MAC & Physical Layer Standards

Physical layer is the lowest layer of the protocol stack and is responsible for trans- mitting raw bits over the hardware transmission medium. For wireless communications, physical layer receives and serialises the frame from data link layer and sends it to the corresponding receiver over electromagnetic radio waves. Physical layer protocols mainly take care of definition of hardware specifications which includes the details of operation of each device such as wireless radio transceiver and network interface card, data encoding and signalling, and data transmission and reception.

The MAC (Medium Access Control) layer is a sub-layer of the data link layer, which provides point-to-point data transmission. The main function of MAC protocol is to al- locate wireless resources to multiple concurrent network devices connected to the same physical medium. The MAC protocol is responsible for achieving efficient resource usage and provides a certain level of reliability to upper layers. MAC layer interacts with physical layer and controls the wireless network interface card (WNIC) which involves a significant impact on the energy consumption involved by the sleep/wake cycles of the WNIC.

Two main functions of MAC layer protocols are: addressing mechanism and access control mechanism. In the addressing mechanism, there is a unique MAC address which is also called physical address of each network interface. The address combined with upper layer IP address enable packets to reach their destination. To be more specific, when a packet is forwarded to the destination sub-network, the IP address is resolved into the physical address of the destination host for successful delivery.

The channel access control is designed to provide access for connection of wireless devices to the shared physical medium. It is critical as packet collisions may easily oc-

cur under wireless conditions if two or multiple wireless hosts transmit data at the same time. Channel access control schemes are categorized according to the way resources are allocated to multiple hosts [59]. The most frequently used channel access methods are Fre- quency Division Multiple Access (FDMA) which divides and allocates frequency bands to users, Time Division Multiple Access (TDMA) which allocates different time slots to pre- vent collisions, and Code Division Multiple Access (CDMA) which allows simultaneous transmission by multiplexing.

IEEE 802.11 [27] is an IEEE Standard for both MAC and physical layers and it is the fundamental technology used by Wi-Fi. It supports both contention based medium access scheme and contention free scheme.

The contention free scheme is Point Coordination Function (PCF) built on infrastruc- ture based scenarios. In this scenario an Access Point (AP) functions as a central node, which receives and forwards all packets sent within its service set. Packet collision is avoided due to the coordination of the AP. It has a built-in Power Saving Mode (PSM) and is the basis of most MAC layer power saving schemes. In PSM, time is divided into bea- con intervals, on a regular base of which the access point broadcasts beacon frames. One component of the beacon announcement is the maximum duration of the contention-free period, CFPMaxDuration. All the associated stations set the Network Allocation Vec- tor (NAV) timer to the maximum duration to lock out Distributed Coordination Function (DCF) based access to the wireless medium. On the mobile nodes side, the standard enables network card to sleep for a fixed duration, i.e. one or several round of beacon intervals, and wakes up listening to beacons. Once the node finds there are packets addressed to it, rounds of polling would take place. For each polling process, one buffered data will be transferred to and acknowledged by the receiver. The whole process ends when all buffered data is retrieved by the wireless node. On the other side, AP buffers packets for sleeping nodes, and notifies those nodes by beaconing regularly. Figure 2.10 illustrates the buffered data retrieval process.

Figure 2.10 Buffered frame retrieval process in PCF

tiple Access with Collision Avoidance (CSMA/CA) to avoid packet collision. CSMA/CA is a contention avoidance scheme which can be deployed in IEEE 802.11-based protocols. Nodes wishing to send packets wake up and listen to the channel for a DCF Interframe Space (DIFS) interval. If the medium is free, transmission will start immediately; other- wise the node will back off and the station defers its transmission. In a network where a number of stations contend for the wireless medium, if multiple stations sense the channel busy and defer their access, they will also virtually simultaneously find that the channel is released and then try to seize the channel. As a result, collision occurs. In order to avoid such collisions, DCF also specifies random backoff, which forces a station to defer its ac- cess to the channel for an extra period. The length of the backoff period is determined by the following equation:

Backof f T ime = random() ∗ aSlotT ime (2.1)

This process repeats until packets are transmitted successfully. DCF also employs an op- tional virtual carrier sense mechanism that exchanges short Request-To-Send (RTS) and Clear-To-Send (CTS) frames between source and destination stations during the intervals between the data frame transmissions. The RTS packet is transmitted after the node senses an idle network for the duration of DIFS, and the receiver will reply with a CTS packet af-

Figure 2.11 Data transmission process in DCF

ter a Short Inter-Frame Space (SIFS). Data packets are only transmitted after the exchange of the pair of packets. Virtual carrier-sensing is provided by the NAV. Most 802.11 frames carry a duration field, which can be used to reserve the medium for a fixed time period. The NAV is a timer that indicates the amount of time the medium will be reserved, in mi- croseconds. The station which is about to transmit data sets the NAV to the time for which they expect to use the medium, so that other stations can count down from the NAV to 0. After this period, the medium should be idle. Power saving under these circumstances can be achieved through preventing packet collision, and turning off the radio while packets transmission is happening among other nodes. Figure 2.11 illustrates the event sequence of DCF.

IEEE 802.16 which is widely known as the supporting technology of WiMAX stan- dardizes air interface and related functions for wireless broadband. The standard divides MAC layer into three sub-layers including MAC security sub-layer which provides data encryption and MAC address based authentication, MAC common part sub-layer which provides medium access, Quality of Service (QoS) etc, and MAC convergence sub-layer which provides interface to various upper layer protocols such as IP and Ethernet. MAC in IEEE 802.16 is a connection-oriented solution with a Base Station (BS) allocating both

uplink and downlink bandwidth. The connection-oriented feature enables strong support of quality of service as different bandwidth allocation to individual sessions is allowed and the connections are unidirectional.

Five sets of services are supported with various specifications on QoS parameters: Un- solicited Grant Service (UGS), real-time Polling Service (rtPS), non-real-time Polling Ser- vice (nrtPS), Best Effort (BE) and extended real-time Polling Service (ertPS). Each of these scheduling services has a mandatory set of QoS parameters that must be included in the service flow definition when the scheduling service is enabled for a service flow, as shown in Table 2.3. UGS is designed to support real time data streams consisting of fixed-size data packets issued at periodic intervals. It is used in applications such as Voice over IP (VoIP) without silence suppression. The ertPS service is added by the 802.16e amendment. It is built on the efficiency of both UGS and rtPS. Similar to UGS, the base stations pro- vides unicast grants in an unsolicited manner, but variable-sized data is allowed. The rtPS type is designed to support real-time data streams consisting of variable-sized data packets that are issued at periodic intervals. The service can be used in Moving Pictures Experts Group (MPEG) video transmission and real time video delivery. It introduces more request overheads than UGS, but supports variable grant sizes for optimum real-time data transport efficiency. The nrtPS scheduling service aims at supporting delay-tolerant data streams consisting of variable-size data packets for which a minimum data rate is required. NrtPS can be employed for applications such as FTP transmissions. The BE service is designed to support data streams for which no minimum service guarantees are required and therefore may be handled on a best available basis.

Power saving in IEEE 802.16 is achieved through the employment of sleep or idle modes of mobile stations (MS). In idle mode, the mobile station is not registered with any base station, but it receives downlink traffic through paging. The sleep mode consists of three classes supporting different quality of service levels. In power save class one the sleeping window of mobile station increases exponentially to achieve maximum energy saving, whereas in power save class two the sleeping window size is fixed. The third power save class is a one-time scheme which means the transceiver of mobile device sleeps for a

Table 2.3 WiMAX Services and QoS Requirements

Service QoS Specifications Application

UGS

• Minimum reserved rate • Maximum sustained rate • Request/transmission policy • Tolerated jitter

• Maximum latency tolerance

VoIP

ertPS

• Minimum reserved rate • Maximum sustained rate • Request/transmission policy • Tolerated jitter

• Traffic priority

• Maximum latency tolerance

Voice over IP without silence suppression

rtPS

• Minimum reserved rate • Maximum sustained rate • Request/transmission policy • Maximum latency tolerance

Streaming audio or video; Tele medicine; E-learning

nrtPS

• Minimum reserved rate • Maximum sustained rate • Request/transmission policy • Traffic priority

FTP, document shar- ing

BE

• Minimum reserved rate • Request/transmission policy • Traffic priority

predefined period and then returns to the normal mode.