4.1 Introduction
WMNs require the coordination and collaboration of mesh APs over multiple hops. Therefore, new MAC features designed specifically for WMNs become necessary to improve the performance of such networks.
Recently, an IEEE 802.11 task group (TGs) was formed to draft a standard for wireless mesh networking. The first baseline draft of 802.11s supports the 802.11 distributed coordination function (DCF) protocol and the 802.11e enhanced distributed channel access (EDCA) protocol with several additional MAC features.
The Optional MAC Enhancements include:
Multichannel MAC protocol, i.e., common channel framework (CCF)
Mesh deterministic access (MDA) scheme that offers better QoS.
An intra mesh congestion control scheme that seeks to relieve the congestion situations among wireless mesh nodes
Power Management 4.2 The MAC frame format
Table 4.1 depicts in general the MAC frame format [10, 18, 32, 34].
Table 4.1: MAC Frame Format
4.2.1 Mesh control field
The mesh control field is a 6 to 24 octet field shown in the Table 4.2 below. The control field helps to support all kinds of unicast, multicast and broadcast traffic. The control field includes:
An 8-bit mesh flag field
Mesh Time to Live
Mesh Sequence Number
Mesh Address Extension
Table 4.2: Mesh Control Field
4.2.2 Mesh flags field
The first octet contains an 8-bit mesh flags field which is used to control mesh specific header processing. As an example, the first two bits indicate the presence of Address Extension (AE).
Another bit indicates the power save level while the rest of the bits are reserved for future use. Table 4.3 shows the Mesh flags Field.
Table 4.3: Mesh Flags Field
4.2.3 Mesh Time To Live
The second octet defines the mesh time to live (TTL). “The field is also 8 bits in length containing an unsigned integer that counts down depending on the number of times remaining that the frame can be forwarded.” It is used to mitigate the frame being forwarded in an infinite loop. Hence once the decremented integer in the frame goes to zero, the next hop device discards the frame.
4.2.4 Mesh Sequence Number field
The third and fourth octet defines the mesh sequence number (SN) field. This sequence number is used by the receiving station to detect duplicate broadcast frames and avoid unnecessary retransmissions.
4.2.5 Mesh Address Extension field
The mesh address extension field works in tandem with the mesh flag field when the entry in the address extension (AE) mode is a non-zero entry value. It contains 6, 12 or 18 octets in length. Table 4.4 shows the mesh address extension field which contains three additional address fields for mesh address extensions.
Table 4.4: Mesh Address Extension Field
“The address 4 field is used in the mesh management frames of subtype multi-hop action to include a fourth address that is missing from the MAC header of management frames.
Address 5 and Address 6 are very useful in cases where the endpoints are non-mesh entities. Their source and destination addresses are transported in this address field”.
4.3 Background
4.3.1 CSMA and CSMA/CA
In Carrier sense multiple access (CSMA), a node first senses the channel to make sure that it is idle before transmitting. If the channel is busy, the node defers its transmission.
Using carrier sensing, a node can successfully avoid collisions with transmitting stations within its carrier sense range.
CSMA with collision avoidance (CSMA/CA) leverages the performance benefits of CSMA and extends CSMA to further reduce the likelihood of collisions.
In a wireless network, as radio signals attenuate over distance, simultaneous transmissions may lead to collisions at the receiver even though both senders have sensed an idle channel.
This is called the hidden node problem. By utilizing two small control packets, i.e., request-to send (RTS) and clear-to-send (CTS), CSMA/CA can effectively mitigate the hidden node problem [1, 3, 31].
4.3.2 IEEE 802.11 DCF Protocol
The IEEE 802.11 standard specifies two medium access methods:
(1) Distributed coordination function (DCF) that builds on CSMA/CA
(2) Point coordination function (PCF) providing contention-free access.
Because PCF requires a central control entity, i.e., a point coordinator, it is rarely used in WMNs.
Additionally, because of its robustness and flexibility, many advanced MAC protocols are also based on the IEEE 802.11 DCF protocol.
The IEEE 802.11 DCF protocol is based on the CSMA/CA principle and it operates in a similar way. A node wishing to transmit first senses the channel. If the medium is sensed busy, it defers its transmission. If the medium is free for a specified period of time called distributed inter frame space (DIFS), the node is allowed to transmit. Upon correctly receiving the data packet, the receiver returns an ACK after a fixed period of time called short inter frame space (SIFS). Receipt of the ACK indicates the correct reception of the data packet.
If no ACK is received, the sender assumes a collision has occurred and doubles the size of its contention window. Then, the sender chooses a random back-off number between 0 and its contention window size. The sender is allowed to retransmit the packet when the channel is free for a DIFS period of time augmented by the random back-off time. The packet is dropped after a given number of failed retransmissions [3, 7, 31].
4.3.3 IEEE 802.11e MAC Protocol
The IEEE 802.11e standard draft defines a number of QoS enhancements to IEEE 802.11. Two main functional blocks are defined in IEEE 802.11e:
(1) The channel access functions
(2) The traffic specification (TSPEC) management.
The channel access function defines a new coordination function called the hybrid coordination function (HCF).
HCF has two modes of operation: a contention based protocol called enhanced distributed channel access (EDCA) and a polling mechanism called HCF controlled channel access (HCCA).
EDCA enhances the original DCF by providing prioritized medium access based on different traffic classes, also called access categories (ACs).
The IEEE 802.11e defines four ACs, each of which has its own queue and its own set of EDCA parameters. The differentiation in priority between ACs is realized by setting different values for the EDCA parameters.
The EDCA parameters include:
(1) Arbitration interframe space number (AIFSN) (2) Minimum contention window (CWmin) (3) Maximum contention window (CWmax) (4) Transmission opportunity (TXOP) limit.
AIFS is the period of time the wireless medium is sensed idle before the start of a frame transmission.
Although, real-time traffic such as video and voice has more aggressive EDCA parameters, which is to ensure QoS traffic
has a better chance to acquire the medium than the best-effort or background traffic. This basic idea of supporting prioritized traffic [1, 3, 7, 9].
4.4 Enhanced Distributed Channel Access
The EDCA is a distributed, contention based medium access mechanism. It is an extension of the DCF. EDCA is the default and mandatory MAC function for all mesh stations in the 802.11s draft standard. It provides MAC QoS enhancement introduced by 802.11e.
In EDCA, stations access the wireless Medium Using 8 different user priorities. This means that packets forwarded by the stations are assigned priority values before access into the MAC based on the information they carry.
These packets are then mapped into four different categories called Access categories (ACs) implemented in EDCA which include [6, 7]:
Voice single hop networks, it has inherent problems like in the case of high load, EDCA devices throttle themselves as unsuccessful transmission leading to increasing contention window sizes and making EDCA less efficient in high medium usage [18].
4.5 Common Channel Framework
An optional CCF is proposed to enable the operation of single-radio devices in a multichannel environment.
The CCF assumes that each node is equipped with a single half-duplex transceiver and nodes in the network or in the same cluster share a common control channel. To legacy devices (STAs and AP) and MPs that do not support the CCF, the common channel appears as any other 802.11 channel and their operation remains unaffected.
Using the CCF, node pairs, select a different channel and switch to that channel for a short period of time, after which they return to the common channel. During this time, nodes exchange one or more DATA frames. The channel coordination itself is carried out on the common channel by exchanging control frames or management frames that carry information about the destination channel. In this way, simultaneous transmission on multiple channels is achieved which in turn results in increased aggregate throughput.
As shown in Figure 4.1, MPs communicate to each other and utilize the common control channel to select an available data channel [3, 10, 25].
Figure 4.1: channel selection on the common control channel.
4.6 Mesh Deterministic Access
MDA allows MPs to access a certain period with lower contention than that in other periods without MDA. Such a period is called an MDA opportunity (MDAOP). Before using MDAOP to access the medium, the owner of this MDAOP, i.e., the transmitter, needs to set up the MDAOP with its receiver [3, 10, 19].
If this period is accessed by the transmitter, i.e., the owner of MDAOP, it attempts to use CSMA/CA but uses new backoff parameters, MDA maximum contention window (MDACWmax), MDA minimum contention window (MDACWmin), and MDA interframe space number (MDAIFSN), to set up an TXOP. However, for a nonowner of TXOP, it has to defer its access by setting its NAV to the end of the MDOAP or by using a carrier sensing scheme [20, 23, 25, 31].
4.7 Intra mesh Congestion Control
In an IEEE 802.11s network intra-Mesh congestion control is achieved by implementing the following three main mechanisms [33]:
Local congestion monitoring
• Each node actively monitors local channel utilization
• If congestion detected, MP notifies previous-hop neighbors and/or the neighborhood
Congestion control signaling
• Congestion Control Request (unicast)
• Congestion Control Response (unicast)
• Neighborhood Congestion Announcement (broadcast)
Local rate control
• Each node that receives either a unicast or broadcast congestion notification message should adjust its traffic generation rate accordingly packets will have to be dropped from the buffer.
The situation is exacerbated by the presence of hidden and exposed nodes on the same channel causing extensive back-off and retransmissions [3, 10].
Congestion control may not be as critical in a wired network as it is in a wireless network, because each individual hop in the wired network is isolated from other hops, also it does not work well across a multihop wireless network largely due to its susceptibility to high packet loss [22, 24, 25, 26].
4.8 Power Management
Many nodes in 802.11s mesh networks always work in an active state since they either need to be an AP or to forward traffic for other nodes. However, there are still other nodes that need to work in power-save mode [10, 24, 25].
All MPs have the capability to operate in power save mode. More specifically, two different power states are considered [33]:
Awake: the MP is able to transmit or receive frames and is fully powered.
Doze: the MP is not able to transmit or receive and consumes very low power.
The manner in which an MP switches between these two power states is determined by the Power Management mode of MP .
These include:
Active mode: the MP shall be in the Awake state all the time.
Power save mode: the MP alternates between Awake and Doze states, as determined by the frame transmission and reception rules.