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MAC protocols for Wireless Mesh Network
Musaab M. JasimFatih university
Computer department
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Outline Introduction of WMN architecture and Important
definitions . Data link layer and MAC definition & classification . Single channel concepts and protocols.
S-MAC . T-MAC . New TDMA-based MAC .
Multi channel concepts and classification . CC-MMAC . SSCH MAC .
Conclusion
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Introduction of WMN architecture and Important definitions .
Introduction of WMN architecture .
First of all , to understand this article , we need know some information about WMN architecture in addition to some important definitions.
WMN consists of three types of componentsMesh routers .Mesh client devices .Router with gateway/bridge functionalities which enable the integration of WMNs with various existing wireless networks.
depending on these components there are three types of WMN architecture .
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1- Infrastructure Meshing
It includes mesh routers that form backbone infrastructure for conventional clients that connect to them . and enable integration with other wireless networks by gateway/bridge functionalities .
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2- Client mesh networking
It consists of mesh clients devices which provides peer-to-peer networking . These devices have to perform additional functions such as routing and self-configuration .
3- Hybrid mesh networking
It is combination from the first two types .
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Depending on these types and because the WMN is Mix of many wireless network . so, in this article we will discuss two classes of protocols which can be used in WMNs , the first one are (S-MAC , T-MAC and TDMA-MAC ) protocols from standard IEEE 802.11 based on CSMA\CA protocols for the single channel , and the second are SSCH-MAC and CC-MMAC for the multiple channel .
Important definitions
To understand this article , we need know some definitions which will be contained in this presentation .•Throughput\ it is the amount of data moved successfully from one place in network to another in a given time .•Fairness \ it means using the system channel and resource in a fair way as possible .
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• Hand shake \ it is transmission\receiving technique which achieved in three levels :I. RTS : Request To Send .
II. CTS : Clear To Send .
III. ACK : Acknowledgment .
With some in-between periods such as- IFS : inter-frame space .
- SIFS : short inter-frame space .
- DIFS : distributed coordination function inter-frame space .
• Duty Cycle \ The fraction of time a system is actually employed in performing its function. The percent of time that the system is active as opposed to the time the system is inactive .
• Overhead \ the processing time required to transmit control packets which uses for error checking and control of transmission .
• ARQ \ error detection method use by link layer to check the error and retransmit the lost packets . And it is abbreviation of ( Automatic Repeat reQuest .
• ATIM \ Ad-hoc Traffic Indication Message.
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Data link layer and MAC definition & classification .
Data link layer & MAC definition
It is the second layer in the WMN stack layer which be responsible of ( Framing , Error detection and correction , Media Access control ,…) , which divide into two sub-layer :
1- logical link control (LLC) .
2- media access control (MAC) .
In this article we will deal with the MAC sub-layer only .MAC \ it is the second sub-layer of data link layer that provide addressing and channel access control protocols and mechanisms to make it possible for several terminals or network nodes to communicate within a multiple access network that incorporates a shared medium . So, it is interface between the (LLC) sub-layer and physical layer.
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MAC protocols classification for WMN
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Mac protocolsMac protocols
distributeddistributed centralizedcentralized
Single Single channelchannel
Multi Multi channelchannel
Multi Multi transceivertransceiver
Multi radioMulti radiosingle single transceivertransceiver
Based-Based-contentioncontention
Schedule-Schedule-contentioncontention
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Single channel concepts and protocols .
Single channel concept and protocols
The Concept-It means that the nodes of WMN use shared single channel for transmission\receiving data . So, only one node can access to media at a time .-The main problem faced by the network is the "collision" which occurs when two nodes in the network send the data at the same time over the common channel and this results in corrupted of data . This problem caused by two reasons :
1- Hidden node .
2- Exposed node .
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Hidden node It is one of the collision problem reasons which occurs
when two nodes have the same neighbor but each one outside the scope of other coverage . As shown in the example below
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1. “A” tend to send data to “B” .
2. “C“ tend to send data to “B“ too .
3. Because of “C” outside the
transmission range of “A” . So, “C”
is hidden node of it .
4. Each “A” and “C” will send the
data to “B” over the same channel
(shared channel) .
5. The receiving data on the “B” will
collide and be corrupted .
Exposed node It is one of the collision problem reasons which means a
node is prevented from sending packets to other node due to a neighboring transmitter. As shown in the example below
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1. “S1” tend to send data to “R1” . So
it send the request packet which
be hearing by “S2”.
2. “S2“ tend to send data to “R2” but
after it heard the “S1” request
packet it will go on the silent state
and prevents to sending the data
until the “S1” accomplish the
transmission , and that results in
latency and reducing in the
network throughput .
To avoid these problems of the single channel network
there are two approaches of MAC protocols :
1. Based-contention protocols .
2. Schedule-based protocols .
In this article , we will deal with two types of based-
contention protocols :
S-MAC protocol .
T-MAC protocol .
and one type of the schedule-based protocol
New TDMA-based protocol .16
S-MAC protocol The objective of the S-MAC is to reduce the collision & overhearing
problem and provide a good energy-efficiency . Its mechanism based on the fixed duty cycle idea and provide
synchronization between the node .
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S-MAC time slot
It contains a periodic time interval in which the node go on one of two modes :- Listen mode\ in which the node goes on the active mode by exchanging the
SYNC packet with the neighbors to prevent the node’s clock from drifting and
maintaining the synchronization by adjusting its timer counters depending on
the SYNC packet value , then check the channel to determine if it needs to
communicate with other node by hand shack mechanism .
- Sleep mode\ after expiration the listen interval the node go into sleep mode
by switch off the radio and sets the timer to awake later .
Synchronizer and Follower node At the start up , the node try to establish the initial schedule by listing for a
certain time if it is not hear schedule from the another node , it will establish private schedule and immediately broadcast this schedule to the neighbor nodes to follow it , so this node called (Synchronizer) and the nodes which follow this table called (Follower ).
But if a node receives a neighbor's schedule after it selects its own schedule , it adopts to both schedules and broadcast its own schedule before going to sleep .
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Collision and overhearing avoidance S-MAC protocol avoids the collision By
using (RTS,CTS,ACK) packet from the carrier sense mechanism.
And by using the network allocation vector (NAV) which contain the transmitted period of the neighbor transmission which is delivered from the duration field in transmitted packet , it can avoid overhearing , every time it tend to transmit data , it check the physical and the virtual carrier sense (NAV) if it are zero (free media) to send.
Message passing technique Consider the previous Example , if
the transmitted data frame is a big and do not reach to the receiver or the ACK packet from receiver lost, the sender will wait for fixed time then use ARQ to retransmit the a big data to receiver again, and this means consuming another time and more power .
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To avoid this state ,S-MAC using message passing technique by dividing the data into small parts called (fragments) . So, when one of these fragments not reach the node will need only less time and power for retransmitted it , but the disadvantage of this technique is reduction the fairness . As shown in the figure where the node “C” will wait a long time to access media.
T-MAC protocol It is contention-based single channel protocol , belongs to CSMA\CA protocols
family from standard IEEE 802.11 . It considered the enhanced version of S-MAC by using adaptive duty
cycle with dynamical listen period called Traffic Aware(TA) instead of fixed duty cycle.
By its mechanisms ,a node will keep listening and potentially transmitting as long as it is in active period and goes to sleeping mode when the active period ends and no activation event has occurred for a time (TA) .
TA must be long enough to receive at least the start of the CTS packet to preventing node from going to sleep mode while its neighbors are still communicating . So, the lower limit on the length of interval TA given in this formula .
TA > C + R + T C : length of the contention interval . R : the length of RTS packet . T : SIFS “ short inter-frame space .
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T-MAC mechanism and early sleeping problem
Mechanism
Consider the example :- A , B , C are nodes in the network .- within (TA) the nodes contend to gain
access to the channel . At the end the A get the channel .
- A send RTS to B and B send CTS to A , and C .
- When C hears the CTS , it stills for the rest of TA then will go to sleeping mode .
- When A receive the CTS packet , it initiates the transmitting data to B.
- when the transmission has accomplished , B send ACK packet to A and C .
- Now C can transmitting its data over the channel .
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T-MAC mechanism and early sleeping problem Early sleeping problem
Consider the example :- A , B , C , D are nodes in the network .- within (TA) the A and C nodes contend to
gain access to the channel .but C loses the contention and A get the channel .
- A send RTS to B and B send CTS to A , and C .
- C tends to sending data to D but because of CTS it waits for random Backoff time.
- But D did not hear any thing , so after (TA) go to sleep mode .
- When the transmission is accomplished between A and B , B will send ACK to A and it be heard by C .
- Now C send RTS to D to initiate the transmitting but D do not response , because of early sleep mode .
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T-MAC mechanism and early sleeping problem Solve “Early sleeping” problem
There are two solution for Early sleeping problem :
1. Future Request To Send (FRTS) packet- When C hear the CTS from B , it will set the
time of the next transmission in the (FRTS) packet and send it to D .
- D immediately sets its timer counter depending on the FRTS to wake up for potential future transmitting .
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2. Taking priority on full buffers- It means that C have to catch the channel
when it hears CTS from B by sending RTS to B to force it to wait for random Backoff time until it accomplish sending data to D and send ACK .
- thus , it avoid the early sleeping problem in D .
Innovative TDMA MAC protocol It is a new TDMA-based (Time Division Multiple Access) MAC protocol
(proposed protocol) compatible for wireless mesh backbone using directional antennas and provide a parallel transmission which means
that the different concurrent transmissions have the same start and end times . It is designed only for uniform grid topologies where each node has
four neighbors and is equipped with four beam antennas which be non-overlapped directional antennas .
the Routers in this protocol can transmit or receive data by using 2 modes (directional or omnidirectional mode) .
Three sets of neighbors are used in this protocol: OO-neighbors/DD-neighbors when both the transmitter and the receiver use omnidirectional/directional mode respectively, and DO-neighbors when either the transmitter or the receiver uses the directional mode.
Its mechanism depending on dividing the time sequence into slots which involve mini-slots.
So, it is designed to be collision free, deafness-free and blocking-free. 24
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Innovative TDMA MAC protocol
Time division and mini-slot assignmentThe time sequence in nodes is divided into slots. Each slot is divided into a
control part and a transmission part.
- Control and transmission part•The goal of the control part is to decide which nodes will transmit in the following transmission part and which nodes will receive.•It is divided into N mini-slot where N represents the number of nodes in the most load two-hop neighborhood. Each mini-slot is divided into three mini-parts.•In order to minimize the control overhead, the transmission part is much greater than the control part.
Innovative TDMA MAC protocol
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- Mini-slot assignment
• The goal of the mini-slot assignment is to
assign a number of nodes to each mini-slot.
• The mini-slot assignment should take into
consideration that two nodes in the same
two-hop neighborhood cannot be assigned
to the same mini-slot , to avoid a collision
when the same two-hop neighborhoods
begin a transmission at the same time .
• the mini-slot assignment achieve by the
algorithm which offers a flexible and
scalable way for assigning time-slots to
wireless routers .
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Innovative TDMA MAC protocol- TDMA frameworkTo explain the operations realized by our protocol and understand it , we need to consider the example in the right side which be a network with 6 nodes in chain topology . Where A,B,C is within the two-hop neighborhoods of the two other. So, each one of them should be assigned to a separate mini-slot . Thus,TDMA assigns a number of nodes to each mini-slot.
here it assigned (A,D),(B,E) and (C,F) to mini-slots respectively .
1.The first mini-slot , D has in the head of its queue a packet intended to transmit it to C. so, what are the procedure to accomplish this transmission ?!
First mini-part\ D transmits a jam signal using the omnidirectional mode.so , C and E know that D will be busy in the transmission part .
Second mini-part\ D transmits a jam signal toward C using the directional mode . So, C knows that it is the corresponding receiver of D and E knows that it is not the corresponding receiver of D .
Third mini-part\ C transmits a jam signal using the omnidirectional mode. So, B know that C will be busy in the transmission part .
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Innovative TDMA MAC protocol- TDMA framework
2.The second mini-slot where B and E existing, During the first mini-slot :
A. If node B has in the head of its queue a packet intended to transmit it to node A, it follows the same procedure of node D.
B. In the other case, if node B has in the head of its queue a packet intended to transmit it to C, it defers the transmission of this packet to the next mini-slot.
C. Then B checks if it has another packet in the queue intended to transmit it to node A. If such packet exists, node B follows the procedure of node D; otherwise B remains idle during its mini-slot.
D. Node E follows the procedure of node B by preventing the transmission toward node D.
2.The same procedure is repeated in the third mini-slot.
The transmission will take place just after the end of the control part. The precedent mechanism is repeated at the beginning of each slot.
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Multi channel concepts and classification
Multi-channel concepts and classification Concept :
It means , exploit the multiple frequencies of the medium for enabling transmissions on different non-conflicting frequencies .
There are many ways to classify the multi-channel protocols :
1. Depending on its mechanism
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Multi-channel MAC Multi-channel MAC protocolsprotocols
Handshake Handshake protocoleprotocole
Hopping Hopping sequencesequence
Directional Directional antennaantenna
Multi-channel concepts and classification
2- Depending on the channel assignment techniques
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Multi-channel MAC Multi-channel MAC protocolsprotocols
Dynamic channel Dynamic channel assignment assignment
Split phaseSplit phase
Static channel Static channel assignment assignment
Dedicated Dedicated control control channelchannel
Common Common frequency frequency hoppinghopping
Multi-channel concepts and classification
3- Depending on the node transceiver
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Multi-channel MAC Multi-channel MAC protocolsprotocols
Multi-RadioMulti-RadioSingle Single
transceivertransceiver Multi- transceiverMulti- transceiver
In this presentation we will discuss two multi-channel protocols- CC-MMAC : Channel Cognitive Multi-Channel MAC protocol. - SSCH : Slotted Seeded Channel Hopping .
CC-MMAC protocol
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CC-MMAC is a multi-channel MAC protocol which be enhanced version of the standard 802.11s with Common Channel Framework (CCF) and (DCF) protocol.
Whereby this protocol , each node uses
A. PCL (Preferable Channel List) to record the usage of channels in the transmission range of the node .
B. NCL (Neighbor Channel List) to record the usage of channel of the neighbor nodes.
It do not need special control channel, and requires a network interface. By using the interactive of the ATIM, ATIM-ACK and ATIM-RES , The Protocol
selects the channel . Where the ATIM frame contain the information of PCL and NCL which sends to the receiving node .
After the ATIM window, data will be transmitted by exchanging RTS/CTS in the selected channel. The RTS/CTS scheme can avoid collision occurred by the different source-destination nodes to use the same data channel at the same time.
In this protocol , the default channel can also be used to transmit data.
CC-MMAC protocol
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In CC-MMAC protocol, each node maintains two data structures.
1. FCL ( Free Channel List) which uses to update PCL part in ATIM and contains the node local view on the channels.
2. NFCL (Neighbor-Free Channel List) which uses to update NCL part In ATIM and contains the node local view on the channels of two-hop nodes .
The channel state types depending on CC-MMAC
There are three states of the channel in this protocol
HIGH preference (Hp, FCL[c] =-1 ) If the channel has already been selected by the node for use in the current beacon interval .
Medium preference (Mp, FCL[c]= 0) If the channel has not yet been taken for use in the transmission range of the host .
Low Preference (Lp, FCL[c] = n, n is the integer except 0) If the channel is already taken for use in the transmission range of the host.
And to avoid the interference cases as much as possible, there is a counter for each channel in the FCL and NFCL to record how many (source-destination) pairs want to use the channel for the current interval.
CC-MMAC protocol
Suppose that node A has packets for B and thus sends an ATIM packet to B during the ATIM window , with A's PCL and NCL included in the packet.
On receiving the ATIM request, B decides which channel to use during the beacon interval, based on its PCL, NCL and A's PCL, NCL.
The selection procedures used by B are :-1. If there is a Hp channel in the receiving node's
PCL and NCL, this channel is selected.
2. Else if there is a Hp channel in the source node's PCL and NCL, this channel is selected.
3. Else if there is not Hp channel, add the counters of the source node's PCL, NCL and the receiving node's FCL, NFCL to calculate a channel that is the smallest interfere for the destination node, or the source node , and the least usage times in source's PCL or destination's PCL will selected.
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Mechanism & Rules for selecting the channel
SSCH MAC protocol
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It is multi-channel single transceiver MAC protocol belongs to Dynamic Channel Assignment \ independent frequency hopping class .
It does not requires dedicated control channel, but needs clock synchronization among nodes.
So, every channel can be regarded as a control channel and wireless devices can employ different channels simultaneously to exchange control information.
And this removes the congestion of control packets and result in increasing the channel capacity .
Each node in SSCH has channel hopping sequence which calculated based on initial channel index and seed type.
And each one can know the other’s hopping schedule by broadcasting their channel schedules .
So, when a node has a packet queued for destination node , it attempts to change the slots value from its channel schedule to match the destination node’s schedule to achieve overlapping channels .
But , what do (slot , channel schedule , seed and channel hopping sequence) mean?!
SSCH MAC protocol
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Important definitions Slot \is defined as the time spent on a single Channel . Channel schedule \is the list of channels that the node plans to switch to in
subsequent slots and the time at which it plans to make each switch . pseudo-random seed or (Seed) \is a deterministic procedure used by
theoretical computer science and cryptography to generate random binary string , in our protocol it used to generate the channel hopping sequence depending on the value of the total channels and current channel and the current seed value to determine the next channel as shown in this formula :
So , for each current channel and seed [(channel , seed) pair] there is a specific hopping sequence .
Channel hopping sequence \ it represents the sequence of channels which the node can hop over it .
To understand the SSCH mechanism we need take an example to how it is work .
SSCH MAC protocol
In this example there are three channels (0,1,2) and two seed (1,2) and tree nodes (A,B,C).
A adopt the first (channel, seed) pair with seed (2) at odd slots and the second pair with seed (1) at event slots .
In this example , A want to generate a traffic to destination C . In a slot 4.
So, A adopts its hopping sequence with node B at the slot 5 with seed (1) .
Now , A and B are synchronous at the odd slot and they are starting the exchanging data .
B also seeks to forward data to the following node C, so can select another non-synchronized pair to synchronize with C.
B iterates the process of receiving data from A at odd slots and forwarding data to C at even slots.
Thus , the transmission A → B → C become more efficient with multi-channel exchanging data of the SSCH MAC mechanism .
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SSCH mechanism
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Conclusions
Conclusions
The design of MAC layer protocol assumes significance in a WMN because achievable capacity depends heavily on the performance of MAC protocol. In addition to a fully distributed operation, the major issues faced by the popular CSMA/CA-based IEEE 802.11 and distributed coordination function (DCF) are: (i) hidden terminal problem, (ii) exposed terminal problem, (iii) location-dependent contention, and (iv) high error probability on the channel.
In order to increase the network capacity, the multiple channels protocols
have to use in WMNs .
As the WMN has a hybrid structure of centralized and ad hoc Architecture , the MAC layer access mechanisms proposed for wireless ad hoc, sensor and WLAN are potentially suitable for wireless mesh networks. There are a lot of papers that study the possibility of implementing existing MAC protocols to Ad hoc network such as CC-MMAC and SSCH MAC protocol for WMN and they give good results .
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The endThe end
At the end At the end
I want to thank dr.Taner for your help and support.I want to thank dr.Taner for your help and support.
And sayAnd say
“ “ thank you to all attendees “ thank you to all attendees “
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