294 VOICE OVER ATM AND MPLS
represents 1, two breaks represent 2, etc. The dial-pulse system was replaced by the dual- tone multi-frequency DTMF
system. When a subscriber presses a key on the keypad, an oscillator inside the telephone generates two simultaneous tones. Each digit is represented
by a particular combination of two frequencies, one selected from a low group of frequencies 697, 770, 852, and 941 Hz and the other selected from a high group of frequencies 1209,
1336, 1477, and 1622 Hz. This allows for sixteen digit values, of which only twelve are used for digits 1, 2, . . ., 9, 0, and special values and . The DTMF frequency combinations
are distinct from naturally occurring sounds in the vicinity of the calling subscriber and also from voice and voiceband data i.e. modem traffic and facsimile.
After the digits have been transferred to the local exchange, the local exchange checks to see if the called party’s phone is free. If it is busy, it sends a busy signal to S1. If it is
free, it sends a ringing signal to alert S2, and it also informs S1 that it is in the process of alerting S2 by sending a ringing tone. When S2 answers the phone, the local exchange
sets up a path through its switch to connect the two subscribers, and at that moment the conversation can begin. Finally, the two subscribers go on-hook, i.e., they hang up, which
generates a clear forward message from the calling subscriber and a clear back message from the called subscriber. The terms on-hook and off-hook come from the days when
the receiver of the telephone was resting on a hook
12.1.2 Channel-Associated Signaling CAS
Let us consider two subscribers i.e. S1 and S2 that are interconnected via exchanges A, B, and C see Figure 12.3. Exchanges A and B, and B and C are connected by a PDH
link e.g. T1E1 or T3E3. Establishing a call between S1 and S2 requires establishing a two-way circuit between S1 and S2. The circuit from S1 to S2 consists of: S1’s subscriber
line to local exchange A; a time slot say time slot i in the frame transmitted from A to B; a time slot say time slot j in the frame transmitted from B and C; and S2’s
subscriber line. In the direction from S2 to S1, the circuit consists of S2’s subscriber line to local exchange C, time slot j in the frame transmitted from C to B, time slot i in the
frame transmitted from B and A, and S1’s subscriber line. These physical resources are entirely dedicated to the call between S1 and S2 and cannot be shared by other calls.
These resources are freed when the call is terminated which happens when either side hangs up.
The channel-associated signaling CAS is used to establish and release calls, and it has been in existence since the beginning of automatic telephony. It was the only
signaling system used until the late 1970s, when the common channel signaling CCS was developed. CAS is still being used, but it is gradually been replaced by CCS. As will
be explained below, CAS uses various signals to establish and release a call. In addition, it uses a number of supervisory bits, which are known as the ABCD signaling bits. The
signals, the digits coded in DTMF, and the ABCD bits are all transmitted through the same circuit, i.e. the same time slots between adjacent exchanges, that are used for the
transmission of the voice traffic of the call. In view of this, CAS is an in-band signaling protocol.
Note that the detailed signaling between S1 and its local exchange A was discussed in the previous section, and is not shown in Figure 12.3. After exchange A receives the
last digit from S1, it seizes a free trunk between A and B which will be used for this new voice call. The trunk is an unused time slot in the frame traveling from A to B.
BACKGROUND 295
S1 Local Exchange
Last digit Wink
First digit S2
Ringing tone Seizure
Answer
Conversation Clear forwarrd
A Local Exchange
B Local Exchange
C
S1 clears Last digit
. .
. Wink
First digit Seizure
Last digit .
. .
Ringing signal Answer
Answer Answer
S2 clears Clear-back
Clear-back Clear forwarrd
Release guard Release guard
. .
.
Figure 12.3 CAS signaling.
Exchange A sends a special seizure signal to B on this time slot. Exchange B responds with a proceed-to-send signal also called a wink signal . This signal is sent on the same
time slot number in the frame traveling in the direction from B to A, to indicate that it is ready to receive the digits of the called party.
Exchange A sends the digits to B, by transmitting one digit in the time slot allocated to this call over a number of successive frames. Exchange B repeats the same procedure as
A. That is, once it has received the entire number of the called party, it seizes an outgoing trunk, i.e., a time slot in the frame traveling from B to C, and sends a seizure signal to
exchange C. Exchange C responds with a wink signal, after which exchange B sends the digits. Exchange C checks to see if the receiver’s phone is idle. If it is idle, it generates a
ringing signal and also sends a ringing tone towards S1. When S2 answers, exchange C sends an answer signal to exchange B, which exchange B forwards to exchange A. The
conversation between S1 and S2 can now begin.
In the example in Figure 12.3, S2 hangs up first. This gives rise to the clear-back mes- sage
sent from exchange C backwards towards exchange A. When exchange A receives the clear-back message, it stops charging for the call and sets up a timer. Exchange A
releases the connection when it receives a clear message from S2 or the timer expires. This is done by sending a clear-forward signal to exchange B, which in turn sends it to
296 VOICE OVER ATM AND MPLS
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
