292 VOICE OVER ATM AND MPLS
operation of circuit emulation services and AAL 1 is described in Section 3.7.1. The ATM trunking using AAL 2 for narrowband services specification is used to transport voice
traffic between two distant private or public telephone networks The MPLS and Frame Relay Alliance has so far defined two different specification
for voice over MPLS. These two specifications use ATM’s AAL 1 and AAL 2 protocols. The first specification deals with circuit emulation services over MPLS, and it makes use
of AAL 1. The second specification deals with the transport of voice over MPLS and it uses AAL 2. Both specifications are described in this chapter.
Below, basic telephony concepts and signaling protocols are reviewed. Subsequently, we describe two ATM specifications: circuit emulation services CES and ATM trunking
using AAL 2 for narrowband services . The latter specification is based on two specially
designed AAL 2 service-specific convergence sublayers AAL 2 SSCS for trunking and segmentation and reassembly SSCS [SEG-SSCS]
. Because these two SSCS specifications play a very important role in AAL 2 trunking, they are also described in detail. The chapter
concludes with the two specifications proposed by the MPLS and Frame Relay Alliance.
12.1 BACKGROUND
To understand the voice over ATM and MPLS solutions, the reader has to be familiar with basic telephony terms and signaling protocols that are used in the existing telephone
system. In this section, we first review some of the basic terms used in telephony and describe the basic signaling steps involved in placing a call. Subsequently, we briefly
present the channel-associated signaling CAS scheme, the signaling system no. 7 SS7, narrowband ISDN N-ISDN
, and the digital subscriber signaling system no. 1 DDS1.
12.1.1 Basic Concepts
Figure 12.1 gives an example of part of a telephone network. Telephone switches indi- cated in Figure 12.1 by A and B are known as exchanges. Exchanges A and B serve a
number of subscribers indicated in Figure 12.1 by circles. C is also an exchange, but it does not have any subscribers. A and B are known interchangeably as local exchanges,
C TG
A B
TG TG
TG TG
Figure 12.1 An example of a telephone network.
BACKGROUND 293
central offices , or end offices; C is known interchangeably as an intermediate, tandem,
toll , or transit exchange.
A trunk is a circuit between two exchanges, and a group of trunks is known as a trunk group TG
. A trunk is nothing else but a channel associated with a time slot in a T1E1 link or in a SONETSDH link, which carries a single voice call. A subscriber,
often referred to as a customer or user, is connected to its local exchange via a subscriber line which is commonly referred to also as the local loop.
A private branch exchange PBX is an exchange owned by an organization, such as a
business, a university, and a government agency. It is located in a building that belongs to the organization and is connected to a nearby local exchange. A PBX enables the
employees in the organization to call each other and also place and receive calls tofrom outside the organization. Typically, a PBX is connected to the telephone network via a
T1 or E1 link.
The main steps involved in placing a call from one subscriber to another are shown in Figure 12.2. For simplicity, assume that both subscribers, S1 and S2, are attached to the
same local exchange. S1 lifts the handset of the telephone from its cradle. This is known as off-hook, which is interpreted by the local exchange as a request for service. The local
exchange generates a dial tone, which indicates to S1 that it is ready to receive the digits. S1 punches the digits, which are transferred one at a time to the local exchange.
Earlier telephones used dial-pulse to generate the digits. The dial is rotated by the sub- scriber, and when it is released it spins back to its rest position producing a string of breaks
in the dc path. Each string of breaks represent a different number. For instance, one break
S1 Local Exchange
Request for service Dial tone
Digit 1 Digit 2
Digit 7 .
. .
S2
Ringing tone Ringing signal
Answer Conversation
Clear forward Clear back
Figure 12.2 Basic signaling.
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.
