Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks

1. 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. Session Outline • Review of last week • Wireless LAN

3. 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. Review, GPRS - Network Architecture Internet or other networks MSC/ VLR HLR GGSN Gateway GSN = packet switch interworks with other networks SGSN SGSN Serving GPRS support node = packet switch with mobility management capabilities BSC/PCU GPRS makes use of existing GSM base stations

5. 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. 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. 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. Review, GPRS , Cont... • For mobility management a new concept is defined, Routing Area RAI = MCC +MNC + LAC + RAC

9. Review, GPRS Interfaces

10. 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. Review, GPRS – Data Connection, Cont… • Detach is the phase when mobile terminates the connection. • GPRS requires subscription

12. BTS BSS SGSN HLR IMSI, P_TMSI+OLD RAI… Update Location Insert Subs. Data Insert Data Ack Update Location GPRS Attach Accepted Review, GPRS Attach Scenario

13. 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. 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. 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. BTS BSS SGSN GGSN 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 Review, PDP Context Activation

17. 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. 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. 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. 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. MAC Protocols • MAC protocol is a sublayer in data link layer • For LANs, data link layer = logical link control (LLC) sublayer + MAC sublayer network LLC data link MAC - defines how stations access the shared medium physical

22. 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. 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. 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. 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. 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. 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. MAC Protocols: Polling • Polling is used in wired network environments but not popular in wireless networks

29. 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. 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. 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. 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)ke- St /k! • Let G = “offered load” = new transmissions and retransmissions

33. MAC Protocol: Pure ALOHA, Cont.. • In equilibrium, throughput (rate of successfully transmitted frames) = rate of new transmissions, S S = GP0 where P0 = probability of successful transmission (no collision) • P0 depends on “vulnerable interval” for frame, 2T

34. - transmission attempt at time 0 frame A - collision if starts in interval (-T,0) frame B - collision if starts in interval (0,T) frame C time -T 0 T MAC Protocol: Pure ALOHA, Cont..

35. MAC Protocol: Pure ALOHA, Cont.. P0 = 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)ke-Gt/k! then P0 = e-2G • By substitution, throughput is S = GP0 = Ge-2G

36. 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. 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. 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. - transmission attempt at time 0 - collision if frame B was ready in interval (-T,0) time -T 0 T MAC Protocol: Slotted ALOHA, Cont.. • “Vulnerable interval” is reduced by factor of 2 to just T frame A frame B

40. MAC Protocol: Slotted ALOHA, Cont.. • Throughput P0 = Pr(no frames ready in previous time slot) = e-G • Now throughput is S = GP0 = 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. 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. MAC Protocol: Slotted ALOHA, Cont.. • Average number of retransmissions per frame increases • Average delay (from first transmission attempt to successful transmission) increases

43. 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. 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. 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. 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. 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. 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. 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. 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