ATM Adaptation Layer 5 AAL 5

PROBLEMS 75 a With what probability a cell is rejected when the HEC state machine is in the correc- tion mode? b With what probability a cell is rejected when the HEC state machine is in the detec- tion mode? c Assume that the HEC state machine is in the correction mode. What is the probability that n successive cells where n 1 will be rejected? d Assume that the HEC state machine is in the correction mode. What is the probability pn that n successive cells will be accepted, where n 1? Hint: Write down the expression for p1 and p2, and express p3 as a function of p1 and p2. Then write down the general expression for pn for any n as a function of pn − 1 and pn − 2. 4. Consider the case of an application running over an ATM network. Assume that each packet generated by the application is carried by n ATM cells, which are transmitted back-to-back. The time to transmit a cell is T and the average time it takes for a cell to traverse the ATM network and reach the destination is D. When all of the ATM cells belonging to the same packet are received by the destination, their payloads are extracted from the cells and are assembled to the original packet. Subsequently, the CRC operation is carried out on the reassembled packet. If the CRC check is correct, the packet is released to the application. Otherwise, a negative acknowledgment is sent back to the source requesting the retransmission of the entire packet. The time it takes to carry out the CRC check is F , and the time it takes for the negative acknowledgment to reach the source is D. Let p be the probability that the ATM cell is received with erroneous payload. a What is the probability that all n cells are received correctly? b What is the probability that exactly m cells are received erroneously, where m n. c Write down the expression for the average time, required to transfer correctly an application packet to the destination. d Plot the average time against the probability p, assuming that n = 30, D = 20 msec, T = 3 µ sec, and F = 0. Vary p from 0.1 to 0.00001. e Discuss the results of the above plot in the light of the fact that there is no data link layer in ATM. 5. Consider the AAL 1 structured data transfer scheme. What is the value of the SDT pointer in the second and third cycle, when the structured block size is N bytes, N = 5, 10, 93, 156, 75, 1000. 6. Let us assume that the SDT pointer is not used in the AAL 1 structured data transfer scheme, i.e. no CS-PDU P format is used within each cycle. Give an example where the receiver might not be able to find the beginning of a structured block. 7. In AAL 2, the receiving CPS retrieves the CPS-packets carried in the payload of a CPS-PDU using the offset field OSF in the header of the CPS-PDU and the LI field in the header of each CPS-packet carried in the CPS-PDU. The use of the OSF might appear redundant Construct an example where the receiving CPS cannot retrieve the CPS-packets carried in a CPS-PDU by using only the LI field in each CPS-packet. Assume no cell loss or corrupted ATM payloads. 8. The following CPS-packets have to be transmitted over the same AAL2 connection: CPS-packet 1 20 bytes, CPS-packet 2 48 bytes, CPS-packet 3 35 bytes, and CPS-packet 4 20 bytes. For simplicity, assume that the length of each of these CPS-packets includes the 3-byte CPS- packet header. a How many CPS-PDUs are required to carry these four CPS-packets? b What is the value of the OSF field in each CPS-PDU? 9. A voice source is active talkspurt for 400 msec and silent for 600 msec. Let us assume that a voice call is transported over an ATM network via AAL 2. The voice is coded to 32 Kbps and silent periods are suppressed. Assume that the SSCS has a timer set to 5 msec. That is, each time the timer expires, it sends the data it has gathered to CPS as a CPS-packet. Assume that the timer begins at the beginning of the busy period. a How long in bytes is each CPS-packet? b How many CPS-packets are produced in each active period? 76 ATM NETWORKS 10. A 1500-byte user-PDU is transported over AAL5. a How many bytes of padding will be added? b How many cells are required to carry the resulting CPS-PDU? c What is the total overhead i.e. additional bytes associated with this user-PDU? APPENDIX: SIMULATION PROJECT: AAL 2 The objective of this simulation project is to establish the value of the timer used in the CPS of AAL 2, that maximizes the average percentage fill of the ATM cells that carry CPS-PDUs, and minimizes the packetization delay in AAL 2. Project Description You will assume that there are two end devices interconnected with a VCC. The end devices support a number of phone calls multiplexed onto the same connection using AAL 2. We will only model the AAL 2 functions in one of the two end devices. The transfer of ATM cells across the ATM connection, the AAL 2 at the receiving end device, and the flow of information in the opposite direction will not be modeled. The number of voice calls multiplexed on the same connection will be provided as input to the simulation. Each voice call is associated with an SSCS, which provides a number of different functions. Of interest to this simulation project is the voice encoding algorithm and the size of the audio packet. We will assume that pulse code modulation PCM is used which produces 8 bits per 125µsec. PCM is defined in ITU-T’s standard G.711. The output of the G.711 coder is accumulated over an interval of 1 msec to yield a sequence of 8 bytes, referred to as the encoded data unit EDU. Further, five EDUs are blocked together into an audio packet of 40 bytes every 5 msec. SSCS also detects the beginning of a silence period and it transmits as special silence insertion description SID code. CPS serves multiple SSCSs – each associated with a different phone call. The SSCSs operate asynchronously from each other. When CPS receives an audio packet, it encap- sulates it into a CPS-packet and places it into the payload of a CPS-PDU which equals 47 bytes. CPS keeps a timer T1. When the timer expires, it encapsulates whatever CPS- packets it has received into a CPS-PDU, adds padding if necessary, and sends the resulting CPS-PDU to the ATM layer. The amount of time it takes to carry out these tasks is assumed to be zero. If the CPS has not received any CPS-packets when T1 expires, then it does not create a CPS-PDU. T1 is reset immediately each time it expires. If the CPS payload is filled up before the timer expires, CPS prepares a CPS-PDU, sends it to the ATM layer, and resets the timer. Each voice will be assumed to go through a talkspurt followed by a silence period. Only the bytes generated during each talkspurt are transported by AAL2, since the silence periods are suppressed. The rate of transmission of a voice during a talkspurt is 64 Kbps. The length of the talkspurt is assumed to be exponentially distributed, with an average of 400 msec. The length of the silence period is also assumed to be exponentially distributed, with an average of 600 msec. The Simulation Structure Simulation of a single SSCS Since a voice alternates between a talkspurt and a silence period, the first thing to do is to generate the length of a talkspurt. When the talkspurt expires, generate a silence