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Table 12.1 ABCD signaling
scheme.
Frame number
Transmitted bit
6 A
12 B
18 C
24 D
30 A
36 B
42 C
48 D
exchange C see Figure 12.3. The release guard signal is used to indicate to the receiving exchange that it can use the same trunk for another call.
In addition to the signals mentioned above e.g. seizure, wink, answer, clear-forward, clear-back, and release guard, and the dialed digits which are transported using the DTMF
scheme, a number of supervisory bits are also used to indicate supervisory line states e.g. on-hook, off-hook, idle, and ringing. Supervisory bits are known as the ABCD signaling
bits . The ABCD bits are transferred by robbing the 8th bit of the time slot associated with
the voice call every six frames. The robbed bit is used to transmit the A, B, C, and D bits, respectively see Table 12.1. Let us consider a time slot associated with a particular voice
call. Then, the A bit will be transmitted by robbing the 8th bit of the slot in the 6th frame, the B bit will be transmitted by robbing the 8th bit of the time slot in the 12th frame, the
C bit will be transmitted by robbing the 8th bit of the time slot in the 18th frame, the D bit will be transmitted by robbing the 8th bit of the time slot in the 24th frame, and so
on. The four bits provide for a 16-state signaling scheme for each voice channel.
12.1.3 Signaling System No. 7 SS7
As mentioned in the previous section, CAS is an in-band signaling protocol. In the common-channel signaling CCS
, all signaling information for establishing and releasing a phone call is carried in messages over a separate packet-switching network. This packet-
switching network is known as the signaling network. It consists of signaling points SP and signaling transfer points STP, which are interconnected by signaling links SL. An
SP originates, receives, and processes signaling messages. It can be part of either a tele- phone exchange or a database, which is accessed via CCS messages. An STP is an SP
that simply switches messages from an incoming SL to an outgoing SL. That is, it does not originate or process signaling messages.
The first generation common-channel signaling protocol was signaling system no. 6 SS6
, which was introduced in the 1970s. SS6 was followed by signaling system no. 7 SS7
about ten years later. SS7 consists of several layers see Figure 12.4. The message transfer part MTP
provides a reliable transfer service to the protocols running above it. It is divided into three parts: MTP1, MTP2, and MTP3. These three parts occupy the
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TC-users Signaling point function
MTP1 level 1 MTP2 level 2
MTP3 level 3 SCCP level 4
TCAP ISUP level 4
TUP level 4
Figure 12.4 The SS7 stack.
first three levels of the SS7 hierarchy. The telephone user part TUP is a protocol for telephony call control and for trunk maintenance. The integrated service user part ISUP
is a protocol for call control and trunk maintenance for both the telephone network and N-ISDN see following Section 12.1.4. It supports signaling for calls in which either
one or both parties are N-ISDN users. The signaling connection control part SCCP is a protocol that provides functions for the transfer of messages that are not trunk related.
The combination of MTP and SCCP corresponds to the OSI Layers 1, 2, and 3. The trans- action capabilities application part TCAP
provides for operations that are not related to individual trunks and involve two signaling points. TCAP provides an interface to
TC-users.
12.1.4 Narrowband ISDN N-ISDN
The integrated service data network ISDN was a concept of a future network put forward in the 1980s. This future network will be capable of providing a wide range of services
for voice and non-voice applications, and all of these services will be provided by one network, rather than different networks. ISDN was seen as evolving progressively from
the digital telephone network to include additional functions and network features, such as circuit-switching and packet-switching for data. The fundamental building block of
ISDN was the 64-Kbps connection.
298 VOICE OVER ATM AND MPLS
TE TE
TE .
. .
Local exchange
ISDN user
Digital subscriber line DSL
Figure 12.5 An N-ISDN user.
The first generation of ISDN is referred to as the narrowband ISDN N-ISDN. In this network, ISDN users can communicate with each other in circuit-switching mode and
packet-switching mode. As shown in Figure 12.5, an ISDN user can operate multiple 64- Kbps digital terminal equipment TE of differing types, such as digital telephone PCM,
high-speed facsimile terminal, and high-speed computer modem. TEs are connected to the local exchange via a digital subscriber line DSL. The DSL is a two-wire or four-wire
line that allows simultaneous transmissions in both directions. The transmission rate in one direction is 144 Kbps.
The 144-Kbps bit stream is divided into two 64-Kbps B-channels and one 16-Kbps D-channel. The B-channels are used for circuit-mode communications. They can also
be used to carry digital information, such as digitized voice, computer data, and video. Both B-channels can be used at the same time, permitting the user to operate two phones
or a phone and a computer modem at the same time. The two B-channels can also be linked together to provide an aggregate 128-Kbps data channel. The D-channel is used
for signaling between the user and the local exchange. The digital subscriber signaling system no. 1 DSS1
is used see Section 12.1.5. In addition, the D-channel can be used for low speed packet-switching. The B-channels and the D-channel are full-duplex.
This basic access scheme is known as the basic rate and it was standardized by ANSI in 1988. Because it makes use of two B-channels and a D-channel, it is also known as
2B + D. The basic rate was intended for residential users and very small offices. As in the TDM schemes see Section 2.1, the basic rate access transmission is orga-
nized into fixed-length frames that repeat continuously. Each frame is 48 bits long and it repeats every 250 µsec. The frame structure from the N-ISDN user to the local exchange
is shown in Figure 12.6. Fields B1 and B2 contain data from the two B-channels. They are 8-bit long, and they repeat twice in the frame. The remaining fields D, F, and L
are 1-bit long. D repeats four times within the frame, and is used to carry data from the D-channel. F and L carry a framing bit and a dc balancing bit, respectively. The F-L
combination is used to synchronize the receiver at the beginning of the frame. Each frame contains 16 bits from each B-channel, and four bits from the D-channel. The bit rate is
144 Kbps, but due to the additional overheads bits, the total bit rate is 192 Kbps.
In addition to the basic rate, the primary rate was defined for users with greater requirements for bandwidth, such an organization with a digital PBX or a local network.
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