The document discusses MAC protocols for broadcast networks. It covers topics like CSMA, CSMA/CD, and how they work. It explains assumptions like the collision model and how protocols determine channel access. It also compares the performance of different protocols in terms of channel utilization and delay under varying load levels. Collision-free protocols are more efficient at high loads while contention-based protocols have lower delay at low loads.

1. Chapter 4

2. In broadcast networks (Multi-access/random-access channels) The key issue is how to determine who gets to use the channel when there is competition for it. MAC=Protocol to determine who goes next on channel It’s important for LANs, WANs are point-to-point. The Channel Allocation Problem: Static Channel Allocation in LANs and MANs FDM and TDM Dynamic Channel Allocation in LANs and MANs

3. The traditional (phone) way of allocating a single channel is Frequency Division Multiplexing. FDM works fine for limited and fixed number of users. Disadvantages: Inefficient to divide into fixed number of chunks. May not all be used, or may need more. Doesn't handle bursty traffics of computer systems. From queuing theory (Poisson distribution for C and T) : T = mean time delay for a channel C = capacity (Bits/Sec.) λ = arrival rate (Frames/Sec.) 1/ μ = mean length of a frame 1 T = ----------  C - 

4. Example: C=100Mbps, 1/ μ =10000 Bits, λ =500 Fps T=200 Micro Sec. If divide this channel into N sub-channels, each with capacity C/N. Input rate on each of the N channels is λ /N. So: N times worse for FDM In example for N=10 => T=2 Mili Sec. Same arguments can apply for TDM 1 N T(FDM) = ----------------- = ------------ = NT μ (C/N) - λ /N μ C - λ

5. Assumptions (1): Station Model: Assumes that each of N "stations" (packet generators, Terminal ) independently produce frames. The probability of producing a packet in the interval Δ t is λ . Δ t where λ is the constant arrival rate. That station generates no new frame until that previous one is transmitted. Single Channel Assumption: There's only one channel all stations are equivalent and can send and receive on that channel. Collision Assumption: If two frames overlap in any way time-wise, then that's a collision . Any collision is an error, and both frames must be retransmitted. Collisions are the only possible error.

6. Assumptions (2): Continuous/ Slotted Time: Time is not in discrete chunks. Frame transmission can begin at any instant. Alternatively, in slotted, frame transmissions always begin at the start of a time slot. Any station can transmit in any slot (with a possible Collision.) Carrier/No-Carrier Sense: Stations can tell a channel is busy before they try it. NOTE - this doesn't stop collisions. LANs have this, satellite networks don't.

7. Carrier Sense Multiple Access (CSMA) 37% utilization is low yet! Stations can listen for a carrier and there is no transmission send it’s data. Persistent and non-persistent: persistent: When channel is found to be busy, keep monitoring to find THE instant when it becomes free. non-persistent: When channel is found to be busy, don't keep monitoring to find THE instant when it becomes free. Instead, wait a random time and then sense again.

8.  1-persistent CSMA Station listens. If channel idle, it transmits. If collision, wait a random time and try again. If channel busy, wait until idle. If station wants to send AND channel == idle then do send. Propagation delay has an important effect on transmission. Success here depends on transmission time - how long after the channel is sensed as idle will it stay idle (there might in fact be someone else's request on the way.)  Non-persistent CSMA (equivalent to 0-persistent CSMA) Same as above EXCEPT, when channel is found to be busy , don't keep monitoring to find THE instant when it becomes free. Instead, wait a random time and then sense again. Leads to better utilization

9.  p-persistent CSMA For slotted time channels If ready to send AND channel == idle then send with probability p and with prob. q = 1 - p defers to the next slot. Example: 0.5-persistent , 0.01-persistent Lower probability is better for higher frame transmission per frame-time

10. Comparison of the channel utilization versus load for various random access protocols. Higher load

11. CSMA with Collision detection (CD) = CSMA/CD CSMA protocols are clearly improved over ALOHA CSMA protocols can improve if stations abort their transmission as soon as they detect a collision. Used with LANs . In CSMA/CD, when a station detects a collision, it stops sending, even if in mid-frame. Waits a random time and then tries again.

12. CSMA/CD can be in one of three states: 1-Contention, 2-transmission, 3-idle. conceptual model

13. t 0 , a station has finished transmitting its frame. Any other station having a frame to send may now attempt to do so. If two or more stations decide to transmit simultaneously, there will be a collision. Collisions can be detected by looking at the power or pulse width of the received signal and comparing it to the transmitted signal.

14. Details of the contention algorithm. Suppose that two stations both begin transmitting at exactly time t 0 . How long will it take them to realize that there has been a collision? answer is: vital to determining the length of the contention period and hence what the delay and throughput will be. The minimum time to detect the collision is then just the time it takes the signal to propagate from one station to the other. worst-case scenario Let the time for a signal to propagate between the two farthest stations be t . At t 0 , one station begins transmitting. At t -Ԑ , an instant before the signal arrives at the most distant station, that station also begins transmitting. Of course, it detects the collision almost instantly and stops, but the little noise burst caused by the collision does not get back to the original station until time 2t - e . In other words, in the worst case a station cannot be sure that it has seized the channel until it has transmitted for 2t without hearing a collision.

16. 4.2.4 Limited-Contention Protocols two basic strategies for channel acquisition in a cable network: contention , as in CSMA collision-free methods . Basic bit-map method Reservation protocols Binary Countdown Each strategy can be rated as to how well it does with respect to the two important performance measures , delay at low load and channel efficiency at high load . Under conditions of light load, contention (i.e., pure or slotted ALOHA) is preferable due to its low delay. As the load increases , contention becomes increasingly less attractive, because the overhead associated with channel arbitration becomes greater. Just the reverse is true for the collision-free protocols. At low load, they have high delay, but as the load increases, the channel efficiency improves rather than gets worse as it does for contention protocols.

18. First divide the stations into (not necessarily disjoint) groups. Only the members of group 0 are permitted to compete for slot 0. If one of them succeeds, it acquires the channel and transmits its frame. If the slot lies fallow or if there is a collision, the members of group 1 contend for slot 1, etc. By making an appropriate division of stations into groups, the amount of contention for each slot can be reduced, thus operating each slot near the left end of SEE NEXT SLIDE

20. The trick is how to assign stations to slots. Before looking at the general case, let us consider some special cases. At one extreme, each group has but one member. Such an assignment guarantees that there will never be collisions because at most one station is contending for any given slot. We have seen such protocols before (e.g., binary countdown). The next special case is to assign two stations per group. The probability that both will try to transmit during a slot is p 2 , which for small p is negligible. As more and more stations are assigned to the same slot, the probability of a collision grows, but the length of the bit-map scan needed to give everyone a chance shrinks. The limiting case is a single group containing all stations (i.e., slotted ALOHA). What we need is a way to assign stations to slots dynamically, with many stations per slot when the load is low and few (or even just one) station per slot when the load is high.

21. The Adaptive Tree Walk Protocol It is convenient to think of the stations as the leaves of a binary tree, as illustrated in Fig. 4-9 . In the first contention slot following a successful frame transmission, slot 0, all stations are permitted to try to acquire the channel. If (one of them does so), fine. then (there is a collision) { if(slot==1) /*during slot 1 */ only those stations falling under node 2 in the tree may compete. If (one of them acquires the channel) /* no collision*/ { /*fine, reserve the frame under node 3 */ the slot following the frame is reserved for those stations under node 3. } else(collision) If (two or more stations under node 2 want to transmit) then there will be a collision during slot 1, in which case it is node 4's turn during slot 2. }

22. CSMA with Collision detection (CD) = CSMA/CD What is contention interval- how long must station wait after it sends until it knows it got control of the channel? It's twice the time to travel to the furthest station. Collision-free protocols: Bit-map protocol Binary countdown

23. IEEE 802.2: Describes the upper part of the data link layer, the LLC (Logical Link Control). Hide the differences between the various kinds of MAC protocols by providing a single format to network layer IEEE 802.2 standard Descriptions of the physical and lower part of the DLL are (MAC): IEEE 802.3 CSMA/CD LAN IEEE 802.4 Token Bus IEEE 802.5 Token Ring

24. Ethernet LAN standard (IEEE 802.3) 1-persistent CSMA/CD + Binary Exponential Back-off Architecture of the original Ethernet.

25. Ethernet Cabling (1) 10Base=10 Mbps, Base-band signaling

26. Ethernet Cabling (2) Transceiver : Electronic circuits that handle carrier detection and collision detection 10Base5, 10Base2, 10Base-T.

27. Ethernet Cabling (3) In Binary coding there's no way to distinguish a 0 bit from nothing-happening. Need to know when is middle of bit WITHOUT a clock => Manchester Encoding  In Manchester: Bit 1= High-Low Bit 0= Low-High Used in Ethernet  Differential Manchester Bit 1 indicated by absence of transition at the start of interval Better noise immunity Used in Token ring

28. Ethernet Cabling (4) Manchester Encoding

29. Ethernet MAC Protocol Preamble == 7 bytes of 10101010 for synchronization SOF: Start of Frame == 1 byte of 10101011 for compatibility with 802.4 and 802.5 Dest. Add. == 6 bytes of mac address multicast == (47 th bit=1) sending to a group of stations. Broadcast == (dest. = all 1's) to all stations on network Source Ad. == 6 bytes of MAC address Length/Type == number of bytes of data/Type of protocol Data == comes down from network layer Pad == ensures 64 bytes from Dest. Add. addr thru checksum. Checksum == 4 bytes of CRC. DIX Ethernet IEEE 802.3.

30. 1500 data byte restrict Max. frame length to 1526 bytes Also need Min. frame length ! Why?  To distinguish valid frames from garbage (Made by collision) need at-least 64 byte frame. If data in frame is less than 46 [=64-18] the Pad field is used to filled out frame to Min. frame length.  To prevented a station from completing the transmission of a short frame before the first bit has reached the far-end of the cable, where it may collide Transmitter need 2 τ time to detect noise of collision, where τ is propagation time of a frame to reach another end of cable.

31. Collision detection can take as long as 2 τ For a 10Mbps LAN with max. length of 2500 meters and 4 repeaters the round-trip time= τ ≈ 50 μ sec. So Min. frame length=500 bits => 64 bytes As the network speed goes up, the Min. frame length must go up or Max. length of cable come down.

32. Binary exponential back-off algorithm: Determine how randomize is done when collision occurs. After a collision, station waits 0 or 1 slot. If it collides again while doing this send, it picks a time of 0,1,2,3 slots. If again it collides the wait is 0 to 2 3 -1 times. In general after i collisions an random number between 0 and 2 i -1 is chosen and that number of slots is skipped. Max time is 2 10 -1 (or equal to 10 collisions.) After 10 collisions, an error is reported. Slot is determined by the worst case times; 500 meters + 4 repeaters = 512 bit times = 51.2 μ sec. Algorithm adapts to number of stations.

33. Ethernet Performance (1) Channel efficiency depends on F: Frame length, B: Network Bandwidth, L: Cable Length, C: Speed of signal propagation, e: optimal number of contention slots per frame. T= Time for transmission a frame = F/B τ = is propagation time of a frame thru cable = L/C A= Probability that a station acquires the channel in a slot with contention, Optimum=1/e T 1 channel efficiency = ------------------ = --------------------------- T+2 τ /A 1 + 2 B L e / c F

34. Ethernet Performance Efforts focus on improving both B and L, both of which will decrease efficiency. In all theatrical researches on performance evaluation of Ethernet, it’s assumed that traffic is Poisson but in real data they are not Poisson, but self similar. Efficiency of Ethernet at 10 Mbps with 512-bit slot times:

35. IEEE 802.3u: An addendum of existing 802.3 (1995) Idea: keep all old frame formats, interfaces and procedural rules but just reduce bite time from 100 nsec. to 10 nsec. Commonly use twisted pair cabling 100Base-T4: Use Ternary (3-level) signaling and UTP-Cat3 cabling 100Base-TX: Use 4B/5B (use 5 bits for transmit 4 bits) signaling and UTP-Cat5 cabling

36. IEEE 802.3z: Another addendum of existing 802.3 (1998) Goal: make 10 times faster + Compatible with existing Ethernet standard Support unacknowledged datagram service in both unicast and multicast Configuration is point-to-point rather than multi-drop (Use Hub or Switch)

37. Two different modes: Full-Duplex: Allows traffic in both directions at same time Is used when there is a switch connect to computer or other switches All lines are buffered  Contention is impossible  CSMA/CD is not used Half-Duplex When computers connect to Hub rather than Switches Don’t buffer frames, is just like classic Ethernet  Need CSMA/CD protocol  Reduce length of cable Carrier Extension : Tell hardware to add its own padding to extend frame length to 512 bytes.

38. Frame Bursting : Allow a sender to transmit a concatenated sequence of frames in a single transmission. This is efficient and perfect over carrier extention Cabling Use 8B/10B signaling for Fiber optic Use different encoding for 1000Base-T

39. User generate some data: Data are passed to Transport layer and adds a header, i.e. TCP header The resulting unit passes down to Network layer , adds headers, IP packet Then goes to DLL which adds its own header (CRC) Resulting frame given to Physical layer These devices operate in different layers 

40. Physical layer: Repeaters: Analog devices that are connected two cable segments, Amplifying incoming signal from one side and send out other. Don’t understand frames, packets or headers, Just understand Volt In classic Ethernet, to extend max. cable length from 500 meters to 2500 meters, 4 repeaters was allowed. Hubs: Has a number of input lines that it joins electrically. Frames arriving on any of the lines are send out on all others. If two frames arrive at the same time  Collision occurs Are like repeater and don’t understand frames, but usually don’t amplify incoming signal

41. Data Link Layer: Bridges: Connect two or more LANs. Use des. Add. In frame header to determine destination Switches: Are like Bridges and use des. Add. To find the route. Used to connect individual computers  need more number of ports, each port has own collision domain. Store & Forward : Get entire a frame then transmit Cut-through-switches : start forwarding the frame as soon as the des. Add. Field has come in.

42. Network layer: Routers: When a packet comes into, the frame header and trailer are stripped off and packet located in the frame’s payload field is passed routing software. Transport and Application layer: Transport gateway: Connect two computers that use different connection-oriented transport protocols. Ex. TCP/IP and ATM Application gateway: Understand the format and content of data and translate message from one format to another one.

43. Logical rather than physical configuration in hubbed or switched Ethernet Reasons? Security, Load, Broadcast,

44. In VLAN:  How many VLANs there will be?  Which computer will be on which VLAN?  What the VLANs will be called? Four physical LANs organized into two VLANs, gray and white, by (a) two bridges. (b) by switches.

46. ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols Limited-Contention Protocols Wavelength Division Multiple Access Protocols Wireless LAN Protocols

47. A wireless LAN. (a) A transmitting. (b) B transmitting.

48. The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A.

49. Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.

51. A simple example of switched Ethernet.

52. The original fast Ethernet cabling.

53. (a) A two-station Ethernet. (b) A multistation Ethernet.

55. WI-FI WiMAX Indoor - deals with mobility- 802.11 was designed to be mobile Ethernet Outdoor -provides service to buildings, and buildings are not mobile. They do not migrate from cell to cell often- so 802.16 was designed to be wireless, but stationary building distances involved can be several meters distances involved can be several kilometers LAN – more secure MAN - open communication over a city means that security and privacy are essential and mandatory 802.11 can use half-duplex communication , (single radio –which switch back & forth between Tx & Rx mode)something avoids to keep the cost of the radios low & that's why we have CSMA/CA & not CD. Uses spread spectrum 802.16 can use full-duplex communication - Traditional narrow-band radio is used with conventional modulation schemes QAM, QPSK etc. Indoor technology-Less users-required less bandwidth-use 2.4 and 5 GHz ISM range – less error prone Outdoor technology - Supports more users – need more bandwidth-uses 10-66 GHz ISM range- millimetre waves – use line of sight –absorb by rain – more error prone-need completely diff Tx-Rx tech-so completely diff physical layer- 802.11 is Omni directional Millimetre waves can be focused into directional beams , so multipath propagation is major issue. QoS- quality of service - 802.11 provides some support for real-time traffic (using PCF mode), it was not really designed for telephony and heavy-duty multimedia usage 802.16 is expected to support these applications completely because it is intended for residential as well as business use.

56. The protocol stack Physical layer radio transmission techniques MAC sublayer protocol Frame structure and Services

59. Two modes CSMA/CA- A contention based protocol. In 802.11 this mode is known as Distributed Coordination Function (DCF) Priority-based access – A contention free access protocol usable on the infrastructure mode. Known as Point Coordination Function (PCF) CS352 Fall,2005

60. Wireless LAN adaptors cannot detect collisions. Carrier Sensing – Listen to the media to see if it is free. Collision Avoidance – Minimize chances of collision by starting a random back off timer, when medium is free and prior to transmission CS352 Fall,2005

61. 802.11 standard specifies 3 transmission techniques allowed in the physical layer. The infrared method television remote controls do. The other two use short-range radio, using techniques called FHSS and DSSS . Both of these use a part of the spectrum that does not require licensing (the 2.4-GHz ISM band). EX: Radio-controlled garage door openers , Cordless telephones and microwave ovens use this band. All of these techniques operate at 1 or 2 Mbps and at low power In 1999, two new techniques were introduced to achieve higher bandwidth. These are (1) OFDM and operate @ up to 54 Mbps (2) HR-DSSS operate @ 11 Mbps In 2001, a second OFDM modulation was introduced, but in a different frequency band from the first one. Each of the five permitted transmission techniques infrared method ,FHSS,DSSS,OFDM,HR-DSSS makes it possible to send a MAC frame from one station to another. They differ, however, in the technology used and speeds achievable.

62. The infrared option: It uses diffused (i.e., not line of sight) transmission at 0.85 or 0.95 microns. Two speeds are permitted: 1 Mbps and 2 Mbps. @ 1 Mbps, an encoding scheme is used in which a group of 4 bits is encoded as a 16-bit codeword containing fifteen 0s and a single 1 - called Gray code . This code has the property that a small error in time synchronization leads to only a single bit error in the output. @ 2 Mbps, the encoding takes 2 bits and produces a 4-bit codeword Infrared signals cannot penetrate walls , so cells in different rooms are well isolated from each other. due to the low bandwidth this is not a popular option.

63. (2) FHSS (Frequency Hopping Spread Spectrum) Frequency-hopping spread spectrum ( FHSS ) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels , using a pseudorandom sequence known to both transmitter and receiver. It uses 79 channels, each 1-MHz wide, starting at the low end of the 2.4-GHz ISM band. A pseudorandom number generator is used to produce the sequence of frequencies hopped to. The amount of time spent at each frequency called the dwell time , is an adjustable parameter, but must be less than 400 msec. Pros: It provides a security since an intruder who does not know the hopping sequence or dwell time cannot eavesdrop on transmissions. Over longer distances, multipath fading can be an issue, and FHSS offers good resistance to it. It is also relatively insensitive to radio interference, which makes it popular for building-to-building links. Cons: its low bandwidth.

65. A spread-spectrum transmission offers 3 main advantages over a fixed-frequency transmission: Spread-spectrum signals are highly resistant to narrowband interference . The process of re-collecting a spread signal spreads out the interfering signal, causing it to recede into the background. Spread-spectrum signals are difficult to intercept An eavesdropper would only be able to intercept the transmission if they knew the pseudorandom sequence. Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference . The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.

66. ( 3) DSSS (Direct Sequence Spread Spectrum) In telecommunications, direct-sequence spread spectrum ( DSSS ) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. DSSS is also restricted to 1 or 2 Mbps. The scheme used has some similarities to the CDMA system but differs in other ways. Each bit is transmitted as 11 chips, using what is called a Barker sequence . It uses phase shift modulation at 1 Mbaud, transmitting 1 bit per baud when operating at 1 Mbps and 2 bits per baud when operating at 2 Mbps.

67. bps = baud per second x the number of bit per baud The number of bit per baud is determined by the modulation technique. Here are two examples: When FSK ("Frequency Shift Keying", a transmission technique) is used, each baud transmits one bit; only one change in state is required to send a bit. Thus, the modem's bps rate is equal to the baud rate: When we use a baud rate of 2400, you use a modulation technique called phase modulation that transmits four bits per baud. So: 2400 baud x 4 bits per baud = 9600 bps Such modems are capable of 9600 bps operation.

68. Features Of DSSS: It phase-modulates a sine wave pseudo randomly with a continuous string of pseudo noise (PN) code symbols called "chips", each of which has a much shorter duration than an inform ation bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate. It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal. pseudo noise (PN) code: In cryptography, pseudorandom noise (PRN) is a signal similar to noise which satisfies one or more of the standard tests for statistical randomness. Although it seems to lack any definite pattern, pseudorandom noise consists of a deterministic sequence of pulses that will repeat itself after its period. In cryptographic devices, the pseudorandom noise pattern is determined by a key and the repetition period can be very long, even millions of years.

72. Chipping code

74. (4) OFDM (Orthogonal Frequency Division Multiplexing) The first of the high-speed wireless LANs, 802.11a to deliver up to 54 Mbps in the wider 5-GHz ISM band. As the term FDM suggests, different frequencies are used— 52 of them=( 48 for data + 4 for synchronization) Since transmissions are present on multiple frequencies at the same time, this technique is considered a form of spread spectrum Splitting the signal into many narrow bands has some key advantages over using a single wide band, including better immunity to narrowband interference and the possibility of using non-contiguous bands. A complex encoding system is used , based on phase-shift modulation for speeds up to 18 Mbps and on QAM above that. At 54 Mbps, 216 data bits are encoded into 288-bit symbols.

75. OFDM is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme such as Quadrature Amplitude Modulation Phase-Shift Keying at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth In mathematics, two vectors are orthogonal if they are perpendicular, i.e., they form a right angle.

79. Quadrature amplitude modulation (QAM) is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. These two waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers or quadrature components — hence the name of the scheme. The modulated waves are summed, and the resulting waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying (ASK), or in the analog case of phase modulation (PM) and amplitude modulation. In the digital QAM case, a finite number of at least two phases, and at least two amplitudes are used. PSK modulators are often designed using the QAM principle, but are not considered as QAM since the amplitude of the modulated carrier signal is constant.

81. Quadrature phase-shift keying (QPSK) Sometimes known as quaternary or quadriphase PSK, 4-PSK, or 4-QAM[6], QPSK uses four points on the constellation diagram, equispaced around a circle. With four phases, QPSK can encode two bits per symbol, shown in the diagram with Gray coding to minimize the BER — twice the rate of BPSK. Analysis shows that this may be used either to double the data rate compared to a BPSK system while maintaining the bandwidth of the signal or to maintain the data-rate of BPSK but halve the bandwidth needed. As with BPSK, there are phase ambiguity problems at the receiver and differentially encoded QPSK is used more often in practice.

83. (5) HR-DSSS (High Rate Direct Sequence Spread Spectrum) Another spread spectrum technique It uses 11 million chips/sec to achieve 11 Mbps in the 2.4-GHz band. It is called 802.11b but is not a follow-up to 802.11a. Data rates supported by 802.11b are 1, 2, 5.5, and 11 Mbps. The two slow rates run at 1 Mbaud, with 1 and 2 bits per baud, respectively, using phase shift modulation (for compatibility with DSSS). The two faster rates run at 1.375 Mbaud, with 4 and 8 bits per baud, respectively, using Walsh/Hadamard codes. In practice, the operating speed of 802.11b is nearly always 11 Mbps. Although 802.11b is slower than 802.11a, its range is about 7 times greater, which is more important in many situations.

84. Limiting factor for speed is the fast fading due to multipath propagation In wireless telecommunications, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings. The effects of multipath include constructive and destructive interference, and phase shifting of the signal. In wireless systems, fading may either be due to multipath propagation In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media.

85. The presence of reflectors in the environment surrounding a transmitter and receiver create multiple paths that a transmitted signal can traverse. As a result, the receiver sees the multiple copies of the transmitted signal, each traversing a different path. Each signal copy will experience differences in attenuation, delay and phase shift while travelling from the source to the receiver. This can result in either constructive or destructive interference, amplifying or attenuating the signal power seen at the receiver. Strong destructive interference is frequently referred to as a deep fade and may result in temporary failure of communication due to a severe drop in the channel signal-to-noise ratio. A common example of multipath fading is the experience of stopping at a traffic light and hearing an FM broadcast degenerate into static, while the signal is re-acquired if the vehicle moves only a fraction of a meter. The loss of the broadcast is caused by the vehicle stopping at a point where the signal experienced severe destructive interference. Cellular phones can also exhibit similar momentary fades.

86. Diffracted Signal Reflected Signal Transmitter Receiver Direct Signal Building Scattered Signal

87. The 802.11 MAC sub layer protocol is quite different from that of Ethernet With Ethernet, a station just waits until the ether goes silent and starts transmitting. If it does not receive a noise burst back within the first 64 bytes, the frame has almost assuredly been delivered correctly. With wireless, this situation does not hold. To start with, there are 2 problems. (1)Hidden station problem Not all stations are within radio range of each other transmissions going on in one part of a cell may not be received elsewhere in the same cell. In this example, station C is transmitting to station B. If A senses the channel, it will not hear anything and falsely conclude that it may now start transmitting A B C

88. (2)Exposed station problem B wants to send to C so it listens to the channel. When it hears a transmission, it falsely concludes that it may not send to C, even though A may be transmitting to D . In addition, most radios are half duplex, meaning that they cannot transmit and listen for noise bursts at the same time on a single frequency. As a result of these problems, 802.11 does not use CSMA/CD, as Ethernet does.

89. Hidden Terminal Problem Exposed Terminal Problem Hidden Terminal problem: - Z can’t sense X; Tx to Y and collision with X Exposed terminal problem: - W senses Y but can’t send to X X Y Z X Y Z W

90. MACA uses signaling packets for collision avoidance RTS (request to send) -Sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send) -Receiver grants the right to send as soon as it is ready to receive Signaling packets contain Sender address Receiver address Duration Variants of this method are used in IEEE 802.11

91. MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal Now C does not have to wait, as it cannot receive CTS from A CTS CTS A B C RTS CTS RTS B C A RTS

93. DCF (Distributed Coordination Function) PCF (Point Coordination Function) CSMA/CA (CSMA with Collision Avoidance). RTS (Request To Send) CTS (Clear To Send) NAV (Network Allocation Vector) beacon frame Interframe spacing in 802.11 SIFS (Short InterFrame Spacing). PIFS (PCF InterFrame Spacing) DIFS (DCF InterFrame Spacing) EIFS (Extended InterFrame Spacing)

94. DCF (Distributed Coordination Function), does not use any kind of central control (similar to Ethernet). PCF (Point Coordination Function), uses the base station to control all activity in its cell. All implementations must support DCF but PCF is optional. Logical Link Control Contention service Contention-free service MAC Layer Point Coordination Function (PCF) Distributed Coordination Function (DCF) IEEE 802.11b PHY Layer Infrared 1 Mbps 2 Mbps IEEE 802.11 Access Point IEEE 802.11a Ad hoc 2.4 GHzOFDM 1, 2, 5.5, 11 Mbps 6, 9, 12, 18, 24, 36, 48, 54 Mbps 2.4 GHz FHSS 1 Mbps 2 Mbps 2.4 GHz DSSS 1 Mbps 2 Mbps 5 GHz OFDM 6, 9, 12, 18, 24, 36, 48, 54 Mbps 2.4 GHz DSSS 1, 2, 5.5, 11 Mbps IEEE 802.11g

96. virtual channel sensing , see fig. In this example, A wants to send to B. C is a station within range of A (and possibly within range of B, D is a station within range of B but not within range of A. The protocol starts when A decides it wants to send data to B. It begins by sending an RTS frame to B to request permission to send it a frame. When B receives this request, it may decide to grant permission, in which case it sends a CTS frame back. Upon receipt of the CTS, A now sends its frame and starts an ACK timer. Upon correct receipt of the data frame, B responds with an ACK frame, terminating the exchange. If A's ACK timer expires before the ACK gets back to it, the whole protocol is run again. D B A C

97. Now let us consider this exchange from the viewpoints of C and D. C is within range of A, so it may receive the RTS frame. If it does, it realizes that someone is going to send data soon, so for the good of all it desists from transmitting anything until the exchange is completed. From the information provided in the RTS request, it can estimate how long the sequence will take, including the final ACK, so it asserts a kind of virtual channel busy for itself, indicated by NAV (Network Allocation Vector) in Fig. 4-27 . D does not hear the RTS, but it does hear the CTS, so it also asserts the NAV signal for itself. Note that the NAV signals are not transmitted; they are just internal reminders to keep quiet for a certain period of time.

98. The use of virtual channel sensing using CSMA/CA.

99. A fragment burst. Once the channel has been acquired using RTS and CTS, multiple fragments can be sent in a row These sequence of fragments is called a fragment burst. Fragmentation increases the throughput by restricting retransmissions to the bad fragments rather than the entire frame

100. transmission order is completely controlled by the base station, So In PCF mode, no collisions ever occur basic mechanism base station to broadcast a beacon frame periodically (10 to 100 times per second). This beacon frame contains system parameters, such as hopping sequences and dwell times (for FHSS), clock synchronization, etc. It also invites new stations to sign up for polling service. Once a station has signed up for polling service at a certain rate, it is effectively guaranteed a certain fraction of the bandwidth, thus making it possible to give quality-of-service guarantees.

101. Central control and distributed control operating at the same time. It works by carefully defining the interframe time interval. After a frame has been sent, a certain amount of dead time is required before any station may send a frame. Four different intervals are defined, each for a specific purpose. The four intervals are depicted SIFS (Short InterFrame Spacing). PIFS (PCF InterFrame Spacing) DIFS (DCF InterFrame Spacing) EIFS (Extended InterFrame Spacing)

102. The shortest interval is SIFS (Short InterFrame Spacing). It is used to allow the parties in a single dialog the chance to go first. This includes letting the receiver send a CTS to respond to an RTS , letting the receiver send an ACK for a fragment or full data frame, and letting the sender of a fragment burst transmit the next fragment without having to send an RTS again. There is always exactly one station that is entitled to respond after a SIFS interval. If it fails to make use of its chance and a time PIFS (PCF InterFrame Spacing) elapses, the base station may send a beacon frame or poll frame. This mechanism allows a station sending a data frame or fragment sequence to finish its frame without anyone else getting in the way, but gives the base station a chance to grab the channel when the previous sender is done without having to compete with eager users.

103. If the base station has nothing to say and a time DIFS (DCF InterFrame Spacing) elapses, any station may attempt to acquire the channel to send a new frame. The usual contention rules apply , and binary exponential backoff may be needed if a collision occurs. The last time interval, EIFS (Extended InterFrame Spacing), is used only by a station that has just received a bad or unknown frame to report the bad frame . The idea of giving this event the lowest priority is that since the receiver may have no idea of what is going on it should wait a substantial time to avoid interfering with an ongoing dialog between two stations.

104. Interframe spacing in 802.11.

105. The 802.11 data frame.

106. The 802.11 standard defines three different classes of frames on the wire: data, control, and management. Each of these has a header with a variety of fields used within the MAC sublayer. In addition, there are some headers used by the physical layer but these mostly deal with the modulation techniques used First comes the Frame Control field. It itself has 11 subfields. The first of these is the Protocol version , which allows two versions of the protocol to operate at the same time in the same cell. Type (data, control, or management) and Subtype fields (e.g., RTS or CTS). The To DS and From DS bits indicate the frame is going to or coming from the intercell distribution system (e.g., Ethernet). The MF bit means that more fragments will follow. The Retry bit marks a retransmission of a frame sent earlier. The Power management bit is used by the base station to put the receiver into sleep state or take it out of sleep state. The More bit indicates that the sender has additional frames for the receiver. The W bit specifies that the frame body has been encrypted using the WEP (Wired Equivalent Privacy) algorithm. Finally, the O bit tells the receiver that a sequence of frames with this bit on must be processed strictly in order.

107. second field of the data frame, the Duration field , tells how long the frame and its acknowledgement will occupy the channel. This field is also present in the control frames and is how other stations manage the NAV mechanism. The frame header contains four addresses , all in standard IEEE 802 format. The source and destination are obviously needed, other two for - frames may enter or leave a cell via a base station. The other two addresses are used for the source and destination base stations for intercell traffic. The Sequence field allows fragments to be numbered. Of the 16 bits available, 12 identify the frame and 4 identify the fragment. The Data field contains the payload, up to 2312 bytes, followed by the usual Checksum.

108. Management frames have a format similar to that of data frames, except without one of the base station addresses, because management frames are restricted to a single cell. Control frames are shorter , having only one or two addresses, no Data field, and no Sequence field. The key information here is in the Subtype field, usually RTS, CTS, or ACK.

109. The 802.11 standard states that each conformant wireless LAN must provide 9 services. These services are divided into two categories: 5 distribution services and 4 Intercell station services . The distribution services relate to managing cell membership and interacting with stations outside the cell. the station services relate to activity within a single cell.

110. Association Disassociation Reassociation Distribution (how to route the frame directly- via adhoc N/w) Integration Frame Translation service - from 802.11 to non 802.11 standard N/w 5 Distribution Services

111. Association. This service is used by mobile stations to connect themselves to base stations. Typically, it is used just after a station moves within the radio range of the base station. Upon arrival, it announces its identity and capabilities. The capabilities include the data rates supported, need for PCF services (i.e., polling), and power management requirements. The base station may accept or reject the mobile station. If the mobile station is accepted, it must then authenticate itself. Disassociation . Either the station or the base station may disassociate, thus breaking the relationship. A station should use this service before shutting down or leaving, base station may also use it before going down for maintenance.

112. Authentication(secret key) Deauthentication Privacy (Ency-Dec) Data Delivery (data Txmission) Intracell Services

113. ISM: Industry, Science, Medicine unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz

114. and 802.11b/g 802.11a

116. IEEE 802.11 variants DSSS: direct sequence spread spectrum FHSS: frequency hopping spread spectrum OFDM: orthogonal frequency division multiplexing 802.11a 802.11b 802.11g 802.11 Standard approved Sep. 1999 Sep. 1999 June 2003 July 1997 Available bandwidth 300 MHZ 83.5 MHZ 83.5 MHZ 83.5 MHZ freq. of operation 5.15-5.35G 5.725-5.825G 2.4-2.4835G 2.4-2.4835G 2.4-2.4835G No. of non-overlapping Ch. 4 3 3 3 Rate per channel (Mbps) 6,12,24,36,48,54 1, 2, 5.5, 11 1, 2, 5.5, 11, 6, 9, 12, 18, 24, 36, 48, 54 1, 2 Range ~150 feet (indoor) 225 (outdoor) ~225 feet ~225 feet ?? Modulation OFDM DSSS/CCK DSSS/CCK; DSSS/OFDM DSSS, FHSS

117. Lets refresh a bit…. CSMA IN Wired n Wireless Diff between Wifi and WiMAX

143. Comparison of 802.11 and 802.16 The 802.16 Protocol Stack The 802.16 Physical Layer The 802.16 MAC Sublayer Protocol The 802.16 Frame Structure

145. W orldwide I nteroperability for M icrowave Acc ess Brand licensed by the WiMax Forum. “ a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL” WiMAX was seen as more of a Metropolitan Area Network (MAN) technology providing a much larger coverage. Based on IEEE 802.16

146. WiMAX, in fact, comes in two forms, a so called ‘ fixed WiMAX’ and ‘ mobile WiMAX’. WiMAX in its fixed form is seen as a possible alternative to expensive cable and fibre deployment. It is faster to deploy and less expensive and it also offers operators more flexibility in terms of deployment time frame and possible installation areas. 3G or other cellular network operators could see this as a potential substitute or as a complement to their cellular product.

147. Wi-Fi has grown from being just a LAN cable replacement technology to a public wireless access technology. Cheap and readily available equipment. WiFi has been viewed as complementary to 3G and other mobile standards as it has worked to enhance mobile services offered by operators. It’s coverage is not as great as that of 3G, but it gives a much higher transmission rate than mobile technology. Handoff between WiFi access points is still not possible and, therefore, it is known more as a wireless access technology than a mobile technology.

148. WI-FI WiMAX Indoor - deals with mobility- 802.11 was designed to be mobile Ethernet Outdoor -provides service to buildings, and buildings are not mobile. They do not migrate from cell to cell often- so 802.16 was designed to be wireless, but stationary building distances involved can be several meters distances involved can be several kilometers LAN – more secure MAN - open communication over a city means that security and privacy are essential and mandatory 802.11 can use half-duplex communication , (single radio –which switch back & forth in Tx & Rx mode)something avoids to keep the cost of the radios low & that's why we have CSMA/CA & not CD. 802.16 can use full-duplex communication Indoor tech-Less users-less bandwidth-use 2.4 and 5 GHz ISM range – less error prone Outdoor tech - Supports more users – need more bandwidth-uses 10-66 GHz ISM range- millimetre waves – use line of sight –absorb by rain – more error prone-need completely diff Tx-Rx tech-so completely diff physical layer 802.11 is omnidirectional Millimeter waves can be focused into directional beams , so multipath propagation is major issue. QoS- quality of service - 802.11 provides some support for real-time traffic (using PCF mode), it was not really designed for telephony and heavy-duty multimedia usage 802.16 is expected to support these applications completely because it is intended for residential as well as business use.

150. Physical layer signal property & Different Tx Schemes. broadband wireless needs a lot of spectrum supports more users Uses 10-to-66 GHz range. In this range waves are in millimetre So they travel in straight line As a consequence, the base station can have multiple antennas, each pointing at a different sector Each sector has its own users and is fairly independent of the adjoining ones signal strength in the millimetre band falls off sharply with distance from the base station, the signal-to-noise ratio also drops with distance from the base station. For this reason, 802.16 employs three different modulation schemes, depending on how far the subscriber station is from the base station. For close-in subscribers, QAM-64 is used, with 6 bits/baud. For medium-distance subscribers, QAM-16 is used, with 4 bits/baud. For distant subscribers, QPSK is used, with 2 bits/baud. For example, for a typical value of 25 MHz worth of spectrum, QAM-64 gives 150 Mbps, QAM-16 gives 100 Mbps, and QPSK gives 50 Mbps. In other words, the farther the subscriber is from the base station, To support broadband uses FDD (Frequency Division Duplexing) and TDD (Time Division Duplexing).

151. The 802.16 transmission environment.

152. The base station periodically sends out frames. Each frame contains time slots. The first ones are for downstream traffic. Then comes a guard time used by the stations to switch direction. Finally, upstream traffic. The number of time slots devoted to each direction can be changed dynamically to match the bandwidth in each direction to the traffic. Downstream traffic is mapped onto time slots by the base station. The base station is completely in control for this direction. It uses Hamming codes to do forward error correction in the physical layer.

153. The downstream channel is fairly straightforward. The base station simply decides what to put in which sub frame. The upstream channel is more complicated since there are competing uncoordinated subscribers that need access to it. Its allocation is tied closely to the quality-of-service issue. Two forms of bandwidth allocation : per station - subscriber station aggregates the needs of all the users in the building and makes collective requests for them per connection - base station manages each connection directly Four classes of service are defined as follows: Constant bit rate service. Real-time variable bit rate service. Non-real-time variable bit rate service. Best-efforts service. Frames and time slots for time division duplexing.

154. Constant bit rate service. Constant bit rate service is intended for transmitting uncompressed voice such as on a T1 channel. This service needs to send a predetermined amount of data at predetermined time intervals. It is accommodated by dedicating certain time slots to each connection of this type. Once the bandwidth has been allocated, the time slots are available automatically, without the need to ask for each one. Real-time variable bit rate service Real-time variable bit rate service is for compressed multimedia and other soft real-time applications in which the amount of bandwidth needed each instant may vary. It is accommodated by the base station polling the subscriber at a fixed interval to ask how much bandwidth is needed this time. Non-real-time variable bit rate service. Non-real-time variable bit rate service is for heavy transmissions that are not real time , such as large file transfers. For this service the base station polls the subscriber often , but not at rigidly-prescribed time intervals. A constant bit rate customer can set a bit in one of its frames requesting a poll in order to send additional (variable bit rate) traffic. Best-efforts service. best-efforts service is for everything else . No polling is done and the subscriber must contend for bandwidth with other best-efforts subscribers. Requests for bandwidth are done in time slots marked in the upstream map as available for contention. If a request is successful, its success will be noted in the next downstream map. If it is not successful, unsuccessful subscribers have to try again later. To minimize collisions, the Ethernet binary exponential backoff algorithm is used.

155. Security sub layer The bottom one - deals with privacy and security, because its public outdoor networks It also manages encryption, decryption, and key management. MAC sub layer common part. channel management In WiMax model - it is base station that controls the system. It can schedule the downstream (i.e., base to subscriber) channels very efficiently and plays a major role in managing the upstream (i.e., subscriber to base) channels as well. it is completely connection oriented so QoS guaranteed for telephony and multimedia communication. where in Wi-Fi its CSMA - contention The service-specific convergence sublayer LLC protion. provides function to interface to the network layer. Mapping at correct service- connection oriented & connectionless 802.16 was designed to integrate datagram protocols (e.g., PPP, IP, and Ethernet) and ATM. The problem is that IP packet protocols are connectionless and ATM is connection oriented.

156. Generic frame structure The EC bit tells whether the payload is encrypted. The Type field identifies the frame type, mostly telling whether packing and fragmentation are present. The CI field indicates the presence or absence of the final checksum. The EK field tells which of the encryption keys is being used (if any). The Length field gives the complete length of the frame, including the header. The Connection identifier tells which connection this frame belongs to. The HeaderCRC field is a checksum over the header only, using the polynomial x 8 + x 2 + x + 1. The checksum is also optional due to the error correction in the physical layer Bandwidth request frame It starts with a 1 bit instead of a 0 bit and is similar to the generic header except that the second and third bytes form a 16-bit number telling how much bandwidth is needed to carry the specified number of bytes. Bandwidth request frames do not carry a payload or full-frame CRC.

157. Channel bandwidths can be chosen by operator (e.g. for sectorization) 1.5 MHz to 20 MHz width channels. MAC designed for scalability independent of channel bandwidth MAC designed to support thousands of users. Wide, fixed (20MHz) frequency channels MAC designed to support 10’s of users 802.16 802.11

158. 802.16a ~5.0 bps/Hz ~2.7 bps/Hz 54 Mbps 20 MHz 63 Mbps* 10, 20 MHz; 1.75, 3.5, 7, 14 MHz; 3, 6 MHz 802.11a Channel Bandwidth Maximum bps/Hz Maximum Data Rate * Assuming a 14 MHz channel 802.16a is designed for metropolitan performance

159. 802.16 is designed for market coverage Optimized for outdoor NLOS performance Standard supports mesh network topology Standard supports advanced antenna techniques Optimized for indoor performance No mesh topology support within ratified standards 802.16 802.11

160. 802.16 is designed for distance Optimized for up to 50 Km Designed to handle many users spread out over kilometers Designed to tolerate greater multi-path delay spread (signal reflections) up to 10.0μ seconds PHY and MAC designed with multi-mile range in mind StandardMAC;Sectoring/MIMO/AMC for Rate/Range dynamic tradeoff Optimized for ~100 meters No “near-far” compensation. Designed to handle indoor multi-path(delay spread of 0.8μ seconds). Optimization centers around PHY and MAC layer for 100m range. Range can be extended by cranking up the power – but MAC may be non-standard. 802.16 802.11

161. 802.16a is designed for carrier class operation Grant-request MAC Designed to support Voice and Video from ground up Supports differentiated service levels: e.g. T1 for business customers; best effort for residential. TDD/FDD/HFDD – symmetric or asymmetric Centrally-enforced QoS Contention-based MAC (CSMA/CA) => no guaranteed QoS Standard cannot currently guarantee latency for Voice, Video Standard does not allow for differentiated levels of service on a per-user basis TDD only – asymmetric 802.11e (proposed) QoS is prioritization only 802.16a 802.11

162. 802.16a maintains fixed wireless security Triple-DES (128-bit) and RSA (1024-bit) Existing standard is WPA + WEP 802.11i in process of addressing security 802.16a 802.11

166. 802.11 and 802.16 both gain broader industry acceptance through conformance and interoperability by multiple vendors 802.16 complements 802.11 by creating a complete MAN-LAN solution 802.11 is optimized for license-exempt LAN operation 802.16 is optimized for license-exempt and licensed MAN operation.

167. WiMAX will not replace WiFi completely, but work TOGETHER Intel is currently integrating WiMAX and WiFi into a single Centrino chip. WiFi’s primary role will always be autonomous hotspot service areas (indoor and outdoor 0 ft. < cell radii <500 ft.). WiMax will ultimately replace WiFi in large-scale (greater than 1 mi.Sq.) commercial and public roles.

172. Bluetooth Architecture Bluetooth Applications The Bluetooth Frame Structure

173. The basic unit of a Bluetooth system is a piconet, which consists of a master node and up to 7 active slave nodes within a distance of 10 meters. Also there can be up to 255 parked nodes in the net parked nodes are devices that the master has switched to a low-power state to reduce the drain on their batteries. In parked state, a device cannot do anything except respond to an activation or beacon signal from the master. Multiple piconets can exist in the same (large) room and can even be connected via a bridge node, as shown in Fig. An interconnected collection of piconets is called a scatternet . Piconet is a centralized TDM system, with the master controlling the clock and determining which device gets to communicate in which time slot. All communication is between the master and a slave; direct slave-slave communication is not possible. Two piconets can be connected to form a scatternet.

174. The Bluetooth profiles.

175. A typical Bluetooth data frame.

176. Access code identifies the master - so that slaves within radio range of two masters can tell which traffic is for them. 54-bit header containing typical MAC sublayer fields. data field , of up to 2744 bits (for a five-slot transmission). For a single time slot, the format is the same except that the data field is 240 bits. Header Fields in Detail: The Address field identifies which of the eight active devices the frame is intended for. The Type field identifies the frame type (ACL, SCO, poll, or null), the type of error correction used in the data field, and how many slots long the frame is. The Flow bit is asserted by a slave when its buffer is full and cannot receive any more data. This is a primitive form of flow control. The Acknowledgement bit is used to piggyback an ACK onto a frame. The Sequence bit is used to number the frames to detect retransmissions. The protocol is stop-and-wait, so 1 bit is enough. 8-bit header Checksum . The entire 18-bit header is repeated three times to form the 54-bit header shown in On the receiving side, a simple circuit examines all three copies of each bit. If all three are the same, the bit is accepted. If not, the majority opinion wins. Thus, 54 bits of transmission capacity are used to send 10 bits of header. The reason is that to reliably send data in a noisy environment using cheap, low-powered (2.5 mW) devices with little computing capacity, a great deal of redundancy is needed.

177. Various formats are used for the data field for ACL frames. The SCO frames: the data field is always 240 bits. Three variants are defined, permitting 80, 160, or 240 bits of actual payload, with the rest being used for error correction. In the most reliable version (80-bit payload), the contents are just repeated three times, the same as the header. Since the slave may use only the odd slots, it gets 800 slots/sec, just as the master does. With an 80-bit payload, the channel capacity from the slave is 64,000 bps For the least reliable variant (240 bits/slot with no redundancy at this level), three full-duplex voice channels can be supported at once, which is why a maximum of three SCO links is permitted per slave.

178. Bridges from 802.x to 802.y Local Internetworking Spanning Tree Bridges Remote Bridges Repeaters, Hubs, Bridges, Switches, Routers, Gateways Virtual LANs

179. Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN.

180. Operation of a LAN bridge from 802.11 to 802.3.

181. The IEEE 802 frame formats. The drawing is not to scale.

182. A configuration with four LANs and two bridges.

183. Two parallel transparent bridges.

184. (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree.

185. Remote bridges can be used to interconnect distant LANs.

186. (a) Which device is in which layer. (b) Frames, packets, and headers.

187. (a) A hub. (b) A bridge. (c) a switch.

188. A building with centralized wiring using hubs and a switch.

189. (a) Four physical LANs organized into two VLANs, gray and white, by two bridges. (b) The same 15 machines organized into two VLANs by switches.

190. Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not.

191. The 802.3 (legacy) and 802.1Q Ethernet frame formats.

192. Channel allocation methods and systems for a common channel.