4
Wireless protocols
A MAC protocol for a wireless LAN provides two types of data-transfer Service Access
Points (SAP): network and native. The network SAP offers an access to a legacy network
protocol (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 Quality of Service (QoS) parameters within the cell service area.
Broadband Radio Access Integrated Network (BRAIN) is used for millimeter wave
band multimedia communications. In BRAIN, all Access Points (APs) need to have only
an optical/electrical (OE) converter because BRAIN incorporates radio on fiber technolo-
gies, which allow for transmitting radio signals through optical fiber cables.
The Hybrid and Adaptive MAC (HAMAC) protocol integrates fixed assignment Time
Division Multiple Access (TDMA) protocols, reservation-based protocols, and contention-
based protocols into a wireless network, simultaneously and efficiently supporting various
classes of traffic such as Constant Bit Rate (CBR), Variable Bit Rate (VBR), and Avail-
able Bit Rate (ABR) traffic. The HAMAC protocol uses a preservation slot technique to
minimize the packet contention overhead in Packet Reservation Multiple Access (PRMA)
protocols, while retaining most isochronous service features of TDMA protocols to serve
voice and CBR traffic streams.
Adaptive Request Channel Multiple Access (ARCMA) is a Demand Assignment Multi-
ple Access (DAMA) protocol with dynamic bandwidth allocation. This scheme is designed
to function in a cell-based wireless network with many Mobile Stations (MSs) commu-
nicating with the Base Station (BS) of their particular cell. Transmissions are done on
a slot-by-slot basis without any frames. Each slot is divided into a Transmission Access
(TA) slot and a Request Access (RA) minislot. The RA channel in ARCMA is capable
of carrying additional information for different classes of Asynchronous Transfer Mode
(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.
Mobile Telecommunications Protocols For Data Networks. Anna Ha´
c
Copyright 2003 John Wiley & Sons, Ltd.
ISBN: 0-470-85056-6
56 WIRELESS PROTOCOLS
4.1 WIRELESS PROTOCOL REQUIREMENTS
The general requirements for wireless protocols supporting wireless LANs are as follows:
•The low cost is achieved by simple implementation and the use of standard multipurpose
modules and components. Modularity and reconfigurability in all stages of system
design are the key elements to meet these requirements.
•The QoS requirements for the data-transfer service of the MAC protocol include support
for user-defined traffic types and connection parameters. The protocol must support
real-time data-transfer services.
•ThewirelessLANcanbeusedbothasanextensionandasanalternativetoawired
LAN. Therefore, for interoperability requirements, the changing topology of a wire-
less network, inadequate security and reliability of the medium, and protocol-specific
management functionality must be hidden from the network user, that is, from legacy
Transmission Control Protocol/Internet Protocol (TCP/IP) applications.
•Wireless medium does not provide the same level of confidentiality and user identifica-
tion as a wired system. A wireless coverage area cannot be reliably defined or restricted.
Actions at the MAC layer have to be taken to provide a secure data-transfer service.
•An unlicensed and globally available frequency band must be selected for the system.
•The architecture of the MAC protocol should follow a master–slave hierarchy as the
centralized control and management enables an easy and efficient support of QoS
parameters and an access point for outside network resources.
•To guarantee the low cost, efficient resource management and guaranteed QoS, the
number of simultaneous users in a single wireless LAN cell can be restricted according
to the target environment.
•The requirement for low power consumption follows from the usage of battery powered
portable network equipment, for example, laptops. A wireless network adapter should
not significantly shorten the operating time of a portable terminal. Therefore, the MAC
protocol should be capable of turning off the transceiver during idle periods without
missing any relevant transmission.
4.2 MAC PROTOCOL
Hannikainen et al. present a MAC protocol for a wireless LAN that provides two types
of data-transfer SAP: network and native. The network SAP offers an access to a legacy
network protocol (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.
The data processing block converts the user data into a more suitable form for the
wireless medium. Encryption is performed for confidentiality while fragmentation and
Forward Error Correction (FEC) coding functions are added for better protection of the
data against transmission errors. The frame queuing and Automatic Repeat Request (ARQ)
retransmissions are controlled according to assigned QoS.
MAC PROTOCOL 57
The control and management functionality consists of state machines that adapt to the
inputs from the management interface and data processing modules while producing an
output according to the current state of the system. The operational parameters are stored
in the Management Information Base (MIB), which can be accessed and modified through
the station management-user interface.
Both the Portable Station (PS) and BS functionality are assembled using the same
functional modules. The BS functionality is achieved by adding the base-specific modules
(data processing, control, and management) on top of the PS functionality. A set of BS
functions can be included into a PS capable of executing them (for instance, a laptop).
Thus, an ad hoc networking is enabled if no permanent BS service is available.
Hannikainen et al. present a connection-oriented wireless MAC protocol that uses a
reservation-based TDMA scheme with the shared medium. The medium access cycle is
divided into time slots that form CBR channels. Four types of channels are distinguished
by their purpose, direction, and bandwidth. These are data, contention, control, and beacon
channels. The data channel includes also acknowledgements.
The data channel can be reserved by a PS for uplink transmission of user data. The
data is forwarded by the BS; however, a direct data transfer between two PS under a BS
control is possible. The data channels remain reserved during data transmission and are
released by the PS request, or by the BS in the case of an idle reserved channel.
Another uplink control channel is formed by the Acknowledgement (ACK) messages
that follow each unicast transmission destined to a single station. The protocol uses a
store and wait flow-control scheme to enable short retransmission delays and fast adapta-
tion to the varying quality of radio link. The acknowledgements carry information about
successful or unsuccessful reception, and control information for bandwidth requirements,
which consists of the amount and priority of data queued in a PS for transmission. This
information is used by the channel-scheduling function of a BS to determine the uplink
or downlink direction of the reserved data channels and the possible requirements for an
extra bandwidth for each PS.
The PS transmits uplink control messages, such as channel reservation and association
requests, in a contention-based channel at the end of the access cycle. The contention
channel is constructed by a series of short contention slots that are monitored for a signal
carrier or energy for Carrier Sense Multiple Access (CSMA)–based transmissions. The
amount of idle contention slots to be detected before transmitting enables various control
message priorities.
The BS transmits downlink control messages in the control and beacon channels.
The control channel is a data channel that can be reserved for control and management
information transfer. Otherwise, the channel is used as a VBR data channel. The beacon
channel is used by the BS only for beacon messages. A beacon broadcasts the current
channel reservation state for the following access cycle. Beacons also carry cell-specific
information, such as a cell identification, structure of the access cycle, and indications for
a required confidentiality with the association and data transfer. A beacon frame indicates
the beginning of the access cycle, thus providing a TDMA cycle synchronization for
PS. The beacon carries indications for buffered data to those PS that use power-save
functionality. These stations power on their receivers only periodically to receive the
possible announcement with the beacon.
58 WIRELESS PROTOCOLS
4.3 BROADBAND RADIO ACCESS
INTEGRATED NETWORK
Inoue et al. present the BRAIN for millimeter wave band multimedia communications. In
BRAIN, all APs need to have only an OE converter because BRAIN incorporates radio-
on-fiber technologies that allow for transmitting radio signals through optical fiber cables.
Reservation Based Slotted Idle Signal Multiple Access (RS-ISMA) is a wireless access
protocol designed for wireless multimedia communications and implemented in the BRAIN
indoor-LAN prototype. In addition, a compact Radio Frequency (RF) module composed of
flat antennas and a Monolithic Microwave Integrated Circuit (MMIC) was employed for
each remote station and AP. The use of large capacity Field Programmable Gate Arrays
(FPGA) decreased the number of signal processing boards. System parameters such as the
packet format were optimized for Internet Protocol (IP) datagram transport to support all
applications based on IP. The function of Negative Acknowledgement (NACK) sensing
was added to RS-ISMA to ensure an efficient and smooth wireless multicast in a multiple
access environment.
BRAIN covers service areas with multiple Basic Service Area (BSA), which includes
an AP and a number of fixed and/or quasi-fixed stations (ST). ST usually employs a
directional antenna in millimeter wave band communications and communicates via AP.
Traffic generated from or arriving at the BSA passes through the AP, and thus the indoor
system is a centralized control system.
RS-ISMA is a wireless MAC protocol that is an integration of reservation-based ISMA
and slotted ISMA, and it is basically a combination of random access protocol and polling
protocol. During the reservation step, an ST transmits a short frame to make a reservation
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.
RS-ISMA was modified to carry IP datagram most efficiently and to support wireless
multicast. The MAC frame format has a fixed length to increase the signal processing
speed, resulting in increased radio transmission speed. The payload of modified RS-ISMA
is 64 octets and the header is 4 octets.
A Stop and Wait (SW) ARQ with a limited number of retransmissions is used for
both stream traffic and data traffic in the modified RS-ISMA because combining the TCP
error-recovery mechanism with SW ARQ allows for low-frame error rate necessary for
reliable transmission of data traffic.
In multimedia wireless LAN, the retransmission scheme for downlink frame transmis-
sion should enable broadcast and multicast of multimedia traffic to multiple users without
errors. The AP, after sending a data frame, transmits a polling signal whose control sig-
nal field indicates Acknowledgement Request (ACKR). In response to the ACKR frame
station, ST1 does not send a frame because it has received the data frame successfully.
Station ST2 sends an ACK frame that informs the AP that the data frame has not been
received successfully. The AP, which senses any carrier from STs during one time slot
following the ACKR, detects a carrier from station ST2. The AP generally does not know
which ST has transmitted a signal since there may be more than two STs that are sending
HYBRID AND ADAPTIVE MAC PROTOCOL 59
ACK frames because they have not received the downlink data frame without errors.
After detecting any carrier, the AP retransmits the data frame, which will be received by
both stations ST1 and ST2 but will be ignored by ST1.
4.4 HYBRID AND ADAPTIVE MAC PROTOCOL
Wang and Hamdi propose a MAC protocol HAMAC, which, integrates fixed assignment
TDMA protocols, reservation-based protocols, and contention-based protocols into a wire-
less network, simultaneously 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.
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.
In the HAMAC protocol, the uplink frame consists of three segments. The first segment
is used by the mobile devices to upload the CBR data using a TDMA round-robin scheme.
There are two types of slots in this segment: the preservation slot and the normal slot.
The preservation slot is used to preserve the position for a CBR connection when it is in
a silent state. The length of the preservation slot should be as short as possible. During
the transmission of the preservation slot, all mobile devices in the same cell should have
enough time to recognize the existence of preservation slot or the existence of silent CBR
connection. The preservation slot is not useful for the BS, and it is discarded by the BS
and does not appear in the downlink frame.
When the preservation slot of a CBR connection is present, the remaining bandwidth of
the connection is free. When the CBR connection becomes active again, the preservation
slot is replaced by the normal slots and the allocated bandwidth for the connection cannot
be used by the other connections and mobile devices. The HAMAC protocol avoids the
reservation operation before the transmission of an active talk spurt, and the BS is not
aware of the state transition of the CBR connection. As a result, there is no need to
make the presence or absence of the preservation slot known to mobile devices using a
downlink frame. The preservation slot can appear or disappear without any notification.
The HAMAC protocol uses the continuous bit to compress the header information
of consecutive slots when they belong to the same traffic source. In the continuous bit
technique, the position of the slots allocated to the connections can float in the uplink
frame, rather than having the slots allocated to a connection being assigned to a fixed
location. In HAMAC protocol, the location of the slots allocated to the connection, defined
as an access point, is assigned as the function of the number of continuous bits rather