission channels [5] At signaling points A and B, modems M, and M, provide the interface between the signaling data link and the signaling terminals ST, and ST, For transmission from A to B, modem

MA converts the digital bit stream (b) from ST, into an analog signal (a) that is suitable for transport on the voiceband channel, and modem M, converts signal (a) back into a bit stream (b) A similar conversion sequence takes place in the other direction

SS6 originally used V.26.bis modems, defined by CCITT [6] The relation between analog signal (a) and bit stream (b), at respectively the input and output

of a modem, is shown inFig 6.1-2 Signal (a) is an 1800 Hz sine wave that changes its phase at intervals T, of l/1200 s The power spectrum of this signal is essentially contained within a 600-3000 Hz frequency band There are four phase changes (with respect to the previous interval TB), each of which represents a specific combination of two consecutive data bits in the bit stream:

signaling links carry messages for 1500 trunks The 3000 trunk load occurs only in failure conditions, when the link carries its normal signaling traffic, and the traffic

0 a -t

I

Figure 6.1-2 Relation between analog signal and digital bit stream (a): analog signal (shown

as modem input) (b): bit stream (shown as modem output) (From Rec V.26.bis Courtesy of ITU-T.)

of a failed companion link Operation at this load increases the queuing delays (the time spent in output buffer OB) of the messages On signaling links with

4800 bits/s modems, the capacities are 3000 trunks and 6000 trunks, respectively

Working signaling links transmit a continuous stream of adjacent signal units (SU) in each direction Each SU has a 20-bit information (INF) field, followed

by an eight-bit check bit (CB) field for error detection (Fig 6.1-3) Most SUs start with a heading field (H) and a signal information field (SI) Usually, H identifies a group of SU types, and SI defines a SU type within this group Some

SU types are defined completely by their H field, and do not have a SI field The lengths of H fields are different in CCITT No.6 and CCIS

We distinguish “message” signal units (MSU) that carry information between the processors P, and P, of Fig 6.1-1, and “link” signal units that convey information originated by signal terminal ST, and intended for ST,, and vice versa (52.2)

Twelve consecutive signal units form a block The last SU in a block is an

SUs that contain acknowledgments of the signal units in a received block

(SYU) A SYU is a “link” SU that is sent by a signaling terminal when no MSU

Figure 6.1-4 Block structure of signal units

is waiting to be transmitted SYUs are also sent when a signaling link has to be synchronized (6.1.4)

SS6 error control consists of error detection, acknowledgments, and retrans- mission of MSUs received with errors, and takes place in the signaling terminals

at both ends of the links [7]

Error Detection Cyclical redundancy checking (see Section 5.2.3) is used The check bits for each SU are calculated by mod 2 division of the number in INF by the divisor = 100010111 The eight bits representing the result (remainder), are inverted, and placed in the CB field of the SU Signal units received error-free are accepted by the signaling terminal, and positively acknowledged Signal units received with errors are discarded, and negatively acknowledged

ledged by the acknowledgment unit (ACU) in a block sent by STB, and vice versa

Bits -

c

Figure 6.1-5 Acknowledgment signal unit (ACU) (From Rec Q.259 Courtesy of ITU-T.)

The layout of the ACU is shown in Fig 6.1-5 The ACU heading is 011 Bits 4-14 indicate positive- or negative acknowledgments (0 or 1) of SU,-SU,, in a previously received block

A signaling terminal identifies each outgoing block by a block completed number BC (bits 18-20) This number is incremented cyclically, from 0 through

7, for consecutive transmitted blocks When terminal STB has received a block with say BC = 3 from terminal ST,, it acknowledges the SUs in this block in an ACU in which it sets the block acknowledged number (bits 15-17) to BA = 3 Retransmission Each ST retains all sent messages in its retransmission buffer until they have been positively acknowledged Most SS6 messages are one-unit messages (fitting in one message SU) However, there are also multi-unit mes- sages that are transferred in a number of consecutive message SUs When a

sent, it retransmits the entire message of which the negatively acknowledged SU

is a part

In SS6, a MSU transmission error results in out-of-sequence message delivery (5.2.5) Out-of-sequence delivery of call-control messages pertaining to the same call causes call-processing problems

SS6 signaling terminals have to besynchronized (or aligned) with their incoming bit streams, so that they can determine the start points of SUs and blocks Synchronization units are used to acquire synchronization when the link is turned on, and re-synchronization after a disturbance on the link [8] The format

of a SYU in CCITT No.6 is shown in Fig 6.1-6 The combination H = 11101, SI

= 1101 identifies the signal unit as a SYU Bits 6-16 are coded 1100011 Bits 17

20 contain a number (N) that indicates the position of the SYU within a block SYUs in CCIS have a slightly different format

When a signaling link is turned on, both terminals start by sending blocks with

MESSAGES, LABELS, AND ROUTING 131

11 SYUs and one ACU The “receive” part of a ST has a counter that steps up one unit with each bit in the incoming bit stream, and recycles when it has counted 12 x 28 = 336 bits (the number of bits in a block) When a terminal is aligned, the counter value should be 1 on receipt of the first bit of the blocks In the initial part of the synchronization procedure, the signaling terminal (ST)

pattern, it knows that it has received bits 1 through 16 of a SYU Moreover, from the value ofN, it knows the place of the SYU in the block, and thus can initialize the counter In the second step of the procedure, both terminals inform each

When the procedure completes successfully, the signaling link is put in service

On a working signaling link, the STs keep verifying their alignment by looking for the fixed pattern 011 (the ACU heading) during steps 309-311 of the counter

6.2.1 Message Structure

As shown in Fig 6.24, there are two SS6 message sizes A one-unit message occupies a single signal unit, called a lone signal unit (LSU) Multi-unit

signal unit (ISU) followed by one or more subsequent signal units (SSU)

A call-control message pertains to a particular trunk, which is identified by

TG

(BN = 102)

Figure 6.2-2 Band number assignments for trunk group (TG)

the label in the message The label consists of two parts: a band number

trunk within a band The band and trunk numbers in CCITT No.6 occupy seven bits and four bits respectively A CCITT No.6 label can thus identify up to 128 bands with up to 16 trunks in each band, for a theoretical label capacity of

2048 trunks CCIS has nine-bit band numbers, and can identify up to 512 bands

of up to 16 trunks (theoretical label capacity: 8192 trunks)

The BN in a message is also used by signal transfer-points (STP) to route the message to its destination (the exchange at the distant end of the trunk) A BN thus cannot be shared by trunks in different trunk groups As a consequence, a group of for example 20 trunks requires two band numbers, but uses only 20 of

can be identified on a signaling link in actual networks

Since the number of available BNs is limited, they have to be “reused” in large signaling networks, such as the CCIS network in the U.S Therefore, a particular

BN value usually identifies different bands of trunks in different parts of the signaling network, and a particular band of trunks is usually identified by different band numbers on the various links in its signaling route When a call- control message arrives at a signaling point, the affected band of trunks is determined from two data items: the BN in the message, and the identity of the signaling link on which the message came in

Figure 6.2-2 shows an example for a trunk group TG with at most 16 trunks (one band) On the “A” signaling between EX, and the STPs of region 1, the group is identified by BN = 34 On the “B” signaling links between the STPs in regions 1 and 2, the BN = 102 On the “A” signaling links between the STPs of region 2 and EX, the BN = 65 This is because these BNs were available at the time that the trunk group was installed

We now explore the BN translations and outgoing link selections at the signaling

MESSAGES, LABELS, AND ROUTING 133

points for messages from Ex, to Ex,, which relate to a specific trunk T of group

TG, in Fig 6.2-2 [4] The trunk has TN = 5 (this number does not change when the messages traverse a STP) We assume that all signaling links are operational, and that the messages for TG are to be load-shared evenly by the four normal signaling routes for the group:

R,: AI-B,-Af R,: A1 -B2-A4 R,: AZ-B3-A3 R,: AZ-B4-A4

All messages relating to a specific trunk have to use the same route Otherwise, two consecutive messages for that trunk, say M, (sent first, on route

because, at the time that the messages are sent, the queuing delays at the signaling links in R, are large, and small on R, Out-of-sequence delivery of messages relating to a trunk can cause problems in call processing

The selection of outgoing signaling links that accomplishes load sharing of signaling links, and also associates a signaling route with a particular trunk, can

be done in several ways, for example:

At exchanges, the primary outgoing A link for messages with odd or even TN

exchange If the primary link fails, the messages are diverted to the other (alternative) A link In this example, the messages for trunk T normally go out

on A,, and reach STP,,

Every STP has a table for each attached signaling link, with entries for all incoming band numbers Each entry contains the outgoing band number (BN,), and the identity of a primary and alternative outgoing link

For messages received by an STP on its A links, the value of BN, indicates the destination region for the message In addition, when BN, is odd, the primary and alternative outgoing B links go to respectively the odd- and even-numbered STP of that region If BN, is even, the primary and alternative B links go to

STP, 1 for messages received on link Al with BN = 34 thus indicates:

Outgoing band number: BNo = 102,

Primary outgoing SL: B, (to STP,,),

Alternative outgoing SL: B, (to STP,,)

For messages received by an STP on its B links, the BN identifies an exchange

in the region of the STP The primary outgoing link is the A link to the exchange, and the alternative link is the C link to the other STP in the region

At STP,,, the table entry for messages received on B, with BN = 102 thus indicates:

Outgoing band number: BN, = 65,

Primary outgoing SL: A4,

Alternative outgoing SL: C,

The normal route for messages from Ex, to Ex, for trunk T is thus A,-B,-&

At destination exchange Ex,, the label (BN = 65, TN = 5) indicates that the message concerns trunk T

Applying the same rules to messages relating to trunk T, and sent by exchange Ex,, the signaling route is AJ-B3-AZ Messages relating to a trunk, and sent in opposite directions thus may traverse different signaling routes

When a primary signaling link fails, messages normally sent out on that link are diverted to the alternative link Consider again the messages sent by Ex,, relating to trunk T On failure of link A,, Ex, diverts the messages to link A, On failure of link B,, STP,, diverts the messages to link B,, and on failure of link A4, STP,, diverts the messages to link C2 (and STP,, sends the messages on A&

parable to those of CCITT No.5 and R2 international signaling (Chapter 4) All signaling is link-by-link

The most important call-control messages [9] are described in Sections 6.3.1 through 6.3.4 In CCITT No.6 documents, these messages are usually denoted

by three-character acronyms The coding of the heading and signal information fields of the messages are listed in Table 6.3-l

Signaling procedures are outlined in Sections 6.3.5 and 6.3.6

The initial address message (IAM) is the first forward message in a call It indicates the seizure of a trunk, and contains the initial digits (in overlap address signaling), or all digits (in en-bloc address signaling), of the called number, and parameters that affect the routing and processing of the call

The IAM layout is shown in Fig 6.3-l Label L in the initial signal unit (ISU) identifies the trunk for which the message is intended The subsequent signal units (SSU) have a heading code H = 00, and a length indicator (LI) that represents the number of SSUs beyond SSU,

SSU, contains a number of international routing indicators, which we have already encountered in No.5 and international R2 signaling

Country Code indicator (c) This indicates whether the called number is an international number (C = 1; country code included), or a national number(C = 0; no country code included)

AD1 Address incomplete

AFC Address complete, subscriber free, charge

AFN Address complete, subscriber free, no charge

ANC Answer, charge

ANN Answer, no charge

BLA Blocking acknowledgment

IAM Initial address message

LOS Line out of service

RAN Reanswer (after CLB)

RLG Release-guard

SAM Subsequent address message

SEC Switching equipment congestion

Nature of Circuit hdicator (N) This indicates whether the connection built

up so far includes, or does not include, a satellite trunk (N = 1, or N = 0) This indicator is used by incoming exchanges When a call is received with N = 1, the exchange avoids routing the call on another satellite trunk

Echo-suppressor lndicat~r (E) This indicates whether the connection built

up so far includes, or does not include, an outgoing half-echo suppressor (E = 1,

or E = 0) The incoming exchange uses this indicator to determine whether to insert an echo suppressor on its incoming or outgoing trunk

An IAM can include up to 4, 8, 12, or 16 address digits (depending on the number of SSUs) The final SSU may contain one or more unused digit slots, which are coded as 0000 (filler code)

On international calls dialed by outgoing operators (who can indicate “end of dialing” from their consoles), the last digit of the called number is followed by an end of address (ST) digit (coded 1111) On subscriber-dialed international calls, the ST digit is not included because outgoing international exchanges (ISC) have no information about the lengths of foreign national numbers

If overlap address sending is used, the outgoing ISC sends an IAM, which includes enough digits (usually four) to allow the incoming ISC to make a route decision The later digits of the called number are sent in a subsequent address message (SAM)

calling party category (CPC) = 1101 (test equipment) In this case, the called address consists of one digit (D1) that specifies a test termination at the incoming ISC For example:

signaling and transmission test line

consists of an ISU, and one or more SSUs, with the format of SSU, in Fig 6.3-l

CCI-IT NO 6 CALL CONTROL 137

All other forward call-control messages are one-unit messages, often called

“signals.” They are carried in LSUs

Continuity Signal (COT” This is used to report a successful continuity test (see 6.3.5)

Clear-forward Signal (CLF) This requests the release of the trunk

request the assistance of an inbound operator in a foreign country

All backward messages are one-unit messages (signals) and are carried in LSUs

(heading code: 11000) consists of the backward signals for successful calls, and are the equivalents of the backward line signals in channel-associated signaling systems

Answer, Charge (A/UC) This indicates that the call has been answered, and should be charged

charged

Reanswer (RAN) This is sent after a CLB, when the called party has gone off- hook again

a trunk It indicates that the exchange has released the trunk at its end

Signals of the second group (heading code: 11011) indicate that the call can not be set up

congested

is available

Call Failure (CFL) The call has failed (reason not specified)

Confusion &OF) The response to a received message that is not “reason- able,” given the state of the call