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Module 1: Data Link Protocols

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Wireless LANs
We are discussing on Computer Networks and Internet Protocol and as last couple of orlast few lectures, we were discussing with layer 2 phenomena. So, what I thought thattoday will discuss overview of WLAN or wireless LAN. Though as many of youunderstand that wireless LAN or WLAN is a topic itself it is a separate course if notmore than one course itself. And it has different consideration to be handled at theespecially at the physical and layer 2 of the MAC layer or data link layer right. So, at thelayer 2 and layer 1 there are separate consideration for enabling the wireless and to therest of the things are may not be affected that much or are not affected that that way.So,so, that is why wireless LAN is a separate consideration, but nevertheless what wethought that a basic overview of this wireless LAN or WLAN at phenomena at data linklayer. And also vis a vis little bit of physical layer may be good for the overall for theoverall course structure, and especially those who are not exposed to this technologies orthis particular well listen and phenomena will be it will be helpful for that right. With thismotivation will be having a basic overview of wireless LAN phenomena at the layer 2level right.(Refer Slide Time: 02:05)
So, as we see that WLANs especially due to huge purification of our different mobiledevices and obvious LAN mobile applications on those devices. So, there is a need for aomnipresent network to be there right. So, rather the if we look at that bandwidthavailability with, in a wireless LAN network or data network par to say our mobiledevices, it is increasing day by day right. And with technologies like 4G and 5G trying tobecome operational and it becomes a different ball game to handle those scenarios right.So, not only that with easy availability of this band width at a lower cost, it also makes itmore feasible to have a infrastructure with wireless LAN right. Even we are these dayshaving a lab starting full working fully on wireless; like, what you require is only thepower to power the systems, the network configuration is wireless. And it also is helpingin reducing this overall cable infrastructure and management of the specific cable andtype of things right. So, its becoming a de facto standard for operations and type ofthings. So likewise if you want to in intraoperative wireless, similarly IEEE 802standards come into play. And if we look at that different set of IEEE 802 standards it isnot only wireless.(Refer Slide Time: 03:56)
So, specifically if you see 802 dot 11 is the wireless LAN working group right. So, thereis a 802 dot 15 wireless personal wide PAN group and so on and so forth. So, there are ifwe see. So, there are lot of activities on the standardization point of view also right. And
to be very specific so these are from that 802 standardization committee’srecommendation and their documents.(Refer Slide Time: 04:35)
And if you look at the 802 standard part say, see we see a series of development cameinto play right. So, 802 dot 11 a wireless network bearer operating at 5 gigahertz ISMband up to 54 Mbps, 11 e quality of service and prioritization, 11 f handover, wirelessnetwork bearer operating in 2.5 gigahertz ISM data rates up to 54 Mbps g, and so and soforth right. And there are different standards which came up, though popular are a b gand n become more popular for practical deployment and type of things. So, what we tryto look at, what we see that there is a lot of effort from the standardization whichobviously, there is based on the demand of this type of deployment.
(Refer Slide Time: 05:35)
Now, if you see that wireless LAN that types of broad categories of wireless LAN. So,one is something base station based right all communication through access point right.So, its a base station based so, you have a access point and communicate, infrastructurewireless base station is connected to the wired back bone all right. So, its a more of athing its a wired backbone and then infrastructure then base stations are connections.These are more controlled because I have that measure control over the wired back bonebetween the segments and etcetera and management is much better and so and so forth.There is a Ad hoc connections where there is no per say there is no central AP sort ofthing, but there is a connection which is Ad hoc and there is MANETs right mobile Adhoc networks. So, Ad hoc network again on the movement and there are differentvariants of different types of networks which are coming of this like, one may be thewhen it declared Ad hoc network the network where the where the vehicles communicateeach other in the things So, it says that there are these are major categories of thenetwork, which where this wireless proliferations is there.
(Refer Slide Time: 07:07)
Now, so, to specifically one with the base station which is wired backbone, another is theAd hoc may be the broad thing type of configurations what we are looking for.(Refer Slide Time: 07:23)
and if we look at that from other perspective so, there is a concept of BSS Basic ServiceSet, where within that particular basic service set the stations can be there theycommunicate with each other. Either on Ad hoc basis without any access point or there isa access point within the basic service set right. And there is a concept of extended
service set, where the service set is extended to the other BSS through some accesspoints.(Refer Slide Time: 07:56)
So, extended service set so, there are basic service set to access point they are extendthey are into different BSS. So, there can be different the stations, this particular stationscan be different type, one is it can be stationary that in other sense it is within the BSSonly. Other is there may be movement of the things that is one BSS to another in side toanother BSS.So, there can be movement from one BSS to another BSS or that can be ESS type ofthings right. So, move from one extended service to another. So, usually what happen?This AP’s are connected on a distributed on a backbone, which is a distribution systems,which in turn connected to a server or gateway, which allows it to different type ofservices starting from different network level services right. So, this is the typicalstructure which make more practical and says that I have BSS different BSSs they areAPs, APs as a backbone where we say distribution systems and go on other things.So, this is the extended service set and there can be movement within the BSS, withintwo BSS, under the one ESS, or across ESS one ESS to another so these are all thosethings are possible. But however, 8 not 2 dot 11 does not guarantee that communicationis continuous during the move right. So, the protocol does not guarantee that thecommunication will effect full during the overall movement.
(Refer Slide Time: 09:33)
And if we look at the physical and data link layer structure of the whole stack. So, it isthis there are at the bottom line there are several physical layer standards, over there,there is a distributed coordination function or DCF, over that the point coordinationfunction PCF. And there is a there are issues of contention free service, contentionservices and over and above there are 802 dot 1 that is the LLC sub layer.So, we have this sort of structure for the wireless or WLAN or more specifically 802 dot11 standard, above this are network transport etcetera that remains same right. Thatwhatever the standards or IP and other things whatever is working as in a same lessfashion so, that the bottom lines are considered.
(Refer Slide Time: 10:40)
So, if you just go little look at little quickly at the different wireless physical layerconsideration so, physical layer confirms to OSI. So, there are 8 naught dot 11 infraredFHSS, DHSS, 802 dot 11 a, OFDM. and 802 dot 11 b, HR-DSSS, and 802 dot 11 g,OFDM so, these are the different things. So, 802 dot 11 infrared 2 capacities 1 Mbps or 2Mbps typically, range from 10 to 20 meters and cannot penetrate wall right. That is whyinfrared cannot penetrate typical wall and does not work outdoors, but its cancommunicate with a low range thing right. And 802 dot 11 FHSS frequency hoppingspread spectrum, again a physical layer consideration we will see some of the aspects ofphysical layer.But may not in our subsequent lecture, but may not go deep into the physical layerconsideration, these are more communication oriented phenomena. So, there is a multipath feeding, 89, 79 non overlapping channels and so and so forth and these are differentcharacteristics of that. So, 802 dot 11 DSSS the direct sequence spread spectrum. So,spread signal over entire spectrum using pseudo random sequence and I have thebandwidth achieved is 1 or 2 Mbps. 802 dot 11 a OFDM orthogonal frequency divisionmultiplexing so, comfortable with some European hyper LAN 2 can go for 254 Mbpswith wider 5.5 gigahertz and there are these are the different consideration into the thing.
(Refer Slide Time: 12:37)
Then we have a variant of 802 dot 11 b, though there are incompatibility between 11 aand 11 b up to up to 11 Mbps for 2.4 gigahertz and with 11 million chips per second. Thebandwidth of all this protocols updates with interference from the several other homeappliances right like microwave oven etcetera. So, 11 b rangers much higher than the 11a so, it can communicate to a higher range.(Refer Slide Time: 13:08)
Then we have 11 802 dot 11 g orthogonal frequency division multiplexing, it isbackward compatible with 802 dot 11 b. That is why in several devices you see that 802
dot 11 b slash g up to 54 Mbps uses 2.4 gigahertz frequency for greater range. So, this assum of this physical consideration, this is primarily to have a idea that what are thetechnologies or what are the standards at the physical level.(Refer Slide Time: 13:41)
So, if we again come back to this 802 dot 11 Mac layer Mac sub layer protocol. So, thatits not which we have a problem of this having this sizing the channel, which is not in the8.3 dot wired network we will see that some of the things. Two major problem whichcomes up here is, the hidden terminal problem and exposed terminal problem. We willsee that this is in channel problem will not come in a wired network where things are notlike this.So, to deal this two problems 802 dot 11 supports two operations; one is DCF orDistributed Coordinated Function and PCF Point Coordination Function. So, these arethe two things which are supported by 802 dot 11 right. So, implementation of the PCF isoptional, but DCF is there its all implemented and support DCF right another PCF ismuch more complicated to handle also.
(Refer Slide Time: 14:55)
So, what is this hidden station and exposed station problem? We will presented in severalliteratures and internet resources. So, this is a hidden station problem on the A figure Aand figure B is the expose station; A wants to send to B, but cannot hear that B is busyright. So, it is a hidden station problem there are range of C radio is not up to the A. Buthere B wants to sends to C, but mistakenly things that the transmission will fail right. So,because it is in the A’s range and it is exposed station problem right.(Refer Slide Time: 15:41)
So, in hidden station problem wireless stations have transmission range, but not allstations are within the radio range like C that it is in the transmission range of B is in theC, but the A is not there. Simple CSMA will not work carrier sensing will not work, Ctransmit to B sorry. And A senses the channel it will not here transmission and falselyconclude A can begin the transmission to B right. So, A senses the channel and see thatthe communication to the B is free because it is not within the range of the C and it maybegin transmission to B. So, this is the challenge with the hidden terminal problem wereshown in the figure A.(Refer Slide Time: 16:31)
On the other hand on the flip side or the so to say on the inverse problem is the exposestation problem, B wants to sends to C listens to the channel. B here is that Atransmission B falsely assume that it cannot sends to C right. In this case B wants tosends to C and since the channel and what it sees that it assumes that here’s Atransmission. And because it is in the radio range of the thing and then it cannot sends toC so, this is a exposed station problem.
Wireless LANs- Part 2
So, this are being tried to handle in the wireless scenario by to as we have been the twoapproaches or two functions. Like there is one distributed coordination function or DCF,uses CSMA/CA, CSMA with collision avoidance, both physical and career sensing right.So, virtual carrier sensing so, it is not that we will see that how things it done. So, twomethods are supported; one we have to say multiple access with collision avoidance withvirtual carrier sensing and one persistent carrier physical carrier sensing so, we will justsee that what it does.(Refer Slide Time: 17:53)
So, Multiple Access Collision Avoided on MACA for wireless has to solve the hiddenexposed station problem or these two challenges by, two type of mechanisms; like one isready to send and clear to send. So, before sending the things it says a RTS signal andwaits for a CTS thing. So, RTS, CTS helps determine who else in the range or the busyare in the range or busy so, that it can avoid collision.Now, we will see that detection collision it may not be false feasible here out here.Because this first of all this channels are pretty noisy, you need to have major resourcesand bandwidth to handle this sort of sensing. Because you are sensing and trying totransmit at the same time like. But whether still collision occurs yes, whether in spite ofthis collision occurs will see that in spite of this to may collision occurs. But neverthelessthe based on that CTS or RTS not receiving the channel will go for again retransmission.(Refer Slide Time: 19:17)
So, if we look at that MACA for WLAN added acknowledge and CSMA no RTS at thesame time. So, this is A send a request to tell me to B and in the range of A so, B sends aCTS and responses CTS clear to transmission and transmits. So, it is really does not; itable to handle this hidden channel, both hidden channel, and exposed channel issues. So,we have a virtual sensing with CSMA CA like in the C in the range of A receives a RTSright.
(Refer Slide Time: 20:04)
C in the range of A receives RTS and based on information in the RTS creates a virtualchannel busy NAV right. So, what is a NAV? So, Network Allocation Vector; the timeperiod set by all other waiting station before sensing the medium for idleness right. So itis the time period like if it is A sends a RTS for B, then it is the other stations who arehearing this will wait for, will set there NAV that and it also in the RTS their informationis there how much time it may required to transmit the data. So, for that time this a otherstations who are hearing this, will wait for that and for that period before looking for thechannel whether it is idle or not. So, D in the range of B receives the CTS and creates ashorter NAV right.So, if D also in the range of B received the CTS and creates a shorter NAV. The virtualimplies that is source stations sends duration field in the data frame of the RTS and CTS.So, and so that is why by reading those its creating a wet time, which is an sensing aftersome after that wet time by setting their own NAVs. So, it is creating some sort of avirtual science channel since sensing right. So, it is not sensing the channel based on thisinformation wise is the RTS and CTS. Station, then adjust the NAV accordingly right so,the network allocation vector accordingly and wait for the things right.So, let me repeat it when a channel wants to A wants to send to B it sends a RTS, theother channels we are listening to this RTS, the RTS also contains the information, howmuch time it required to sends the data. So, based on that sensing, the other channels way
set their network allocation vector to that time period after which it will look thatwhether the channel is idle or not right.(Refer Slide Time: 22:18)
So, there is a another one persistent physical carrier sensing, the station senses thechannel when it wants to send right. So, change the station senses the channel when itwants to send, if idle station transmits right. So, station does not sends channel whiletransmitting. If the channel is busy station defaults until idle and then transmits right,upon collision wait a random time using binary exponential back off. So, there is a onepersistent physical sensing, in a previously it was there it is not physically sensing, butbased on the information in the RTS CTS it is setting up that after what time it will againcheck the idleness of the thing.But in one persistent physical carrier sensing, what it is doing that station senses thechannel when it wants to send some data right. If it is idle that is the station transmits.So, if it finds the station it find the channel is idle that nobody is within that rangebecause, the revaluation. So, there is no wired channel and then the station does notsense the channel while transmitting, while transmitting it will not sense the channel. It isgo on sensing there, if the channel is busy that station defaults still the idle and thentransmit. So, upon collision wait for a random time using binary exponential back offperiod right; so, this is the bottom line of this one persistent sensing.
(Refer Slide Time: 24:01)
Now, point coordinated coordination function that PCF uses a base station to poll othersstation to see if there are frames to be sent. So, in this in PCF in this case it requires abase station to poll other station, to see that it is a polling operation goes on, if they haveframes to send so no collisions occurs per say right. So, there is no collision occurs. Basestation sends beacon frame periodically. So, base station can tell other station to sleep, tosave batteries and base station holds the frame for sleeping station right.So, it is a more coordinated or point coordinated because they are the base station ispolling and seeing that if they are frames to send. No collision occur, base station sendsbecome frame periodically, base station can tell other station to sleep to save thebatteries, and other and base station holds the frame. So, this is the point coordinatedfunction little bit complicated and not over not it is optional for things that you may notfor. But DCF is considered mandatory for all communications with those features ofRTS, CTS sort of things.
(Refer Slide Time: 25:18)
So, DCF, PCF coexistence distributed and centralised control can coexisting using interframe spacing. Like there are three type of thing one is short IFS is the time waitedbetween the packets of ongoing dialog like RTS, CTS, data, acknowledgement, nextframe. So, this is the short IFS; so, some short interstine spacing and PIFS PCF IFS whenno SIFS response the base station can issue a beacon or poll alright. So, for that itrequires that is the PCF inter frame spacing. There is a DIFS that is DCF IFS when noPIFS any station can attempt to acquire the channel alright.So, that is a distributed that is DC DCF, IFS and there is a extended IFS or EIFS lowestpriority interval used to report bad and unknown frames right. So, that is a extended IFSor EIFS which has a much lower priority and it is primarily to report bad unknownframes. So, these are different inter frame spacing’s which are standardized, which areused for this communication.
(Refer Slide Time: 26:38)
So, we come back to our CSMA/CA. So, as we have discussed the WLAN canimplementing CSMA/CD there are lot of problems in case of WLAN. Collision detectionrequires send data and receive signal and the same time, required resource poll stationsand higher bandwidth which is a difficult for several this type of mobile devices; whichare communicating, which are not that resource poll both the in terms of application andother resources. And, there are issues of hidden and expose station problems alright.So, if there are hidden and expose station problems which are things. And, in number ofcases this channels are noisy, sometimes the devices which are communicating are faraway which creates a problem of fading. So, the signal fades off or the signal strengthcomes down which makes this CSMA/CD to implement much difficult right. So, as wehave discussed we have DIFS: Distributed InterFrame Space, RTS: Request to Send; acontrol frame, SIF: Short FrameSpace, CTS: Clear to Send; with this thing if we just welook at the things.So, at the start it is a send set the back off to 0, it sends the channel if it is not free somepersistence strategy is deployed. So, wait for DIFS or Distributed InterFrame Space thenthe sense the. So, it is it waits for a time period and then it sends the RTS or a to requestto send to its destination station. After that it after it sends a timer that so that it islooking that within that particular time period, the CTS should be reserved that is the
clear to send signals, it should receive from the destination. If it is received yes, then waitfor again for a short time period or SIFS and then send the frame.Again set a timer so, long this acknowledgement is not received and if it is successful ifit is acknowledgement received is success. So, sense the channel, wait for a persistencechannel deploy a persistence strategy. If it is channel is busy wait for some time, wait fora distributed or DIFS, sends RTS, wait for CTS. If it is received within the time period,wait for a again small periods, set the send the frame it is on the sender to the CTS isreceived from the destination. And, then it sends the frame sorry, sends the frame set atimer if acknowledge is received is successful right.On the other hand if you see, if there is no CTS that it does not depend with the things, itmay increment the back off, back off time limit, wait for the back off time and go for thethings. Again if acknowledgement is also so, if the CTS is not received within the timeperiod or acknowledgement is not received the entrepreneur is goes for a back off timeright so this way it goes on running.(Refer Slide Time: 30:03)
So, just to look at in the other perspective so, the source waiting for a DIFS sends a RTSwaiting for a SIFS sends a the destination sends a CTS. And, it again waits for a SIFSsends the data or the information to the destination and this waits again for a SIFS andsends the acknowledgement. So, this is the whole process and there in that this phasethere is a NAV or that is Network Allocation Vector which are set by the all the other
station within the sensing zone. Like the wait they because this RTS, CTS have that thingthat how much time they required to send the data. So they have they wait for the otherthings before checking that whether this channel is idle or not. So, this way it handles theproblem of this hidden station expose station problem.Now, whether still collision not cannot happen or type of things, yes it may still happenright. Still happen there are this is a totally mobile, this is a wireless environment thereare mobile devices etcetera. There are noisy channels these are channel so, though it mayhappen. If there is a things either the things will be lost RTS acknowledgement, CTSacknowledgement etcetera will be lost or corrupted. So, it is within the time period it isnot reached it all goes for the back off things.So, with this let us conclude our overview a short discussion of this how this wirelessLAN or WLAN have there the phenomena works in the layer 2 or data link layerinfrastructure right. Again as I mention that the wireless LAN is a separate topic orsubject all together may be it requires couple of courses to handle the all aspects ofwireless LAN. So, it is a basic overview of the wireless LAN with respect to the data linklayer that what are the basic consideration following the 802 dot 11 standard.So, we will continue our discussion on this overall networking topic in our subsequentlecture, let us conclude today.Thank you.