Channel-Associated Signaling CAS BACKGROUND

VOICE OVER ATM SPECIFICATIONS 301 CALLPRC SETUP SpeechData TE Local Exchange Called user TE SETUP ALERT Calling user D-channel D-channel ALERT CONN CONN CONNACK CONNACK DISC DISC RLSE RLSE RLCOM RLCOM Figure 12.8 An example of DSS1 signaling.

12.2 VOICE OVER ATM SPECIFICATIONS

ATM is a technology well-suited for voice over packet, since it can guarantee the end-to- end delay, a necessary QoS parameter for the delivery of voice, and is also well-established in the backbone and in access networks, such as ADSL-based access networks. The key to implementing voice over ATM is a device known in ATM as the inter- working function IWF . In IP networks, an IWF is known as a media gateway. An IWF implements the functionality defined in a voice over ATM specification. It can be a stand- alone device or part of a bigger device. An IWF can offer a variety of services, such as the transport of PCM voice i.e., 64-Kbps voice, encoded voice, facsimile, telephone signaling, and circuit mode data i.e., T1E1 and fractional T1E1. The ATM Forum has defined a number of different specifications for voice over ATM. These specifications can be classified into two groups. The first group, referred to as ATM trunking for voice , deals with the transport of voice between two telephone networks. The second group of specifications deals with how to provide voice to a user at a desktop or to a user over ADSL. The following four specifications have been defined for ATM trunking for voice: • ATM trunking using AAL 1 for narrowband services • Circuit emulation service s CES • Dynamic bandwidth circuit emulation service s DBCES • ATM trunking using AAL 2 for narrowband service s 302 VOICE OVER ATM AND MPLS The first three specifications are based on AAL 1, whereas the fourth one is based on AAL 2. The first specification ATM trunking using AAL 1 for narrowband services addresses the interconnection between two PBXs over a private or public ATM network. The cir- cuit emulation services CES , as shown in Section 3.7.1, emulates a point-to-point TDM circuit, and is used to connect TDM interfaces such as such as T1, T3, E1, E3, and J2 the J system is used in Japan, over an ATM network. The dynamic bandwidth cir- cuit emulations service s DBCES specification provides circuit emulation services for fractional T1E1, as in the CES specification, but in addition it permits the bandwidth allocated to the ATM connection that carries the fractional T1E1 signal to be dynamically changed. Finally, the ATM trunking using AAL 2 for narrowband services specification was designed so that it can interconnect two distant public or private telephone net- works over an ATM network. It can be used, for instance, to connect distant PBXs to a central office over an ATM network, or to interconnect two distant PBXs. This specifi- cation is based on two AAL 2 service-specific convergence sublayers: the AAL 2 SSCS for trunking and the segmentation and reassembly service-specific convergence sublayer for AAL 2 SEG-SSCS . The AAL 2 SSCS for trunking specification was designed to pro- vide a variety of services, such as audio service, circuit-mode service, and frame-mode data service. The SEG-SSCS specification was designed to transport packets whose size is bigger than the maximum length of 45 bytes permitted in the payload of the CPS packet. The following specifications have been defined for the second group of specifications, which deals with how to provide voice to a user at a desktop or to a user over ADSL: • Voice and Telephony over ATM to the Desktop • Loop Emulation Servic e LES using AAL 2 The first specification, as its name implies, deals with providing voice to the desktop, and the second specification deals with providing narrowband services to a user over ADSL, hybrid fiber coaxial HFC , or a wireless connection. Below, we first review briefly the circuit emulation services specification and then we describe in detail the ATM trunking using AAL 2 for narrowband services specification. The reader is assumed to be familiar with the material on ATM adaptation layers 1, 2, and 5; if needed, review Section 3.7.

12.3 THE CIRCUIT EMULATION SERVICES CES SPECIFICATION

Circuit emulation services CES emulates a point-to-point TDM circuit. CES is used to connect TDM interfaces such as such as T1, T3, E1, E3, and J2, over an ATM network. CES is based on AAL 1 and it uses the CBR service category in order to guarantee the end-to-end delay. Both the unstructured and structured data transfer pro- tocols of AAL 1 CS, described in Section 3.7.1, are used. The following services have been defined: • Structured DS1E1 N × 64 Kbps fractional DS1E1 • Unstructured DS1E1 1.544 Mbps2.048 Mbps • Unstructured DS3E3 44.736 Mbps34.368 Mbps • Structured J2 N × 64 Kbps fractional J2 • Unstructured J2 6.312 Mbps THE CIRCUIT EMULATION SERVICES CES SPECIFICATION 303 TDM circuit ATM network CES IWF CES IWF TDM circuit Figure 12.9 Reference model for CES. These services are named according to whether they use the unstructured or structured data transfer protocol of the AAL 1 CS. Only the first two services are described in this section. The reference model is shown in Figure 12.9. The two CES IWFs are connected by an ATM connection using the CBR service category. Each CES IWF is connected to a TDM circuit such as T1, T3, E1, E3, and J2. The two IWFs extend transparently the TDM circuit across the ATM network.

12.3.1 Structured DS1E1J2 N × 64 Kbps Service

This service is intended to emulate point-to-point fractional DS1, E1 and J2 circuits, where N takes the values: 1 ≤ N ≤ 24 for DS1; 1 ≤ N ≤ 31 for E1; and 1 ≤ N ≤ 96 for J2. CES must maintain 125-µsec frame integrity across the ATM connection. For example, given a 2 × 64 Kbps circuit, the 2 bytes that are sent to the input of the IWF in each frame have to be delivered at the output of the egress IWF in one frame and in the same order. The structured DS1E1J2 N × 64 Kbps service also requires synchronous circuit timing. The IWF service interface provides 1.544 MHz timing to external DS1 equipment, 2.048 MHz timing to external E1 equipment, and 6.312 MHz timing to external J2 equipment. The structured DS1E1J2 N × 64 Kbps service can support signaling in two modes: with CAS and without CAS. With the former, the CES IWF has to be able to recognize and transport to the egress IWF the ABCD signaling bits. In the non-CAS mode, also known as basic mode, there is no support for CAS. This basic mode can be used for applications not requiring signaling or those that provide signaling using CCS as used in N-ISDN. The AAL 1 CS structured data transfer protocol is used to transport fractional DS1E1J2. This protocol is described in detail in Section 3.7.1. A structured block is created by collect- ing N bytes, one from each of the N time slots, and grouping them in sequence. The SDT pointer points to the beginning of such a block. A significant source of delay is due to the packetization delay – that is, the amount of time it takes to collect enough data to fill a cell. This can be reduced by sending partially filled cells. This reduction in the delay is at the expense of higher cell rate. Partial fill is an optional feature of the CES IWF. The number of bytes that are sent in each cell can be set at the time the connection is established. When padding is used, the SDT pointer applies to both payload and padding. If a cell loss is detected at an egress IWF, a dummy cell is inserted. The content of the inserted cell is implementation dependent. If too many cells have been lost, the AAL 1 receiver will locate the next AAL 1 STD pointer to re-acquire framing.

12.3.2 DS1E1J2 Unstructured Service

A large number of applications use DS1E1J2 interfaces that make use of the entire band- width of the TDM circuit. The unstructured service emulates a point-to-point DS1E1J2