PERFORMANCE MEASURES

The performance measures are the Channel Throughput (TPC) and the Average Transmis- sion Delay (DAVG). The Average Queue Length (LAVG) of the mobile’s buffer illustrates

the effects on CBR traffic. The performance parameters are defined as follows:

1. Channel Throughput (TPC):TPC is defined as the ratio of the total number of trans- mitted packets and the total number of time slots. That is,TPC =PT/TT L, wherePT

number of time slots. TheTP_C is measured as the number of packets transmitted per time slot.

2. Average Transmission Delay (DAVG): DAVG is defined as the ratio of the total packet

transmission delay and the number of active mobiles. Hence,DAVG=DT L/M, where

DT L is the total packet transmission delay and M is the number of active mobiles.

DT L is the sum of each packet transmission delay in every active mobile. Each delay

is defined as the time (number of time slots) taken, when a packet first arrives at the mobile’s buffer to the time the packet reaches the BS. DAVG is measured by the

number of time slots.

3. Average Queue Length (LAVG): LAVG is defined as the ratio of the total number of

packets in all the mobiles’ buffer and the number of active mobiles. Thus, LAVG = LT L/M, whereLT L is the total number of packets in all the mobiles’ buffer, andM is the number of active mobiles.LAVG is measured by the number of packets.

Protocol design goal is to reduceDAVG while maintaining a reasonableTPC.

ARCMA protocol offers better performance in terms of channel throughput and aver- age delay under most traffic conditions. It provides better overall channel utilization by reducing contention in the RA channels. Depending on the delay tolerance of the traffic, ARCMA can achieve very highTPC. Future high-speed cellular networks (e.g., picocell) may provide a higher delay (in time slots) tolerance enabling throughput of over 90% under suitable traffic conditions.

ARCMA protocol is designed to efficiently share the limited spectral resources of a wireless network. With the proliferation of multimedia portables, support for integrated multimedia traffic is increasingly important. In addition to the limited wireless bandwidth, new protocols are required to support real-time delay-sensitive traffic. ARCMA protocol is designed to handle some of these requirements in the MAC sublayer. There are few wireless protocols that can satisfy the high bandwidth and low Bit Error Rate (BER) of ATM networks in the wireless environment. Most of them do not provide support for the requirements of different ATM service types. The ARCMA scheme provides better support for delay-sensitive CBR traffic by prioritizing the transmission scheduling policy. In addition, ARCMA improves channel utilization by reducing collision in the request subchannel. ARCMA protocol provides request-free transmission for CBR and bursty traffic (within the same burst). An adaptive request channel can increase the request (without collision) probability by exploiting idle TA channels.

ARCMA performs better than DQRUMA regardless of the traffic load. Under heavy traffic, ARCMA protocol is capable of producing significantly higher channel throughput than DQRUMA. The worst traffic scenario for ARCMA protocol is nonbursty (single packet burst) traffic. Every packet arrival requires transmission request, causing heavy collisions in the RA channel. Conversely, ARCMA performs extremely well with bursty traffic (e.g., VBR) capable of achieving over 85% channel throughput with limited trans- mission delay. The CBR extension enables ARCMA to satisfy the delay-sensitive CBR traffic while reducing collisions in the RA channel. This result justifies the added com- plexity and overhead for CBR support.

Although the ARCMA protocol does not provide direct support to the other time- sensitive traffic (e.g., VBR), the strategies implemented in ARCMA protocol significantly

SUMMARY 69

reduce contention in the RA channel, allowing such traffic to transmit with less delay. ARCMA provides an efficient DAMA that is practical for implementation in a Wireless ATM (WATM) network. It brings us a step closer to designing a complete protocol suite that could be used in the wireless integration of ATM networks.

ARCMA protocol can be extended to provide direct support for other ATM services such as VBR and ABR traffic. Access delay can be further reduced if there exists a mechanism to specifically handle VBR or ABR mobiles. Such a mechanism alleviates the need for retransmitting requests packets through the RA channel. ARCMA protocol does not include services for network management. To provide a complete MAC sublayer support, we need to include services such as call admission and cell handoff.

4.10 SUMMARY

RS-ISMA is a wireless access protocol designed for wireless multimedia communications and implemented in the BRAIN indoor-LAN prototype. In addition, a compact RF mod- ule composed of flat antennas and an MMIC was employed for each remote station and AP. The use of large capacity FPGA decreased the number of signal processing boards. System parameters such as the packet format were optimized for IP datagram trans- port to support all applications based on IP. The function of NACK sensing was added to RS-ISMA to ensure an efficient and smooth wireless multicast in a multiple-access environment.

The HAMAC protocol uses a super frame that is divided into two frames, the downlink frame and the uplink frame. The length of the frames can vary depending on the bandwidth demand. The downlink frame is used by the BS to broadcast the frame configuration information, the connection setup, the allocation information, the request information, and the data to all mobile devices. The information and the data can be broadcast using a single burst because only the BS controls the downlink. Mobile devices can filter out irrelevant information upon receiving them. The first segment of the downlink frame is used for control signaling needed for the frame configuration to be known by all mobile devices before starting the reception and the transmission.

ARCMA implements a dynamic RA channel in which an entire uplink channel can be converted into multiple RA channels. This conversion is done when the Request Table is empty, which in most cases indicates heavy collisions in the request channel. ARCMA uses an algorithm that takes advantage of the random access scheme in the RA channel. We use the slotted ALOHA with BEB as the random access protocol for ARCMA.

The request is made in the RA channel (RA minislot). The request data packet contains the mobile’s b-bit Access ID assigned during setup. In ARCMA protocol, in addition to the Access ID, the request packet also includes the type of service being requested. The protocol provides additional support for periodic traffic (i.e., CBR). Since traffic can be either CBR or non-CBR, only a single bit is required to identify the service type. This bit is transmitted together with the request packet in the RA channel. DQRUMA provides no distinction between requests of different service types. The distinction provided in ARCMA is used by the BS to assign priority to CBR traffic.

PROBLEMS TO CHAPTER 4

Wireless protocols Learning objectives

After completing this chapter you are able to

• demonstrate an understanding of different wireless protocols.

• explain a MAC protocol for wireless LAN.

• explain implementation of BRAIN architecture.

• explain the HAMAC protocol.

• demonstrate an understanding of demand assignment multiple access protocols.

• explain the role of a Request Table in ARCMA.

• explain implementation of multiple RA channels.

Practice problems

4.1: What is the role of network and native service access points? 4.2: What is the RS-ISMA?

4.3: What are the functions of HAMAC protocol? 4.4: How is transmission performed in ARCMA? 4.5: What is the role of a Request Table?

4.6: How is dynamic RA channel implemented?

Practice problems solutions

4.1: A MAC protocol for a wireless LAN provides two types of data transfer SAP: network and native. The network SAP offers an access to a legacy network protocols (e.g., IP). The native SAP provides an extended service interface that may be used by custom network protocols or user applications capable of fully exploiting the protocol specific QoS parameters within the cell service area.

4.2: RS-ISMA is a wireless access protocol designed for wireless multimedia commu- nications and implemented in the BRAIN indoor-LAN prototype. RS-ISMA is a wireless MAC protocol, which is an integration of reservation-based ISMA and slot- ted ISMA, and is basically a combination of random access protocol and polling protocol. During the reservation step, an ST transmits a short frame to make a reser- vation under a random access scheme. In the information transmission step, either an isochronous or an asynchronous polling scheme is used for information transmission depending on the QoS requirements.

4.3: The HAMAC protocol integrates fixed assignment TDMA protocols, reservation- based protocols, and contention-based protocols into a wireless network, simultane- ously and efficiently supporting various classes of traffic such as CBR, VBR, and ABR traffic. The HAMAC protocol uses a preservation slot technique to minimize the packet contention overhead in PRMA protocols, while retaining most isochronous service features of TDMA protocols to serve voice and CBR traffic streams.

PROBLEMS TO CHAPTER 4 71

4.4: ARCMA is a DAMA protocol with dynamic bandwidth allocation. This scheme is designed to function in a cell-based wireless network with many MSs communicating with the BS of their particular cell. Transmissions are done on a slot-by-slot basis without any frames. Each slot is divided into a TA slot and an RA minislot. The RA channel in ARCMA is capable of carrying additional information for different classes of ATM service (e.g., CBR, VBR, etc.). This additional information is used by the BS to provide better QoS support for different classes of traffic. Transmission from CBR traffic may reserve an incremental series of slots in the duration of their transmission. No further request is needed until the CBR transmission finishes. 4.5: The BS maintains a Request Table to keep track of all successful requests and assigns

permission to mobiles for transmission at different time slots. In ARCMA protocol, the BS inspects the service class of a request and gives transmission priority to delay-sensitive data (e.g., CBR). A piggyback bit is used in the uplink channel to reduce contention in the RA channel. This is especially beneficial for bursty traffic. 4.6: ARCMA implements a dynamic RA channel in which an entire uplink channel can be converted into multiple RA channels. This conversion is done when the Request Table is empty, which in most cases indicates heavy collisions in the request channel. ARCMA uses an algorithm that takes advantage of the random access scheme in the RA channel. We use the slotted ALOHA with BEB as the random access protocol for ARCMA.

ARCMA improves the spectral efficiency by reducing collisions in the RA channel while improving support for the various classes of ATM services.

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