Signaling System No. 7 SS7

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 304 VOICE OVER ATM AND MPLS circuit across an ATM network. The incoming bits from the DS1 circuit are simply placed sequentially into the payload of the AAL 1, without regard to framing, using the unstructured data transfer protocol see Section 3.7.1.

12.4 THE ATM TRUNKING USING AAL 2 FOR NARROWBAND SERVICES

SPECIFICATION The ATM trunking using AAL 2 for narrowband services specification was designed to interconnect two distant public or private telephone networks over an ATM network. It can be used, for instance, to interconnect a distant PBX and a central office over an ATM network, such as PBX A and the central office in Figure 12.10. It can be also used to connect two distant PBXs over an ATM network, such as PBX B and C in Figure 12.10. A PBX or a central office is connected to an IWF over a T1E1 link. This specification is used in cellular telephony to transport multiple voice calls. The protocol stack of an IWF that supports the ATM trunking using AAL 2 for narrow- band service specification is shown in Figure 12.11. As can be seen, an IWF can transport IWF T1E1 IWF ATM network Central office PBX A PBX B IWF IWF PBX C T1E1 T1E1 T1E1 Figure 12.10 ATM trunking using AAL 2 for narrowband services. Circuit mode data services Nx64 Kbps User traffic Voiceband services Inband signaling User traffic PCM voice Compressed voice Facsimile demodulation AAL 2 SSCS for trunking SEG-SSCS Frame mode data services User traffic IWF-IWF CPS AAL 5 ATM layer Figure 12.11 The protocol stack of an IWF.