UNIT-3. Medium Access Control Sub Layer A network of computers based on multi-access medium requires...

UNIT-3

Medium Access Control Sub Layer• A network of computers based on multi-access medium requires a

protocol for effective sharing of the media.

• As only one node can send or transmit signal at a time using the broadcast mode, the main problem here is how different nodes get control of the medium to send data, that is “who goes next?”.

• The protocols used for this purpose are known as Medium Access Control (MAC) techniques .

Goals of MACS :

Medium Access Control techniques are designed with the following goals :

• Initialization : The technique enables network stations, upon power-up, to enter the state required for operation.

• Fairness: The technique should treat each station fairly in terms of the time it is made to wait until it gains entry to the network, access time and the time it is allowed to spend for transmission.

Medium Access Control Sub Layer• Priority: In managing access and communications time, the technique

should be able to give priority to some stations over other stations to facilitate different type of services needed.

• Limitations to one station: The techniques should allow transmission by one station at a time.

• Receipt: The technique should ensure that message packets are actually received (no lost packets) and delivered only once (no duplicate packets), and are received in the proper order.

• Error Limitation: The method should be capable of encompassing an appropriate error detection scheme.

• Recovery: If two packets collide (are present on the network at the same time), or if notice of a collision appears, the method should be able to recover, i.e. be able to halt all the transmissions and select one station to retransmit.

Medium Access Control Sub Layer

• Reconfigurability: The technique should enable a network to accommodate the addition or deletion of a station with no more than a noise transient from which the network station can recover.

• Compatibility: The technique should accommodate equipment from all vendors who build to its specification.

• Reliability: The technique should enable a network to confine operating inspite of a failure of one or several stations.

Medium Access Control Sub LayerChannel Allocation Problem • Data link layer is divided into two sub layers called LLC and MAC .

• In MAC uses protocols to deal with channel allocation problem .

• In broad cast networks a single transmission medium is shared by community of users .

• In this types of networks the key issue is how to determine who gets to use the channel when there is a competition for it .this is called channel allocation problem .


Fig : Approaches to Sharing a Transmission Medium

Medium Access Control Sub LayerStatic Channel Allocation • There are two types of Static channel allocation techniques :

1. FDM 2 . TDM

FDM :• If there are N users , the band width is divided into N equal sized

portions ,each user assigned one portion .

• It is efficient for small number of users .

Problems :• If the spectrum is cut into ‘N’ regions , and fewer than N users are

currently interested in communicating , a large piece of valuable spectrum will be wasted .

• If more than N users want to communicate some of them will be denied .

Medium Access Control Sub LayerTDM • The static allocation can be done on the basis of time division

multiplexing .

• Each user is statically allocated N th time slot .

• If the user does not use the allocated slot ,it just wasted .

Dynamic Channel Allocation • Multiple Access Protocols on Dynamic channel allocation .

• The basic assumptions needed for channel allocation is station model , single channel assumption ,Collision Assumption ,Time Slots ,Carrier Sensing .

1.Station Model :• The network with a station model has ‘N’ number of independent

stations .

• These generate frame transmissions and unless this frame is transmitted on to the channel the station remains idle and is blocked .

Medium Access Control Sub Layer2.Single channel Assumption :• As per Traffic is concern ,we have only one channel and the protocol

has to take care of all the problems associated with it .

3.Collision Assumption :• If more than one user transmit frames simultaneously they over lap in

time and the resulting signal is garbled or distorted this is called collision .

• The sender has to retransmit the collide frames to the receiver .

4.Time Slots :• Time is divided into discrete intervals ,called slots .

• Frame transmission always begins at the starting of the slot .such a time slot may contain one or more frames corresponding to an idle slot ,successful slot or collision slot respectively .

• Other option is frame that can be transmitted at any time .

Medium Access Control Sub Layer5. Carrier Sensing :• Every Station checks the status of the channel before it transmit any

signal.

• If it senses busy ,it waits for the channel to get free .

• In carrier sense station can tell if the channel is in use before trying to use it .

• If no carrier sense ,station can not sense the channel before trying to use it . Only they can determine whether or not the transmission was successful .

Medium Access Control Sub LayerMAC Protocols ALOHA :

• The ALOHA random access scheme had its origins in the university of Hawaii.

• In the 1970’s Norman Abramson and his colleagues derived a new and elegant method to solve channel allocation problem .

• There are two types of ALOHA protocols :

1. Pure ALOHA 2.Slotted ALOHA

1.Pure ALOHA

• Basic idea of ALOHA system is users transmit data whenever they have data to sent .

• From time to time packet transmission will collide , but these can be treated as transmission errors and recovery can take place by re-transmission .

• When the traffic is very light the probability of collision is low .


Fig : Collision in Pure ALOHA

Medium Access Control Sub Layer• If the frame is destroyed , the sender just waits a random amount of

time and sends it again .

• System in which multiple users share a common channel in a way that can lead to conflicts are widely known as contention system .

• Transmission errors that are due to noise affect only a single station .

• In packet collision more than one station is involved and hence more than one re-transmission is necessary .

• ALOHA scheme requires a station to use a random re-transmission time .

• The user in pure ALOHA in two modes :

1. Preparing 2. Waiting

1.Preparing :

• When the frame is ready ,the station transmits it and checks the channel to see if it is successful .

• Initially all users are in Preparing mode .

Medium Access Control Sub Layer2.Waiting

• If the frame is collision then the user goes to waiting state .

• The sender waits random amount of time and re-transmits the frame .

Fig : Vulnerable Period for the Reference Frame

Medium Access Control Sub Layer• The time required to transmit a frame successfully without any

collisions is called vulnerable period .

2. Slotted ALOHA

• The performance of the pure ALOHA scheme can be improved by reducing the probability of collisions .

• This method was invented in 1972 by Roberts , to double the throughput of an ALOHA system.

• In this scheme divide the time up into discrete intervals ,each interval corresponding to one frame .

• This approach requires the users to agree on time slots .

• All the stations keep track of transmission time slots and are allowed to initiate transmissions only at the beginning of a time slot.

• The frames are assumed to be of constant length and take one time slot.

• The Vulnerable period in this system is only X seconds .


Fig : collisions in Slotted ALOHA


Fig :Throughput(S) VS Offered Traffic(G)

Medium Access Control Sub LayerMAC Address • Medium Access Control (MAC) address or a Hardware address refers

to a unique value assigned to a network adaptor on a LAN .

• This Address is used to identify each node in a network in a distinct manner .

• MAC addresses are 48 bits long and is represented as 12 digit hexadecimal number .

AA : AA:AA:BB:BB:BB or AA-AA-AA-BB-BB-BB

• Here first half specifies the manufacturing Id of an adapter and the second half specifies the serial number associated with the adapter .

• This serial number is assigned by the manufacturer to an adapter .

Medium Access Control Sub LayerCSMA (Carrier Sense Multiple Access)• The channel utilization of ALOHA system is very low due to wastage

of band width and collisions .

• This wastage can be reduced by transmissions that are certain to cause collisions .

• If any station is transmitting that is know by the remaining all other stations no other station will transmit .

• It is necessary for a station to know what other stations are doing , that is by sensing the medium for the presence of a carrier signal , a station can determine whether there is an ongoing transmission and act accordingly .

• These protocols are called CSMA Protocols .

• CSMA schemes differ according to the behavior of stations that have a frame to transmit when the channel is busy .

Medium Access Control Sub Layer• There are three types of CSMA protocols

1. 1-persistent CSMA 2. Non – Persistent CSMA

3. P- Persistent CSMA

1. 1-persistent CSMA

• A node having data to send, start sending, if the channel is sensed free. If the medium is busy, the node continues to monitor until the channel is idle. Then it starts sending data.

2. Non – Persistent CSMA

• If the channel is sensed free, the node starts sending the packet. Otherwise, the node waits for a random amount of time and then monitors the channel.

3. P- Persistent CSMA

• If the channel is free, a node starts sending the packet. Otherwise the node continues to monitor until the channel is free and then it sends with probability p.


Fig : P- Persistent CSAM

Medium Access Control Sub Layer• The efficiency of CSMA scheme depends on the propagation delay,

which is represented by a parameter a

Propagation delay

a = Packet transmission time.

CSMA/CD(Carrier Sense Multiple Access with Collision Detection)

• CSMA is improved over the ALOHA schemes because of two reasons:

1. It reduced the vulnerable period to single propagation delay .

2 . No station begins to transmit when it senses the channel is busy .

• In CSMA and ALOHA schemes after collision of frames entire packet s transmitted .

• To reduce the transmission time and wasted band width by aborting the transmission when a collision is detected .


Fig : Reaction time in CSMA-CD


Fig : Binary Exponential Back of Algorithem used in CSMA/CD


Fig : Comparison of MAC Protocols

Medium Access Control Sub LayerCollision Free Protocols • Protocol with out any collision is called collision free protocol .

• There are two types of collision free protocols :

1. Bit Map Protocol

2. Binary Count down Protocol

Bit Map Protocol• It is a first collision free protocol .

• Here each contention period consists of N slots .

• If a station 0 has a frame to send , it transmit a 1 bit during the 0 slot ,no other stations allowed to transmit the frames during this slot .

• Next station 1 will get an opportunity to transmit 1 during slot 1 ,but only if it has a frame in queue .

• Any station K may transmit 1 in the Kth slot only .


Fig : Basic bit map protocol

Medium Access Control Sub Layer• In the above figure we have consider 8 stations ,so that total

contention period is divided in to 8 slots .one slot for each station .

• Stations 2,4 and 7 has the packets and they have reserved their slots ,after wards they start transmitting the frames .

• If a station has more frames it has to take chance in next contention period .

Drawbacks

• If a station has a frame just after its bit slot was passed by to transmit its frame , it has to wait until the completion of transmission by the stations who have reserved in this contention period and the next contention bit map has come .These protocols are called as reservation protocols .

Medium Access Control Sub LayerBinary Countdown • Here a station waiting to use the channel , broadcasts its address as a

binary bit string , starting with the higher-order bit .

• All addresses are assumed to be of same length .

• The bits in each address position from different stations are boolean ORed together .This protocol is called Binary Countdown .

Medium Access Control Sub LayerIEEE 802.X

• IEEE 802.1 High Level Interface ( MAC Bridge)

• IEEE 802.2 Logical Link Control (LLC)

• IEEE 802.3 CSMA/CD , Ethernet

• IEEE 802.4 Token bus

• IEEE 802.5 Token Ring , FDDI

• IEEE 802.6 MAN DQDB

• IEEE 802.7 Broadband LANS

• IEEE 802.8 Fiber Optic LANS

• IEEE 802.9 Integrated Data and Voice network

• IEEE 802.10 Security

• IEEE 802.11 Wireless Networks

Medium Access Control Sub LayerStandard Ethernet (802.3)• There are four different types of Ethernet cables :

Table : Ethernet Cabling

Name Cable Max Segment

Nodes Advantages

10Base5 Thick Coax

500m 100 Good for Back Bones

10Base2 Thin Coax

185m 30 Cheapest

10Base-T Twisted pair

100m 1024 Easy maintenance

10Base-F Fiber optics

200m 1024 Good between Buildings

Medium Access Control Sub Layer1. Thick Ethernet(10 Base5)• First implementation is called 10base5,thick ethernet or

thicknet.uses bus topology with external tranceiver (transmitter/receiver) connected via a tap to thick coaxial cable. Can’t bend .

• Tranceiver is responsible for transmitting,receiving and detecting collisions. . collision occurs in coaxial cable

Medium Access Control Sub Layer2. Thin Ethernet (10 Base2) • Second implementation is called 10base2,thin ethernet or

cheapernet.uses bus topology with thinner and flexible cable.can bend.

• Tranceiver is part of n/w interface card,installed inside station.

• Installation is simpler. collision occurs in coaxial cable .

Medium Access Control Sub LayerTwisted Pair(10 base –T)• Third implementation is called 10baseT, Twisted pair ethernet uses

physical star topology .

• Stations are connected to a hub via 2 pairs of twisted cable.collisions occur in hub.max length is 100m to minimize effect of attenuation.

Medium Access Control Sub LayerFiber Optics (10Base-F)• Uses star topology. stations are connected to a hub via 2 pairs of fiber

optic cable. up to 200m .

• Has good noise immunity .

Medium Access Control Sub LayerFast Ethernet :• It is designed to compete with LAN protocols such as fddi or fiber .

Goals: 1)upgrade the data rate to 100mbps

2) compatible with standard Ethernet.

3)keep same 48 bit address.

4)keep same frame format.

5)keep same min and max lengths.


Fig : Categories of Fast Ethernet

Medium Access Control Sub Layer100Base-TX :

• 100 Base-TX requires two pairs of UTP or STP .

• These pairs are used to connect the node to the hub .

• One cable is for transmit the data another is for giving the response .

Medium Access Control Sub Layer100 Base-FX :

• It consists of two optical fiber cables .

• One cable is used to transmit the data from node to the hub and the other cable is used to give the response from hub to node .

• Supports upto 2000 meters distance .

Medium Access Control Sub Layer100 Base-T4

• Type 3 UTP cable consists of 4 distinct twisted pair wires .

• Among 4 pairs two pairs are bidirectional and two pairs are unidirectional .

• It is designed to avoid rewiring .

Medium Access Control Sub LayerGigabit Ethernet• Need for even higher data rate resulted in design of gigabit

Ethernet(1000mbps) .

Goals 1)upgrade the date rate to 1gbps.

2) compatible with standard Ethernet.

3)keep same 48 bit address.

4)keep same frame format.

5)keep same min and max lengths.

Medium Access Control Sub Layer1000 Base – T : • This physical layer alternative consists of 4 pairs of types 5 UTP

cables .

• These UTP cables are used to provide a range of distance upto 100 meters only .

Medium Access Control Sub Layer1000 Base-CX :• It is better than the 1000 Base-T .

• It allows the nodes to communicate with each other in the range of 1000 Meters.

• For this it attaches a different separate STP cable pairs among different nodes .


1000 Base-LX :• It uses optical fiber medium to provide more transmission range than

1000 Base-CX and 1000Base-T .

• Wave length range from 1270 to 1355 nm .

1000 Base-SX • It also uses the optical fiber medium and the maximum distance it

supports is in the range of 770 to 860 nm .

• It uses short wave laser as a signal .

Medium Access Control Sub Layer10 Gigabit Ethernet • It is defined only for full-duplex mode providing a point-to-point

Ethernet connectivity service with the CSMA-CD algorithm disabled .

• It has two types of physical layers LAN and WAN .

• It differs in framing but supports the same capability in terms of distance .

• LAN is intended to support existing Ethernet LAN applications and WAN is for SONET applications .

Medium Access Control Sub LayerToken Ring (802.5):• Nodes are connected in a ring format .

• Data always flows in one directions around the ring .

• Each node receives the frame from its upstream neighbor and then passes it to the downstream neighbor through a special bit pattern called token .

• This token circulates around the ring .

• If any node wants to send the frame then the node has to remove the token from the ring transfer its frame into the ring .

• Each node along the way simply forwards the frame with destination node saving a copy and forwarding the frame on to the next nodes on the ring ,so that the frame actually comes back to the sender .

• At destination node before it forwards the frame , that host checks the CRC to ensure that there are no errors and then not only accepts the frame but also changes a flag bit in the frame to indicate the receipt of a correct frame .


Fig : Token Ring

Medium Access Control Sub Layer• At the end the frame comes back to the sender after covering the

entire ring .

• At this stage the sender can check the flag bit to verify if the frame was received by the destination successfully or not .

• It has two operating modes : Listen and Transmit

Listen Mode:

• In put bits are copied to out put bits with a delay of 1 bit time .

Transmit Mode :

• It enters only if the token has been removed from the ring .

• It break the interface between the input and output and allows the station to transmit their frames in the ring .

Medium Access Control Sub LayerToken Bus(802.4)• The standard 802.4 describes a LAN called a token bus .

• This is a MAC technique for a BUS or Tree Configuration .

• The stations are connected to the bus and the token is passed on the bus .

• A station receiving the token may transmit the data and then passes the token on to the next station .

Medium Access Control Sub Layer• Each station knows the address of its neighboring stations .

• Token bus follows a priority scheme that is when ever the logical ring is initialized the highest numbered station can send his frame and then it is passed to its neighboring station by a special control frame called a token .

• The token spreads across the logical ring with only token holder being permitted to transmit frames .

• The advantage of token bus is that it gives the priority to the stations so that the highest priority station first transfer the frame .


Wireless LANS(802.11)• A system of portable computers that communicate by radio signals

can be regarded as a wireless LAN .

Requirements

1.Several nodes :Thousands of nodes across multiple cells are connected to wireless LANs .

2.Throughput : It provides an efficient utilization of wireless transmission medium to increase the capacity .

3. Coverage Area : Every wireless LAN should cover an area of 100-300 m in diameter .

4. Power consumption of Battery : A mobile has a long battery life if they are used with wireless adapters .

5.Connecting to Back bone LAN : To interconnect the stations , the high speed back bone wired LAN is needed .

Medium Access Control Sub Layer6.Reliability and Security : The wireless LAN should be reliable so that

it must allow reliable transmission even in a noisy environment . A wireless LAN should provide security from unauthorized users .

7.Roaming : A wireless LAN allows the mobile station to move from one cell to another with the help of a MAC protocols .

LAN Technology :• There are three wireless LAN Technologies :

1. Infrared LANs

2. Spread spectrum LANs

3. Narrowband microwave LANs

1. Infrared LANs• It is used to implement wireless LANs and wireless interfaces to

connect laptops and other portable machines .

• It uses the wavelength band between 780 and 950 nm.

• Its range is limit to a room .

Medium Access Control Sub LayerAdvantages :• It carries a high bandwidth .

• The toll of infrared is relatively inexpensive and simple .

• It does not penetrates through walls so its provides security from eavesdropping .

Disadvantages

• Its coverage distance is very limited .

2. Spread Spectrum LANs :• The structure of Spread spectrum LAN consists of different cells ,

where each cell is attached to other .

• Neighboring cells uses different core frequencies with in the same band to avoid interference .

• Each cell consists of topology which can be either peer-to-peer or hub.

• A peer to peer topology does not contain a hub , instead it uses a CSMA technique to give the access control .


Fig : Structure of Spread Spectrum wireless LAN

Medium Access Control Sub LayerNarrowband Microwave LANs

• The main aim of this technology is to interconnect LANs between buildings .

• The microwave radio frequency band is used for the transmission of the signal with a certain bandwidth .

• The disadvantage of microwave technology is that the frequency band requires licensing by the FCC .

IEEE 802.11 Architecture :

• The IEEE 802.11 architecture consists of components like STA (Station) ,BSS(Basic Service Set),AP(Access Point) ,ESS(Extended Service Set), and DS(Distribution System ).

Station :

• It is used to connect to the wireless medium .

• It provides the service of authentication ,deauthentication ,privacy and delivery of data .


Fig : IEEE 802.11 Architecture

Medium Access Control Sub LayerBasic Service Set :

• A BSS consists of a set of STA that communicate with each other .

Access Point :

• It also like station , it provides the same services of STA .

• In an infrastructure BSS every mobile STA communicate with wired LAN and the wired local relay function for the BSS.

• One advantage of AP is the buffering of congestion for a mobile STA while that station is operating in a very low power state .

Extended Service Set :

• This component extends the range of mobility .

• An ESS consists of infrastructure BSS where one AP communicates with the other AP to move the traffic from one BSS to another .

• It also facilitates to move mobile STA from one BSS to another BSS.

• This is the responsibility of AP to perform this communication through a medium known as Distribution Service (DS)

Medium Access Control Sub LayerDistribution Service(DS):

• This is the back bone of wireless LAN and can be constructed of either wired or wireless networks .

• The DS allows the AP to communicate and to exchange frames for STAs in their BSS .

IEEE 802.11 Services :• Services are of two types :

1.STA Services

2.Distribution Services

STA Services :

1.Privacy

• It is designed to provide a level of Security .

• It secure the data only as they traverse the wireless medium.

• It does not provide a complete protection of data between different applications run on mixed networks .

Medium Access Control Sub Layer2.MSDU Delivery

• This is similar to all the services which are provided by IEEE 802.11 LANs.

• MSDU provides the reliability of sending the data frames from the MAC in one STA to the MAC in one or more other STAs.

3.Authentication

• This service is used to prove the identity of one STA to another .

• If any STA fails to proof this identity then the STA is not allowed to use WLAN for data delivery .

4.Deauthentication

• This service is used to remove a previously authorized user from any further use of network .

Medium Access Control Sub LayerDistribution Services :

1. Integration : It is responsible for address translation and media conversion logic in order to exchange the data .

2. Association :

• This service helps to make a logical connection between a mobile STA and an AP .

• The DS requires this logical connection to know the information regarding where and how to deliver data to the mobile STA .

• The AP also requires this logical connection to accept the data frame from the mobile STA and to allocate resources to support the mobile STA .

3.DisAssociation :

• It is used by AP or STA .AP uses this service to inform one or more STAs that it is disconnecting the logical connection to the WLANs.

Medium Access Control Sub Layer4. Reassociation :

• This service is similar to association service except that it includes the information about the AP with which a mobile STA has been previously associated .

• The reassociation service allows a mobile station to move from one BSS to another .

5. Distribution :

• This service is used by an AP to know how to transfer the data frames it receives .

• When ever a mobile station wants to send a frame to the access point for transferring to another STA then the AP invokes the distribution service to determine whether the frame should be sent back to its own BSS or not .

Medium Access Control Sub LayerMAC Protocols :• There are two protocols which are provided by the IEEE 802.11

standard to the MAC .

1 . Distributed Access Protocol

• It uses CSMA mechanism .

• It is used in adhoc network and it is also usefull in wireless LANs where there is a busy traffic .

2 . Centralized Access Protocol

• It is used in where if we give some priority to over data .

IEEE 802.11 Frame Structure :• It contains the fields like frame control

,duration/connectionID ,address,sequence control,body of the frame ,crc .


Fig : MAC Frame Format

• Frame Control :It is used to identify the type of frame (Data,Control,Management)

• Duration Field : It gives the time that the frame exits .

• Address Field : There are four address fields like source station , destination station, transmitting station and receiving station .

• Sequence Control : It is used to store the sequence of frame numbers .

• Data Unit : It is used to store the data fragment .

• CRC : checks errors .

Medium Access Control Sub LayerIEEE 802.11 Physical Layer :• There are three types of physical layer implementations :

1.Frequency Hopping Spread Spectrum (FHSS)

2.Direct Sequence Spread Spectrum (DSSS)

3.Infrared(IR)

1.FHSS :

• It uses 2.4 GHz Band .it is divided into 75 1-Mhz sunchannels .

• The source and destination agree on a hoping format and exchange their data frames over a sequence of the subchannels .

2.DSSS :

• It uses 2.4 GHz Band .it is divided into 14 12 MHz channels .

• Neighbouring channels overlap one another partially ,with three of the 14 being completely non overlapping .

Medium Access Control Sub Layer3.Infrared :

• This technique is omnidirectional and the maximum range it supports is 20m .

• The modulation scheme prescribed for 1mbps data is often reffered as 16 pulse position modulation(PPM) .

Bridges• Bridge is an electronic device used to connect different networks or

LANs.

• By using bridge we can forward frames from one ethernet LAN to another .

• Bridge gives it access to the physical address of all stations connected to it .

• When a frame enters a bridge ,the bridge regenerates the signal and checks the address of its destination and forwards the new copy to the address which it belongs .


Fig : Bridge

Medium Access Control Sub LayerWorking of Bridge

Medium Access Control Sub LayerTypes of Bridges • There are two types of bridges

1. Transparent 2.source routing

1. Transparent :• Transparent Bridges are also called spanning tree bridges .

• It accepts every frame which it sends on all the LANs to which it is connected .

• It maintains a Hash table of MAC address of frames arrive .

• Ethernet networks use Transparent bridges .

• Duties of Transparent bridges are Filtering of frames, Forwarding and Blocking.

Medium Access Control Sub Layer2.Source Routing• Each station should determine the route to the destination when it

wants to send a frame and there fore include the route information in the header of the frame .

• Address of these bridges includes in the frame .

• Frame contains not only source and destination address but also contains bridge address .

• It is used in token ring networks .

UNIT-3. Medium Access Control Sub Layer A network of computers based on multi-access medium requires...

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