1. #1EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks Lecture 8: Mobile Data, Part III Instructor : Jila Seraj email: jseraj@engr.smu.edu http://www.engr.smu.edu/~jseraj/ tel: 214-505-6303

2. #2EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Session Outline  Review of last week  Wireless LAN

3. #3EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Announcements  Answer to homework #1 is on the web  Homework #2 is on the web. —Deadline for in-campus students October 24 —Deadline for distant students November 7

4. #4EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS - Network Architecture GPRS makes use of existing GSM base stations Serving GPRS support node = packet switch with mobility management capabilities Gateway GSN = packet switch interworks with other networks Internet or other networks GGSN MSC/ VLR SGSN HLR BSC/PCU

5. #5EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS, Cont...  GSM Release’97 introduced general packet radio service (GPRS) for bursty data  Make use of existing GSM network equipment and functions  In Contrast to CDPD, it is integrated into GSM, i.e. dedicated Control channel and data channel.  Requires two new network element, GGSN and SGSN

6. #6EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS, Cont...  GGSN = Gateway GPRS Support Node — External interfaces — Routing  GPRS register maintains GPRS subscriber data and routing information. Normally it is integrated in GSM HLR  PCU (Packet Control Until) is collocated with BSC.

7. #7EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS, Cont... Three class of mobile terminals —Class A: Operates GPRS and Circuit switched service simultaneously —Class B: Monitors the Control channels of GPRS and GSM simultaneously but can operate one set of services at a time —Class C: Only CS or GPRS capable.

8. #8EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS, Cont...  For mobility management a new concept is defined, Routing Area RAI = MCC +MNC + LAC + RAC

9. #9EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS Interfaces

10. #10EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS – Data Connection  GPRS data connection starts with Attach and ends with Detach.  Attach is the phase when the mobile informs the network of its intention to create a data connection  At conclusion of Attach, SGSN is ready to set up data services on behalf of the mobile user.

11. #11EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS – Data Connection, Cont…  Detach is the phase when mobile terminates the connection.  GPRS requires subscription

12. #12EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS Attach Scenario BSS HLR SGSN BTS IMSI, P_TMSI+OLD RAI… Update Location Insert Subs. Data Insert Data Ack Update Location GPRS Attach Accepted

13. #13EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS – Mobile Attach Scenario  Mobile sends Attach message. This message contains P-TMSI or TMSI. It also contains NSAPI (Network Service Point Identifier)  SGSN contacts HLR to verify if the user is permitted to use the service  After authentication, SGSN send back Attach Accepted together with a TLLI (Temporary Logical Link Identity)

14. #14EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS – Mobile Attach Scenario  A database in SGSN is now populated with mobile identity and TLLI. TLLI is used by logical link controller in the SGSN.

15. #15EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, GPRS – Setting Up Packet Data  After attach the mobile is known by SGSN and have an identity there, but it is not known to the external network.  First it needs to create an identity for itself by performing a procedure called PDP Context Activation. PDP is Packet Data Protocol, which could be IP or x.25 protocol.

16. #16EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, PDP Context Activation BSS GGSN SGSN BTS Activate PDP Context Create PDP Context Request NASPI, PDP type PDP, QoS,APN Create PDP Context Response PDP Address, QoS Activate PDP Context Accepted PDP Type, PDP Address, QoS

17. #17EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, PDP Context Activation, Cont..  Mobile requests PDP Context Activation  Based on the information provided, SGSN determines which GGSN to connect to. The GGSN should be capable to support the PDP requested by mobile  GGSN updates its data base and assign a TID to the mobile and SGSN  SGSN updates its data base with the GGSN address and TID. It then send PDP Context Activation Accepted message to mobile

18. #18EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, Actually Sending Data  After PDP Context Activation the mobile is known to the external packet network (PDN)  When SGSN receives data from mobile, it looks up its database and relate the TLLI to NSAPI.  SGSN and SNDPC pad the IP packet and replace the destination address with GGSN IP address and sets GTP header to TID

19. #19EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Review, Actually Sending Data, Cont…  Packets are then sent to GGSN with SGSN as sender  At GGSN, the additional information is removed to get the original packet. The packet can now be routed to its intended destination.

20. #20EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Wireless LANs  Wireless LANs are usually logical bus topology (broadcast medium)  Why wireless LANs? —Saves trouble of rewiring a building —Portable computing devices (laptops, PDAs) are more common

21. #21EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocols  MAC protocol is a sublayer in data link layer  For LANs, data link layer = logical link control (LLC) sublayer + MAC sublayer data link physical LLC MAC network - defines how stations access the shared medium

22. #22EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocols, Cont.. —LLC sublayer builds on MAC sublayer to provide medium-independent communication service to higher layers (makes MAC sublayer transparent) —LLC can provide appearance of connectionless or connection-oriented service

23. #23EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocols, Cont.. Connectionless service treats each message independently. No connection setup and no sequential order Connection-oriented service requires connection setup and preserves sequential order of messages

24. #24EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC protocols, Token Passing  Token ring and token bus —Every station connected to the bus is given a token —The token is passed according to order —When a station has something to send, it keeps the token until it is done, before sending it to the next station.  It is fair and has no contention  The system encounters delays for sending the token.

25. #25EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocols: Token Passing, Cont..  Token passing is another technique to eliminate contention (collisions)  Token is short packet representing permission to transmit —Token is passed from station to station according to an arranged order defining a logical token ring topology —A station with the token can transmit for a limited time —After transmission, token is sent to next station in ring

26. #26EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocols: Polling  Objective to eliminate random contention (collisions) which reduces throughput of system  Polling is centralized control —One station will periodically poll other stations to see if they have data to transmit —A polled station may transmit data, otherwise controller will poll next station in a list

27. #27EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocols: Polling  Polling involves exchange of control messages between stations and controller —Efficient only if roundtrip propagation delay is small overhead due to control messages is small user population is not large and bursty —As population increases with more bursty users, performance of polling degrades

28. #28EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocols: Polling  Polling is used in wired network environments but not popular in wireless networks

29. #29EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Token passing, Cont..  Commonly used in wired LANs (IEEE 802.4 token bus and 802.5 token ring), token passing has not found much adoption in wireless networks  Overhead is increased to improve throughput under heavy load —Issue is efficiency

30. #30EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC protocols: Aloha  Aloha —Stations starts sending when they have something to send —Pure Aloha, no contention resolution, relies on timed-out acks, max throughput 18% —Slotted Aloha, no contention resolution, relies on timed-out acks, only can start sending in the beginning of a slot, max through put 36%

31. #31EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Pure ALOHA, Cont..  Throughput —Assume infinite population of stations generating frames at random times —Each frame is transmitted in fixed time T —Assume average number of transmission attempts is S in any interval T

32. #32EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Pure ALOHA, Cont..  Throughput —Number of new transmission attempts in any interval t has Poisson probability distribution: Pr(k transmissions in interval t ) = (St) k e - St /k! —Let G = “offered load” = new transmissions and retransmissions

33. #33EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Pure ALOHA, Cont.. —In equilibrium, throughput (rate of successfully transmitted frames) = rate of new transmissions, S S = GP 0 where P 0 = probability of successful transmission (no collision) —P 0 depends on “vulnerable interval” for frame, 2T

34. #34EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Pure ALOHA, Cont.. frame A frame B frame C time -T0T - transmission attempt at time 0 - collision if starts in interval (-T,0) - collision if starts in interval (0,T)

35. #35EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Pure ALOHA, Cont.. P 0 = Pr(no other frame in 2T interval) —Assume total number of frames in any interval t is also Poisson distributed, with average G: Pr(k transmissions in t) = (Gt) k e -Gt /k! then P 0 = e -2G —By substitution, throughput is S = GP 0 = Ge -2G

36. #36EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Pure ALOHA, Cont.. —This is maximum at G = 0.5, where S = 1/2e = 0.184 (frames per interval T) Pure ALOHA achieves low throughput

37. #37EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Slotted ALOHA  Slotted ALOHA is a modification to increase efficiency —Time is divided into time slots = transmission time of a frame, T —All stations are synchronized (eg, by periodic synchronization pulse)

38. #38EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Slotted ALOHA  Slotted ALOHA is a modification to increase efficiency —Any station with data must wait until next time slot to transmit —Any time slot with two or more frames results in a collision and loss of all frames – retransmitted after a random time

39. #39EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Slotted ALOHA, Cont..  “Vulnerable interval” is reduced by factor of 2 to just T frame A frame B time -T0T - transmission attempt at time 0 - collision if frame B was ready in interval (-T,0)

40. #40EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Slotted ALOHA, Cont..  Throughput P 0 = Pr(no frames ready in previous time slot) = e -G —Now throughput is S = GP 0 = Ge -G —This is maximum at G = 1, where S = 1/e = 0.368 (frames per interval T) Slotted ALOHA doubles throughput of pure ALOHA

41. #41EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Slotted ALOHA, Cont..  Note that throughput is never very high  Also, at high loads, throughput goes to 0, a general characteristic of networks with shared resources —Number of empty time slots and successful slots decrease, number of collisions increase

42. #42EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Slotted ALOHA, Cont.. —Average number of retransmissions per frame increases —Average delay (from first transmission attempt to successful transmission) increases

43. #43EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: CSMA Carrier Sense Multiple Access (CSMA)  Sense the presence of carrier, sense the channel is free, send data, wait for Ack, re- send if timed-out, if busy back off and try again. Max throughput 60%  Many versions, most popular method in LAN.

44. #44EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: CSMA, Cont..  Family of CSMA protocols defined by rules for backing off with varying degrees of persistence —1-persistent CSMA: stations are most persistent —P-persistent CSMA: persistence increases with value of p —Non-persistent CSMA: stations are not that persistent

45. #45EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: 1-persistent CSMA  Slotted or un-slotted versions  If channel is busy, station will transmit immediately after channel becomes idle  If collision is detected, then back off and try again after a random time  Propagation delay can effect performance – station A takes longer to detect that station B is transmitting —Causes collisions to be more likely

46. #46EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: 1-persistent CSMA, Cont..  Even without propagation delays, collisions are possible —Stations A has the channel, stations B and C are ready and will both transmit after station A is done  Throughput analysis is complicated —Carrier sensing improves throughput over ALOHA —Throughput goes to 0 under very high load

47. #47EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: P-persistent CSMA  If channel is idle, station will transmit with probability p  Otherwise, goes to next time slot and senses if channel is idle  If idle, transmits with probability p or otherwise, goes to next time slot and repeats procedure  Performance depends on choice of p

48. #48EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: Non-persistent CSMA  If channel is idle, station will transmit  If channel is busy, station will wait for random number of time slots before trying again - even if channel is idle meanwhile  Helps avoid collisions right after an active time slot

49. #49EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: MAC protocols, Cont..  Carrier Sense Multiple Access-Collision Detection (CSMA-CD) —Send when carrier is free. —Listen to detect collision —If collision is detected, back off and retry —Second order of improvement to CSMA

50. #50EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: MAC protocols, Cont..  Carrier Sense Multiple Access-Collision Detection (CSMA-CD) —Not possible in wireless LAN environment, the same frequency for sending and receiving (unlike cellular) —CSMA-CA is the method of choice

51. #51EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: CSMA/CD, Cont.. frame transmission idle frame contention: series of time slots for collisions frame time  3 alternating states: (1) transmission (2) contention (3) idle

52. #52EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: CSMA/CD, Cont..  Performance depends on time to detect collision (assume transmissions can be aborted immediately)  If D is worst-case propagation delay between any two stations, then collision detection time is 2D station A A begins transmit time station B B begins transmit just before signal reaches B A detects collision after 2D signal

53. #53EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: CSMA/CD, Cont..  Assume N = number of stations 2D = length of collision time slots T = time to transmit frame (T > 2D, otherwise collisions are not detected) P = probability a station will transmit in idle time slot

54. #54EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: CSMA/CD, Cont..  After successful frame, there is contention period of series of collision time slots (multiple attempts) or idle (no attempts), ended by a successful frame (exactly one attempt)

55. #55EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: CSMA/CD, Cont..  Find P 1 = Pr(exactly 1 attempt in time slot) = NP(1-P) N-1  Maximum when P = 1/N, then  Mean length of contention period: —Pr(j slots with collisions or idle followed by one transmission) = (1 - P 1 ) j P 1

56. #56EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: CSMA/CD, Cont.. —Mean length of contention period is —Maximum utilization is —Note utilization decreases for large D or small T (slots) frame time frame time + contention period =

57. #57EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: CSMA-CA  Carrier Sense Multiple Access-Collision Avoidance (CSMA-CA) —When node A has something to send to node B, it send Request-To-Send (RTS) packet to B with the amount of data to be sent —B responds with Clear-To-Send (CTS) packet with time of transmission and amount of transmission back to node A

58. #58EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Protocol: CSMA-CA  Carrier Sense Multiple Access-Collision Avoidance (CSMA-CA) —When a node has something to send, it should also checks CTS before start transmitting. —Improves CSMA performance

59. #59EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU What Is Hidden Node? A C B A can hear B C can hear B A can not hear C C can not hear A sending data

60. #60EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU MAC Frame Format CRCFrame Control DurationSequence Control Frame Body Address 4Address 1Address 2Protocol Version TypeSub typeTo DS From DS RetryLast Fragment RSVDEPPower Mgt 2 6 6 6 2 2 0-2304 4 224 1 1 1 1 2 1 1

61. #61EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Frame type and subtypes  Three type of frames —Management —Control —Asynchronous data  Each type has subtypes  Control —RTS —CTS —ACK

62. #62EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Frame type and subtypes, Cont..  Management —Association request/ response —Re-association request/ response —Probe request/ response —privacy request/ response —Beacon (Time stamp, beacon interval, TDIM period, TDIM count, channels sync info, ESS ID, TIM broadcast indicator) —TIM (Traffic Indication Map) indicates traffic to a dozing node —dissociation —Authentication

63. #63EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Power Management  AP knows the power management of each node  AP buffers packets to the sleeping nodes  AP send Traffic Delivery Information Message (TDIM) that contains the list of nodes that will receive data in that frame, how much data and when.  The node is awake when it is sending data, receiving data or listening to TDIM.

64. #64EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Authentication  Three levels of authentication —Open: AP does not challenge the identity of the node. —Password: upon association, the AP demands a password from the node. —Public Key: Each node has a public key. Upon association, the AP sends an encrypted message using the nodes public key. The node needs to respond correctly using it private key.

65. #65EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Mobility Management Access point connects other access points via backbone network Backbone Network Access Point

66. #66EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Mobility Management, Cont..  A node can associate when it enters the coverage area of an AP  It shall re-associate when it handoffs to another AP.  AP bridge function keeps track of all nodes associated with it.

67. #67EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Access Point Functions  Access point has three components —Wireless LAN interface to communicate with nodes in its service area —Wireline interface card to connect to the backbone network —MAC layer bridge to filter traffic between sub-networks. This function is essential to use the radio links efficiently

68. #68EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Bridge Functions  Listen to all packets being sent.  Find out which nodes are in which sub-network by analyzing the source address. Store that data in a routing table.  If a packet is addressed to a known node, only repeat the data on that sub-network, otherwise repeat it on all networks.  Age the entries after a timer value has expired since the last communication

69. #69EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Bridge Functions, Cont..  If the timer is too long, we might send data to a node that might have left the sub-network or is turned off or even gone to coverage area of another access point.  If the timer is too short, we remove the user too early and repeat the packet destined to it in all sub-networks.  Other functions of a bridge, buffering for speed conversion, changing frame format between LANs.

70. #70EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Routing  Building routing tables can be done as —Source tree, keeps track where other nodes are and the best way of reaching them. When sending a packet the route is also determined. It must be done in each node and is heavy. —Spanning tree, is built iteratively, each bridge advertises it identity and all other bridges it knows and how many hops it takes to get there. Then each bridge follows a specific algorithm to calculate how get to each bridge with least hop.

71. #71EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU IEEE 802.11 WLAN  1997 IEEE 802.11 working group developed standard for inter-working wireless LAN products for 1 and 2 Mbps data rates in 2.4 GHz ISM (industrial, scientific, and medical) band (2400-2483 MHz)  Required that mobile station should communicate with any wired or mobile station transparently (802.11 should appear like any other 802 LAN above MAC layer), so 802.11 MAC layer attempts to hide nature of wireless layer (eg, responsible for data retransmission)

72. #72EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU 802.11 WLAN, Cont..  1999 IEEE 802.11a amendment for 5 GHz band operation and 802.11b amendment to support up to 11 Mbps data rate at 24 GHz  MAC sublayer uses CSMA/CA (carrier sense multiple access with collision avoidance) - very similar to CSMA/CD except collisions are detected by ACKs after entire packets are transmitted

73. #73EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU 802.11 WLAN, Cont.. —Station will listen to channel if ready to transmit —If channel is idle, begins to transmit —If channel is busy, will wait until channel is free and transmit after a random time (to reduce collisions) —In case of collisions, stations will try again following a random exponential back off

74. #74EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU 802.11 WLAN, Cont..  Random exponential back off: —Stations keep track of contention window parameter, CW Initially CW is a minimum value —When station wants to transmit, it chooses a random (uniformly likely) value between 0 and CW

75. #75EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU 802.11 WLAN, Cont.. —Waits for chosen number of time slots before transmitting —After each collision, CW is doubled (exponential increase) X collisions tries in one of 2 slots Example XX tries in one of 4 slots tries in one of 8 slots

76. #76EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU 802.11 WLAN, Cont..  Other MAC sublayer functions: —Optional “point coordination function”: centralized contention-free multiple access for time-sensitive data (a centralized polling mechanism) —“Association” and “re-association” processes to dynamically establish connections between mobile stations and fixed access points

77. #77EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU 802.11 WLAN, Cont.. —Optional encryption for security —Power management to allow mobile stations to power down (sleep) without losing data (eg, access point will buffer packets for sleeping stations until requested)

78. #78EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU HIPERLAN  WLANs have small share of LAN market now —Higher costs per station —Standards are recent (HIPERLAN, IEEE 802.11) —Rapid growth is projected  1995 ETSI technical group RES 10 (Radio Equipment and Systems) developed HIPERLAN/1 wireless LAN standards using 5 channels in 5.15-5.3 GHz frequency range —Technical group BRAN (Broadband Radio Access Network) is standardizing HIPERLAN/2 for wireless ATM

79. #79EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU HIPERLAN, Cont..  HIPERLANs with same radio frequencies might overlap —Stations have unique node identifiers (NID) —Stations belonging to same HIPERLAN share a common HIPERLAN identifier (HID) —Stations of different HIPERLANs using same frequencies cause interference and reduce data transmission capacity of each HIPERLAN —Packets with different HIDs are rejected to avoid confusion of data

80. #80EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU HIPERLAN, Cont..  Data link layer = logical link control (LLC) sublayer + MAC sublayer + channel access control (CAC) sublayer data link physical LLC MAC network CAC

81. #81EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU HIPERLAN, Cont..  MAC sublayer: —Keeps track of HIPERLAN addresses (HID + NID) in overlapping HIPERLANs —Provides lookup service between network names and HIDs —Converts IEEE-style MAC addresses to HIPERLAN addresses —Provides encryption of data for security

82. #82EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU HIPERLAN, Cont..  MAC sublayer: —Provides “multi hop routing” – certain stations can perform store-and-forwarding of frames —Recognizes user priority indication (for time- sensitive frames)

83. #83EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU HIPERLAN, Cont..  CAC sublayer: —Non-preemptive priority multiple access (NPMA) gives high priority traffic preference over low priority —Stations gain access to channel through channel access cycles consisting of 4 phases:

84. #84EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU HIPERLAN, Cont..  CAC sublayer: 1. Priority phase: if station has data with priority N, will wait for N-1 priority slots and transmit “priority assertion” burst in Nth slot If it hears another station of higher priority, it will give up on this channel access cycle Winning stations of same priority go into next phase

85. #85EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU HIPERLAN, Cont.. 2. Elimination phase: each station will transmit a burst of random length (geometrically distributed number of time slots) and see if channel is idle If channel is idle, it will go to next phase If busy, it will give up on this access cycle

86. #86EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU HIPERLAN, Cont.. 3. Yield phase: surviving stations will listen to channel for a random number of time slots (geometrically distributed) If it hears another station transmitting, it will give up on this access cycle If channel is idle, it will begin to transmit

87. #87EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU HIPERLAN, Cont.. 4. Transmission phase: winning station will transmit CAC is designed to give each station (of same priority) equal chance to access the channel