Adaptive Resource Allocation Mechanisms for Support of Data Traffic over ATM Satellite Networks Introduction The Dynamic Flow Control Scheme

Transcription

1 Adaptive Resource Allocation Mechanisms for Support of Traffic over ATM Satellite Networks Tanja Lang and Nirmala Shenoy, Co-operative Research Centre for Satellite Systems, Institute for Telecommunications Research, University of South Australia, Warrendi Road, The Levels Campus, SA 5095, 1 Introduction Satellite ATM networks will combine the advantages of ATM networks, with its guaranteed Quality of Service (QoS) support, and Satellite systems which have a wide geographic coverage and inherent broadcasting capabilities. Satellite resources however are limited and costly and must be used efficiently. Also the high delay and the losses involved in transmission over satellite could affect the QoS guaranteed by the ATM network. Therefore traffic and congestion control schemes for an integrated Satellite ATM network need to be developed. The integration scenario addressed here uses a Satellite Access network on an ATM backbone to provide ATM like services to users over the Satellite Network. In [1] we had evaluated the performance of the Satellite ATM Network with the Negotiable Call Admission Control (NCAC) Scheme and a Dynamic Bandwidth Allocation (DBA) scheme called the Uniform Reallocation DBA (UR- DBA). This paper discusses the Rate Adaptive Reallocation DBA (RAR-DBA) scheme an enhancement of UR-DBA, which adapts to the varying traffic rates at the mobile user. It further reduces cell loss and improves overall performance in the system. The minimal cell loss and the guaranteed Minimal Cell Rate (MCR) of this scheme is especially desirable for data traffic over TCP/IP since cell loss and timeouts due to long delays would cause TCP to flood of the network with retransmitted packets. As pointed out in [1] the NCAC is very efficient and has been adopted for this work as well. The basics of the proposed scheme can be found in ref [2]. The following section gives a brief overview of the flow control scheme as proposed in [2]. Section 3 gives the system model used in our study. Section 4 gives the two DBA mechanisms proposed by us for comparison purposes. Section 5 gives the simulated model and section 6 discusses the performance of the two systems based on the graphs obtained. Section 7 concludes. 2 The Dynamic Flow Control Scheme A dynamic flow control scheme as proposed in [2] aims to smoothen the bursty ABR traffic entering the network, by regulating it at the point of entrance. The goal is to maintain the ABR traffic close to an average rate. To give a guaranteed MCR to ABR traffic CAC is implemented in this scheme, the number of acceptable ABR users depends on the demand of higher priority classes. In the scheme discussed in this paper, the flow control scheme is implemented through the use of buffer advertisement by the user and a simple DBA scheme at the controller, placed at the entry point to the network. The CAC has the capability to pack in more ABR users and the DBA scheme aims at fairness among these users. The scheme takes maximum advantage of the burstiness of the traffic by using statistical multiplexing technique. In the original scheme [2], the flow control was distributed between the users systems and the edge nodes of an ATM network. In the integrated ATM Satellite network especially if the satellite is a bent-pipe all the control has to be implemented at the Satellite Control Station (SCS), which acts as a Gateway to the ATM Network. 3 The System Model The assumptions made in the simulation study are as follows. 1. The Satellite is a bent-pipe. Hence no processing is associated with the satellite. 2. There is a constant bandwidth Bw available for ABR traffic at the SCS. In a realistic situation this constant Bw changes depending on the high priority calls coming in at that time. For a certain number of calls accepted, the traffic allocated to all ABR calls entering the network at the SCS is maintained close to this constant value Bw. This is in line with what we propose

2 to achieve, which is mainly that all traffic generated by ABR users will be smoothened to an average rate. 3. There is no contention among the users, as we do not intend to study this aspect of the call control. 4. We have not modeled any mobility effects and time adjustment at the mobile users. The NCAC for ABR calls has a simple scheme for admitting more ABR users and details of this scheme can be found in [1]. The DBA algorithm provides fairness among the ABR users and achieves a high statistical multiplexing gain. Together the CAC and the DBA schemes achieve very good results. For access over the satellite network, a Time Division Multiple Access (TDMA) scheme is adopted. The time slot allocation is as specified below. The uplink frame and a connection request packet are shown in fig. 1. The user specifies its estimated Average Cell Rate (ACR) and the MCR acceptable to him. The downlink frame and slot assignment message is given in fig. 2. The slot assignment message carries assignment information for uplink and downlink frames The uplink frame format is from the mobile user to the SCS and the downlink frame is from the SCS to the Mobile user. The users place their request in the request in the uplink frame. The control messages carry information of allocation and any changes in allocation to the different users from the SCS. A total of 26 users were considered, with their calls starting and ending at random times. A total of 40 slot are assumed per time frame. Each slot can accommodate an ATM cell. Request Fig. 1a. Uplink frame User ID ACR MCR 8 bits 4 bits 4 bits Fig. 1b. Connection Request Packet Control Fig. 2a. Downlink frame 4 Dynamic Bandwidth Allocation Mechanisms req 1 req 2 user user cntrl cntrl user. user ID1 ID 40 ID 1 ID 40 uplink assignment Fig 2b. Slot assignment message downlink assignment 4.1 UR- DBA algorithm For the UR- DBA algorithm there are four different buffer states: low, normal, high, overflow and the buffer is partitioned into four equal- sized parts. The buffer flag is set as follows: If (current buffer level <= buffer size/4) Buffer flag = low Else if ((current buffer level > buffer size/4) (current buffer level <= buffer size/2)) Buffer flag = normal Else if ((current buffer level > buffer size/2) (current buffer level <= buffer size*3/4)) Buffer flag = high Else Buffer flag = overflow With every ATM cell sent the user transmits its current buffer state to the control station via 2 bits. The SCS compares the new state to the previous one and can give or take bandwidth to a connection according to the result of this comparison. If a connection has had a low buffer state for longer than one second then some bandwidth will be taken from it and allocated to the free bandwidth pool maintained at the SCS. This will be done every second the connection remains in the low state. A connection will receive extra bandwidth in terms of x if its buffer makes a transition from the normal to the high or from the high to the overflow state. For every second the connection remains in the high or overflow state additional x are given to it. The amount of bandwidth given in the high or overflow state and also when a transition occurs is always the same, also in the case that the

3 connection remains in the low buffer state the same amount of x is taken every second. The pseudo code for this algorithm is given below. If ((previous state == normal) (current state == high)) If ((previous state == high) (current state == overflow)) If (state has been high or overflow during the last second) If (state has been low during the last second) Take x Although this algorithm improved cell loss as well as the usage of the satellite link its main problem is that it does not take into account the rate with which the buffer level increases, therefore the RAR- DBA was developed. 4.2 RAR-DBA algorithm This ais an ongoing algorithm that checks each cell from the users for the buffer-status flag. The buffer state determination was done as indicated below For RAR- DBA algorithm the buffer is partitioned into six distinct levels. The partitions for the low, normal and overflow state are uniform but the high state is further split into two equal- sized parts high1 and high2. Therefore the buffer flag is set as follows: If (current buffer level <= buffer size/4) Buffer flag = low Else if ((current buffer level > buffer size/4) (current buffer level <= buffer size/2)) Buffer flag = normal Else if ((current buffer level > buffer size/2) (current buffer level <= buffer size *5/8)) Buffer flag = high1 Else if ((current buffer level > buffer size*5/8) (current buffer level <= buffer size*3/4)) Buffer flag = high2 Else Buffer flag = overflow This algorithm considers the rate of increase of the buffer-level when assigning new bandwidth to a user. A connection gets extra bandwidth, x, for a transition from normal to high1, from high1 to high2 and from high2 to overflow. It gets the same amount of bandwidth every second as long as it is in the state high1 or high2. In the overflow state a connection also gets extra bandwidth, the same amount of x as in the high1 or high2 state but in intervals of every half a second. When the connection makes a transition from the overflow to the high2 state the speed at which it gets extra bandwidth decreases again to x per second and when it reaches the high1 state no more bandwidth is given to the connection since its buffer level is going down. Because of the non- linear manner a connection receives bandwidth once its buffer level increases it has become necessary to also take the bandwidth from a low buffer state connection in a non- linear manner in order to maintain fairness between the users and to use the bandwidth efficiently, if not a connection will keep a high amount of bandwidth for a considerable amount of time even when it does not need it. Every connection in the low buffer state that still has more bandwidth than required for its ACR looses bandwidth every half a second. Only when the bandwidth allocated to it is lower than needed for ACR it looses bandwidth every second. The amount of bandwidth given and taken in every step is the same. If ((previous state == normal) (current state == high1)) If ((previous state == high1) (current state == high2)) If ((previous state == high2) (current state == overflow)) If (state has been high2 during the last second) If (state has been overflow during the last half second) If ((state has been high1 during the last second) (the last transition has been upward)) If ((state has been low during last half second) (allocated bandwidth > ACR)) Take x

4 If ((state has been low during last second) (allocated bandwidth <= ACR)) Take x 5 Simulation model The mobile user and the SCS have only the MAC layer implemented. The CAC and the DBA form part of the MAC layer at the SCS. The Mobile Users have an ABR traffic generator modeled by superposition of two Poisson Arrival processes. To make the traffic from the source bursty, one Poisson process has a low inter-arrival time and the other has a high interarrival time and they alternate. The Satellite is assumed to be a LEO Satellite and is placed at approximately 1000 kms from the Earth. To accommodate for any timing shifts between the users and to avoid collisions among the transmitted packets, a guard time is provided between the users. Simulations were carried out for a lossy environment and a non-lossy environment and from the results the improved performance of the implemented CAC and flow control scheme is obvious. In fact the results show that the scheme is quite robust and the deterioration suffered under lossy conditions is minimal. 6 Performance Because of page restrictions graphs 1-3 which show the result for NCAC only are not given (for these results see [1]). Graphs 4 to 9 give the performance of the two bandwidth allocation mechanisms proposed with the NCAC scheme. The bandwidth utilisation of the NCAC/UR- DBA s nearly 95% as can be seen in graph 4. Out of 26 call requests 6 were rejected as shown in graph 5. Only three users suffered cell loss and the cell loss suffered by the worst affected user is given in graph 6. The maximum number of lost cells was around 280 cells. The next set of graphs give the performance for the NCAC with the RAR-DBA mechanism. Graph 7 shows the bandwidth utilization for this scheme which is again just over 95%. Graph 8 gives the number of calls rejected to be 5 for the 26 requests that were made. This is slightly better than the NCAC operation with the UR- DBA, because the allocation with the RAR- DBA scheme is more conservative than with the UR-DBA scheme. There was only one user who suffered cell loss and the maximum cell loss suffered by him was approximately 110 cells. This is a much better performance than the UR- DBA mechanism. The graphs shown are for a lossy environment and the difference in performance of the UR-DBA and RAR-DBA mechanism for a lossy and a non-lossy environment was minimal, which goes to show the robustness of the scheme. 7 Conclusions In this paper we have presented two dynamic bandwidth allocation mechanism used with a simple negotiable CAC proposed by us earlier. The two schemes complement the NCAC and provide good performance of the overall system. The scheme was targeted mainly to support data traffic arising from applications like ftp, rlogin and s in an ATM Satellite Network, where the ATM network is the backbone and the Satellite network is an access network. The flow control scheme was adopted from one proposed earlier for an ATM Network. The simulation results were given and the performance of the schemes were discussed. The results show the better performance achieved, especially with a rate adaptive flow control scheme and also the robustness of the scheme under lossy channel environment. References 1. Nirmala Shenoy, Tanja Lang, Support of TCP/IP data over an ATM Satellite Network, accepted in 6 th IEEE International Workshop on Intelligent Signal Processing and Communication Systems, Melbourne 4-6 November Nirmala Shenoy, "Performance of a Deterministic Flow control scheme for Traffic in ATM Network", Proceedings of the IEEE ICICS 97, Singapore. 3. Sastri Kota, Rohit Goyal and Raj Jain, Satellite ATM Network Architectural Considerations and TCP/IP performance, Proceedings of the third Ka BanUtilization Conference, Sorrento, Italy, September Ian F. Akyildiz and Seong Ho Jeng, Satellite ATM Networks: A Survey, IEEE Communications Magazine July Pg Anthony Hung, Marie-Jose Montpetit, George Kesidis and Peter Takats, A Framework for ATM via Satellite, Proceedings of Globecom 96.

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