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Basics of Telephony Digital Signaling Systems
Digital Signaling Systems

The rapid deployment of digital PBXs is resulting in greater interconnectivityto Link Series multiplexers at the T1 and E1 levels. Therefore, an increasedawareness of digital signaling concepts is required. The followingsubsections cover:

- Per-Channel Signaling Schemes - Common Channel Signaling Schemes

The matrix below illustrates the capabilities of analog and digital signalingsystems defined by CCITT:

    |                          | Signaling System  |CCITT | CCITT |CCITT | CCITT |
    |         FEATURE          |-------------------|  R1  |  R1   |  R2  |  R2   |
    |                          |# 3|# 4|# 5|# 6|# 7|Analog|Digital|Analog|Digital|
    |Inband Signaling          | X | X | X | - | - |  X   |   -   |  X   |   -   |
    |Outband Signaling         | - | - | - | - | - |  -   |   -   |  X   |   -   |
    |Common Channel Signaling  | - | - | - | X | X |  -   |   -   |  -   |   -   |
    |Digital Transmission      | - | - | - | X | X |  -   |   X   |  -   |   X   |
    |DTMF                      | - | X | X | - | - |  -   |   -   |  -   |   -   |
    |MF                        | - | - | X | - | - |  X   |   X   |  X   |   X   |
    |Operation over Satellites | - | - | X | X | X |  X   |   X   |  -   |   -   |

Per-Channel Signaling Systems

T1 Channel Bank Signaling

In DS-1 (T1) systems, the signaling bit is transported in the eighth bitposition within the DS0 timeslot, but only during certain 'frames' of the'superframe'.

For D4 channel banks, a four-state signaling scheme is implemented through theuse of an 'A-bit' (transmitted in the 6th frame), and a 'B-bit' (transmittedin the 12th frame).

For D5 channel banks (Extended Superframe), a sixteen-state signaling isimplemented through an 'A-bit' (transmitted in the 6th frame), a 'B-bit'(transmitted in the 12th frame), a 'C-bit' (transmitted in the 18th frame),and a 'D-bit' (transmitted in the 24th frame).

D4 signaling is described in ATT Pub 43801, 'Digital Channel BankRequirements and Objectives'. The mapping of 2 and 4 state signaling schemesto ESF framing (16-state) is also described and tabulated below:

The following pages describe these DS-1 signaling schemes:

CCITT G.732 Signaling

CCITT Recommendation G.732 details a Channel Associated Signaling (CAS)scheme. In this recommendation (which is not a formal standard signalingsystem), DS0 #16 of the G.732 frame is used to transport a multiplexedABCD-Bit status for each channel.

Common Channel Signaling (CCS) Systems

Common channel signaling systems are usually HDLC-based, message orientedsigaling systems. Within the United States PSTN the original implementationof Common Channel Signaling started in 1976, and is known as CCIS (CommonChannel Interoffice Signaling). This is similar to CCITT's Signaling System#6 (SS6). The CCIS protocol operated at relatively low bit rates (2.4K, 4.8K,9.6K), but transported messages that were only 28 BITS long. However, CCIScould not adequately support an integrated voice and data environment.Therefore, a new HDLC-based signaling standard and CCITT recommendation wasdeveloped: Signaling System 7.

First defined by the CCITT in 1980, the Swedish PTT started SS7 trials in 1983and some European countries are now entirely SS7-based.

Within the US, Bell Atlantic began implementing SS7 in 1988, among the firstBOCs (if not the first) to do so.

Currently, a large majority of the Long Distance networks and Local ExchangeCarrier networks have migrated to implementations of CCITT's Signaling System7 (SS7). By 1989, AT&T converted had converted all of its digital network toSS7; and US Sprint is SS7 based. However, many LECs are still in the processof upgrading their networks to SS7. This is because the number of switchupgrades required for SS7 support impact the LECs much more heavily than theICs. THE SLOW DEPLOYMENT OF SS7 WITHIN THE LECs IS ALSO, IN PART, RESPONSIBLEFOR DELAYS IN INCORPORATING ISDN (INTEGRATED SERVICES DIGITAL NETWORK) WITHINTHE U.S.

There are three versions of SS7 protocols at the present time:

- CCITT Version (1980, 1984) detailed in CCITT Q.701 - Q.741
- ATT and Telecom Canada (1985)
- ANSI (1986)

The evolution of SS7 into the U.S. PSTN is detailed below:

Signaling System 7 U.S. PSTN Features

SS7 currently provides support for POTS (Plain Old Telephone Service) throughthe use of a Telephony User Part (TUP). This defines the messages that areused to support this service. An additional ISDN User Part (ISUP) has beendefined that supports ISDN transport. Eventually, since the ISDN User Partincludes translations from POTS to ISDN, the ISDN User Part is expected toeventually solely replace the Telephony User Part.

SS7 Signaling Protocol

While the structure of SS7 is based upon the OSI model, there are somesubstantial differences found within layers 4-7. The following block diagramindicates the basic SS7 components:

Level 1:  SS7 normally operates at rates of 56 KBPS and 64 KBPS.
Level 2:  The SS7 protocol utilizes HDLC (w/ zero insertion) techniques. Messages may be up to 272 bytes (octets) long.
Level 3:  Tranfers 3 message types:  Link Status Signaling Units Message Signaling Units Fill-In Signaling Units The Link
	   Status Signaling Units convey status and connection/disconnection information. The Message Signaling Units carry
          specific switch instructions concerning maintenance and call management functions.
	    The Fill-In Signaling Units are transmitted during idle states.

SS7 Signaling Networks

The SS7 network is accessed by End Offices that communicate to Signal TransferPoints (STPs).

These STPs perform a packet switching function, facilitating highly reliableand cost-effective network architectures.

Physical SS7 network interconnectivity can utilize two schemes:

- Associated Signaling (AS)
- Quasi-Associated Signaling (QAS)

Associated Signaling employs signaling channels that are routed alongside eachparticular transmission path between end-points. This topology results in alarge number of signaling links with relatively low signaling link usage. The STP performs no switching function in this implementation.

In Quasi-Associated Signaling, the signaling links may be separated from thetransmission facility and switched by Signal Transfer Points (STPs). Signaling between offices is therefore supported through logical STP-providedconnections.

SS7 (ATT implementation) networks also employ Action Control Points (ACP)that can provide switching and routing functions and Network Control Points(NCP). NCPs are typically data processing CPUs, that contain detailedinformation on how to handle the call (e.g. Virtual Network digit translation,Calling Card verifications, etc.). Communications between an ACP and a NCPmay traverse several STPs.

Note that the ACPs shown in the example above also serve as End Offices. However, in practical implementation the End Office resides within the LEC'sdomain, with ACP functions residing in the IC's (e.g. ATT) network. This example does hold true for LEC SS7 networks, however.

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