(DRISMA) for Wireless ATM Networks - Semantic Scholar

Credit-based Dynamic Reservation Integrated Services Multiple Access (DRISMA) for Wireless ATM Networks Henry C. B. Chan 1 and Victor C. M. Leung 2 1 2

Department of Computing, Hong Kong Polytechnic University, Hong Kong

Department of Electrical and Computer Engineering, University of British Columbia, Canada

Abstract - In this paper, we extend our previous dynamic reservation integrated services multiple access (DRISMA) protocol for the integration of constant bit rate (CBR), variable bit rate (VBR) and unspecified bit rate (UBR) traffic over wireless ATM networks. DRISMA provides CBR services by pre-assigning slots in a frame and VBR services by reserving slots on a bandwidth-on-demand basis. By using a request-mini-slot method, it also ensures that UBR traffic only utilizes the residual bandwidth of the CBR and VBR traffic. However, our previous DRISMA protocol provides isochronous services only. In this paper, we propose a novel credit-based scheme for supporting bursty VBR traffic more effectively. This paper describes the operation of the credit-based scheme and presents some simulation results to illustrate its performance. I. INTRO DUCT IO N

With the advent of low cost mobile computers and mobile telephones, there is an increasing demand for high quality mobile communication services. The wireless asynchronous transfer mode (WATM) is a potential architecture for future wireless communication networks [1-3], in particular to support broadband multimedia services. A key advantage of WATM is that it facilitates the integration with emerging wired ATM networks. To integrate different types of traffic over WATM networks, an efficient medium access control (MAC) protocol is required. Conventional MAC protocols for cellular networks such as time division multiple access and frequency division multiple access techniques are not suitable for WATM networks because they are not efficient in terms of bandwidth utilization [4]. Although code division multiple access can provide high bandwidth efficiency, it requires more complexity in the base stations and suffers from power control problem and peak bit rate limitations. Random access protocols such as Aloha and carrier sense multiple access are also not appropriate for WATM because they cannot provide guaranteed services and cause significant bandwidth wastage due to access contention. MAC protocols based on contention reservation techniques are attractive for WATM because they can provide bandwidth-on-demand service. A typical example is the packet reservation multiple access (PRMA) protocol proposed by Goodman [5], which has received much interest by This project was supported by Hong Kong Polytechnic University Research Grant PolyU5088/99E (account code G-T041).

researchers. Over the years, numerous proposals have been made to improve the performance of PRMA. These improvements include using mini-slots, adjusting the bandwidth dynamically, combining polling and random access for coordinating packet transmissions, making reservations based on non-collision (or deterministic) techniques, resolving access contention using splitting algorithms, scheduling packet transmissions based on more sophisticated mechanisms, and controlling access priority by means of signature techniques [6-9]. However, they are in general not designed for integrating the complete range of ATM traffic. To support ATMbased services, more advanced scheduling mechanisms and priority control techniques are required. In recent years, there have been a number of MAC protocols proposed for WATM. The following protocols are representative of three well-known WATM projects. Karol et al. of Lucent proposed a distributed queuing request update multiple access (DQRUMA) protocol [11]. This is a frameless protocol specially designed for transporting VBR traffic. Basically, mobile stations (MSs) contend for mini-slots to establish an initial connection with the base station (BS). Subsequent slot requirements are piggybacked via packet transmissions. A major contribution of DQRUMA was the use of mini-slots and piggybacking to enhance slot utilization. However, DQRUMA did not address the integration of CBR, VBR and ABR traffic based on different quality of service requirements. Raychaudhuri et al. proposed a DTDMA/ TDD protocol for the WATMnet prototype system at C&C Research in NJ [12]. It is based on a time division duplex structure with uplink and downlink subframes. Slotted Aloha is used for uplink access to the control slots. Based on the nature of the traffic, slots are assigned via dynamic allocations for ABR traffic, fixed and shared allocations for VBR traffic, and fixed allocations for CBR traffic. A MASCARA protocol is proposed in [13] for the W-ATM Network Demonstrator project supported by the European Community. It employs a variable-length time-frame consisting of uplink and downlink subframes, a cell-train concept to enhance channel efficiency, and a Priority Regulated Allocation Delay-Oriented Scheduling (PRADOS) mechanism that combines priority control with a leaky bucket traffic regulator. Compared to other MAC protocol proposals, MASCARA has a more comprehensive scheduling mechanism for CBR, VBR

and ABR traffic. Besides the contention reservation protocols, MAC protocols based on adaptive polling have also been proposed for WATM [14]. While polling protocols are simpler to implement in general, their performance depends heavily on the polling period, which may be difficult to determine for a mixture of CBR, VBR and ABR MSs. In [15], we have proposed a dynamic reservation integrated services multiple access (DRISMA) protocol for wireless ATM networks. DRISMA provides CBR services by pre-assigning slots in a frame and VBR services by reserving slots on a bandwidth-on-demand basis. A novel method using request-mini-slots is employed to ensure that UBR traffic can only utilize the residual bandwidth of the CBR and VBR traffic. DRISMA also enhances bandwidth utilization by adjusting the reservation bandwidth dynamically. However, our previous DRISMA protocol provides isochronous services only. In this paper, we propose a novel credit-based mechanism for supporting bursty VBR traffic more effectively. The rest of the paper is organized as follows. Section 2 gives an overview of wireless ATM. Section 3 presents the credit-based DRISMA protocol. Section 4 gives an example to explain the operation of the credit-based scheme. Section 5 presents the simulation results and discussions. Section 6 gives the conclusion. II. O VERVIEW

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WIRE LE SS ATM

In a typical WATM scenario, each user carries a mobile station (MS) with integrated communication capabilities. This means that the MS can support multimedia communications by using the ATM-based constant bit rate (CBR), variable bit rate (VBR) and unspecified bit rate (UBR) services. Typical examples for CBR, VBR and UBR services include CBR voice, VBR video, and electronic mail. A cellular architecture is often used to facilitate frequency reuse with each cell controlled by a base station (BS). When an MS enters a radio cell, it needs to register with the corresponding (BS) in order to access the WATM network. If the MS is from another radio cell, appropriate handoff technique is also required to pass the call from the previous BS to the new BS. In each radio cell, the corresponding BS serves MSs over a number of wireless channels. BSs are connected to the ATM switches by means of copper cable or optical fiber. Effectively, each BS serves as an interface for an MS to access the wired ATM networks. Although code division multiple access is becoming popular in recent years because it can provide high bandwidth efficiency, it requires more complexity in the BSs and suffers from power control problem and peak bit rate limitations. Hence, time division multiple access (TDMA) technique is usually employed for WATM networks. The use of TDMA technique also greatly facilitates the integration between wired ATM and WATM networks because each time slot can hold an ATM cell. The communication links from the MS to the BS and from the BS to the MS are called the uplink and downlink, respectively.

Depending on the number of carrier frequencies used by a two-way connection, either frequency-division duplex (FDD) or time-division duplex (TDD) can be used. In FDD, one carrier frequency is used for the uplink and a different one is used for the downlink channel. Hence uplink and downlink transmissions can be carried out simultaneously. Furthermore, if a very small radio cell (micro-cell) is used, immediate feedback on the result of access contention in the uplink channel can be provided. In TDD, only one carrier frequency is used, so the uplink and downlink transmissions are integrated in the same time frame. The downlink channel is usually of a broadcast nature and its time slots can be managed as in a centralized queue since the BS has immediate knowledge of the current traffic requirements. A MAC protocol is required for the MSs to access the uplink channels. To enhance bandwidth efficiency, time slots are assigned or reserved dynamically. Most of the TDMA-based MAC protocols for WATM use a random access protocol for reserving time slots in the uplink channels. In general, an MS first connects to the BS by accessing a reservation slot. To improve efficiency, minislots (slots that are shorter than a normal time-slot) are often used. Once connected, the MS can notify the BS of its traffic requirements. By taking into account the traffic requirements of all stations, the BS can assign uplink transmission slots accordingly and broadcast the assignments on the downlink channel to the MSs. III. O VERVIEW

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CRE DIT -BASED DRISMA

In this section, we give an overview of the enhanced DRISMA protocol. Our focus is on the new credit-based scheme for supporting VBR/CBR traffic. For details of the DRISMA protocol, please refer to [15]. Each wireless channel is slotted and framed. A slot can be used to carry an ATM cell or be partitioned into six mini-slots for conveying reservation requests. As explained in [15], a request-mini-slot is attached to each slot to ensure that UBR traffic can only utilize the residual bandwidth of the channel. Propagation delay is assumed to be negligible (which is the case in emerging wireless ATM systems employing micro-cells) and the uplink slot status is broadcasted by the BS to all MSs via the downlink channel. The use of FDD for two-way connections is considered in this study, such that all MSs can monitor the status of the next upstream slot via the downstream channel and then apply the respective station protocol accordingly. We first describe the protocol for VBR MSs, as the CBR MS can be viewed as a special VBR MS. In general, a VBR MS can be modeled by an on-off traffic source alternating between active and idle states. When a VBR MS becomes active, it first re-connects to the base station by sending a reservation request. This is done by capturing a mini-slot. In this paper, we consider that a mini-slot is captured by using a ppersistent protocol (i.e., an active VBR MS keeps on capturing a mini-slot with a probability p until successful). In the

reservation request, the MS specifies the bit rate requirement as {s,c}, where s is the required number of reserved slots per frame and c is the required credit as explained later. If the request can be entertained, the BS reserves s slots per frame for the MS. Subsequently, the VBR MS will use these reserved slots for its cell transmissions. Furthermore, the VBR MS is also allowed to “borrow” a maximum of c slots from future frames by piggybacking this request in a transmitted cell. If b slots are “borrowed”, b reserved slots have to be released (“repaid”) in future frames so as to maintain an overall reservation of s slots per frame. Note that if a VBR MS specifies its bit rate requirement as {s,0}, it means that perfect isochronous service is required. Therefore the previous DRISMA protocol is in fact a subset of the new protocol. In general, a VBR MS can specify that it requires an average of s slots per frame with a possibility of sending a burst of c cells. At the BS, each MS will be monitored according to the traffic contract {s,c}. Essentially this is similar to the leaky bucket mechanism adapted for a wireless ATM channel. When a VBR MS becomes inactive, the reserved slots will be released. To further improve the system performance, we assume that if a VBR MS cannot use up the reserved slots in a particular frame, the unused slots can be released and borrowed by other MSs subject to the credit constraint. Basically, the protocol for CBR MSs follows a similar approach, except that the reserved slots are assigned during call establishment and released only when the call ends. The protocol for UBR MSs is similar to that presented in [15]. Basically there is a special mini-slot called requestmini-slot at the beginning of each slot. It is used to disable the UBR traffic whenever there are VBR cells waiting for uplink transmissions. Each capturing VBR MS acquires this request-mini-slot with probability one by setting all its bits to one. If the BS receives an idle request-mini-slot (i.e., no capture), it sets a request indicator (RI) to zero; otherwise (i.e., either an MS captures the request-mini-slot successfully or there is a collision) it sets the RI to one. The RI indicates whether there are capturing VBR MSs in the cell such that UBR traffic is switched off until there are no more capturing VBR MSs detected. Based on the traffic situation and the RI, the BS assigns an available upstream slot as follows: • RI = 1 - Assign the slot as mini-slots for reservation requests from VBR MSs only; • RI = 0 and data request queue is not empty - Assign the slot for the next UBR MS in the data request queue; • RI = 0 and data request queue is empty - Assign the slot as mini-slots for reservation requests from any MSs. Note that an upstream slot is available if it has not been used by the reserved MS or borrowed by another MS. Essentially the CBR and VBR MSs always have priority over the UBR MSs in using an idle slot.

IV. EXAMPLE

In this section, we give an example to illustrate the basic operation of the credit-based reservation protocol. For simplicity, we assume that each frame has six slots and cells only arrive at the beginning of each frame. There are three VBR MSs: S1, S2 and S3. Each MS can reserve two slots and borrow one (i.e., the contract is {2,1}). Table 1: Station’s status

Table 2: Buffer status for each mobile station

Table 1 shows the slot status of each frame where A (Arrived), R (Reserved) and B (Borrowed) denote the number of cells that has arrived at the MS, the number of slots reserved by the MS, and the number of slots borrowed by the MS in the respective frame. For example in Frame 1, three cells has arrived at S1’s buffer and S1 reserves two slots and borrows one slot for transmitting the cells. Table 2 shows the buffer status of each MS at the beginning and end of each frame. For example, S1 has three cells in its buffer at the beginning of Frame 1 and all these cells can be transmitted in Frame 1. Fig. 1 shows the slot status as a result of the creditbased reservation protocol where Bx and Ry denote that the respective slot is borrowed by MS x and reserved by MS y, respectively. For example, in Frame 1, the first slot is reserved by MS 1 and the last slot is borrowed by MS 2. Let us describe the operation of the credit-based reservation protocol in detail. In Frame 1, S1 needs to transmit three cells. As it only has two reserved slots, it needs to borrow one slot. Note that this is possible because it has a credit of 1 slot. As shown in Fig. 1, S1 obtains the fifth slot. Consequently, S1 can transmit all the cells in its buffer in Frame 1. Similarly S2 can also transmit all the cells in its buffer in Frame 1. In Frame 2, S1 requires two slots so it can transmit through the reserved slots all the cells that have arrived. S2 only wants one slot so it can repay one slot for the slot it has borrowed in Frame 1. Note that the repaid slot is indicated inside the bracket. As shown in Fig. 1, the 4th slot is repaid accordingly. It is important to remember that the slot is still owned by S2 (i.e., it will be turned back to a reserved slot for S2 in Frame 3) so it can only be borrowed or turned into request mini-slots but not reserved by another MS. In this case, the slot is turned into mini-slots because no MS wants to borrow a slot at that time. In Frame 2, S3 becomes active again and

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needs to reserve two slots. Suppose that S3 captures one of the mini-slots of the 4th slot. Subsequently it gets the 5th and the 6th slots which are released after being borrowed by S1 and S2, respectively, in Frame 1. In Frame 3, three cells arrive at the buffer of S1, but it only has two reserved slots, so it needs to borrow one additional slot. However, since S1 has not repaid the previous borrowed slot, it does not have sufficient credit to borrow any more slots. Therefore, the cell must be retained in the buffer. S2 needs only one slot, so it can release one reserved slot (i.e., the 4th slot). The released slot is borrowed by S3. Note again that S3 cannot reserve this slot because it is only available temporarily. At the end of Frame 3, the buffer status of each MS is as shown in the last column of Table 2. V. SIMUL ATION RESUL TS AND DISCUSSIO NS

We have simulated the credit-based reservation protocol using C++. Our focus is on evaluating the performance of the VBR MSs under the new credit-based reservation protocol. There are five VBR MSs in the radio cell and each MS is modeled as an on-off traffic source alternating between active and idle states. The active and idle periods are exponentially distributed with means of 1.5 s. and 2.25 s., respectively. When an MS becomes active, it first reconnects to the BS by sending a reservation request. This is done by capturing a mini-slot (using a p-persistent protocol). Unless otherwise specified, each MS has a buffer of 1000 cells. There are 120 slots per frame. The simulation time is 200,000 frames. During the active periods, the number of cells arriving at a VBR MS in each frame follows the normal distribution with a mean of 40 cells and a variance that is varied as a parameter. Therefore we set s equal to 40 for each MS. Fig. 2 shows the packet loss ratio (PLR) as the number of credit slots increases. As expected, when the variance is zero

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(i.e., cells arrive in an isochronous manner), the PLR is constant because no slot is released for borrowing. As the variance increases, the VBR traffic becomes more bursty. In this case, the PLR increases if the MSs are not allowed to borrow any slots (i.e. zero credit). If the credit is increased, the PLR is lowered. This confirms that, by adjusting the credit, different quality of services can be provided. However, it is found that when the credit reaches a certain limit, no further improvement is possible. It can be seen that all the curves flatten off after a certain credit. Fig. 3 shows the mean cell delay. Again, it indicates that by increasing the credit, a lower cell delay can be achieved but no further improvement is possible after reaching a certain credit. Fig. 4 shows the slot utilization. A slot is defined as utilized if it is used to carry an ATM cell. The figure indicates that, as the credit increases, the slot utilization converges to about 85.5%. Fig. 5 and Fig. 6 show the PLR and the mean cell delay when the buffer size is increased to 10000 cells. It can be seen that the PLR is decreased at the expense of increasing the mean cell delay. For time-sensitive traffic such as video, which can tolerate some cell losses, it may be more desirable

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VI. CO NCLUSIO NS

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In conclusion, we have presented a credit-based DRISMA protocol for wireless ATM networks. This is an extension of our previous DRISMA protocol to provide nonisochronous CBR/VBR services. Basically, a CBR/VBR MS can reserve s slot(s) per frame while having a credit of c slot(s) to allow it to borrow slots from other MSs. Having borrowed a slot, the MS has to “repay” it through one of its reserved slots. Some simulation results have been presented to illustrate the performance of the protocol. They indicate that, by increasing the credit, better quality of services in terms of lower packet losses and reduced packet delay can be achieved. We are currently investigating how to make use of the credit-based scheme to provide different quality of service for heterogeneous VBR MSs.

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