VOICE OVER ATM SPECIFICATIONS

308 VOICE OVER ATM AND MPLS Circuit mode data At the receiver’s side, it passes through circuit mode data, such as N × 64 Kbps fractional T1E1, and at the receiver’s side it regenerates the circuit mode data. Data frames At the transmitter’s side, it extracts payloads from data frames, and removes flags, bit stuffing and CRC. At the receiver’s side, it regenerates data frames and restores flags, bit stuffing, and CRC.

12.5.2 The Service-Specific Convergence Sublayer

Voice is real-time traffic, that has to be delivered to the destination with minimum jitter. Annoying distortions can result due to jitter variability and brief silence periods can be shortened or lengthened. Modem traffic also has to be delivered with minimum jitter, because abnormal phase shifts can be sensed if the delay varies. An SSCS transmitter passes information from its User to CPS with no delay variation. However, cell delay variation can be introduced at the CPS transmitter during periods of time when voice from too many AAL 2 connections is directed simultaneously onto the same ATM connection. Cell delay variation can be controlled by CAC and by requesting the user to switch to an algorithm with greater compression. As in the case of the AAL 1 convergence sublayer, the receiving SSCS introduces a delay before it delivers the information to the receiving User in order to cancel any delay variations incurred by the network. Type 1 and Type 3 packets A transmitting SSCS passes data to CPS in the form of a packet known as CPS-packet see Section 3.7.2. The structure of the CPS-packet is shown in Figure 3.20. It consists of a 3-byte header and a payload which has a maximum length of 45 bytes. In the AAL 2 SSCS for trunking, the CPS-packet payload has been further defined. Specifically, it can be either a Type 1 packet unprotected or a Type 3 packet fully protected. Type 2 packets are to be defined. In Type 1 packet, the CPS-packet payload is simply made up of data without any additional information that can be used for error detection, such as CRC or parity check. The maximum payload is 45 bytes. The format of the Type 3 packet is shown in Figure 12.13. The maximum payload is 43 bytes, and the remaining 2 bytes are used for the fields: message type and CRC. The message type is a 6-bit field and it contains a code to indicate the contents of the payload. Message type codes have been defined for dialed digits, ABCD CAS bits, facsimile demodulation control data, alarms, and user state control operations. The CRC-10 is a 10-bit field that contains the FCS computed using the polynomial x 10 + x 9 + x 5 + x 4 + x + 1. payload Message type CRC-10 Figure 12.13 Type 3 packet fully protected. THE AAL 2 SERVICE-SPECIFIC CONVERGENCE SUBLAYER 309 Time stam p Redundancy Message-dependent information . . . Message type CRC-10 1 2 3 4 5 6 7 8 Figure 12.14 Common facilities for Type 3 packets. In certain cases, SSCS makes use of a more detailed format of the Type 3 packet, referred to as the common facilities for Type 3 packets see Figure 12.14. It contains information which depends upon the type of the message and the fields: redundancy, time stamp, message type and CRC-10. The message type and the CRC-10 fields are the same as in the Type 3 packet. The common facilities for Type 3 packet is triple redundant. That is, it is transmitted three times. The interval between two successive transmissions depends on the type of message. It is 5 msec for dialed digits and ABCD CAS bits; 20 msec for facsimile demodulation control packets and user state control packets. Each copy of the redundant packet contains the same content, except in the 2-bit redundancy field which is populated with the values 1, 2, and 3 corresponding to the packet’s first, second, and third retransmission. The time stamp field is a 14-bit field and it contains a time stamp. SSCS packets A variety of different packets have been defined to transport the different types of data streams supported by the AAL 2 SSCS for trunking. Below, we examine some of these packets: • Audio packet : The Type 1 packet is used, and the payload contains one or more EDUs. For instance, when the G.711 PCM 64-Kbps algorithm is used, five EDUs are trans- mitted in a payload, making a total of 40 bytes. Also, an audio packet is accompanied by a sequence number that is carried in the UUI field of the CPS-packet header. • Generic silence insertion description SID packet : G.711, G.722, G.726, G.727, G.728 do not contain provisions for voice activity detection, discontinuous transmission, and comfort noise generation tailored to a specific algorithm. In view of this, a generic SID has been developed. Specifically, a generic SID packet is sent immediately after the last active voice packet of a talkspurt. It marks the beginning of the silence and alerts the receiver to expect an absence of active voice packets. The SID can also be sent at arbitrary times during a silence period. The silence period is terminated when the receiver receives an active voice packet. The SID packet is carried in a Type 1 packet. • Circuit-mode data at N × 64 Kbps packet : The Type 1 packet is used. It consists of M × N bytes, where M is the number of multiples of N bytes that will be packed