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 6: Mobie Data 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 • Network Performance Discussion • Primer on Aloha • Mobile Data • Mobites • ARDIS • CDPD • GPRS

3. Review, IS-95 CDMA • Spread spectrum techniques adapted from military (used since 1950) • Narrowband signal is multiplied by very large bandwidth signal (spreading signal) • All users, each with own pseudorandom codeword approximately orthogonal to all other codewords, can transmit simultaneously with same carrier frequency

4. Review, IS-95 CDMA - Radio Aspects • Receiver performs a time correlation operation to detect only desired codeword • All other codewords appear as noise due to decorrelation • Receiver needs to know only codeword used by transmitter • In other words, users are separated by their codes rather than frequency and time slot

5. Review, IS-95 CDMA , Features • Multiple users can share same frequency • Soft capacity limit: more users raises noise floor linearly, no absolute limit on number of users - performance degrades gradually for all users • Multipath fading is reduced by signal spreading

6. Review, IS-95 CDMA Features • Spatial diversity provides soft handoff: MSC monitors signal of a user from multiple base stations and chooses best version of signal at any time • Self-jamming is a problem: because spreading sequences of different users are not exactly orthogonal • When despreading, other users can Contribute significantly to receiver decision statistic

7. Review, IS-95 CDMA, Features • Near-far problem: if power of multiple users are unequal, strongest received mobile signal will capture demodulator at base station • Power Control to ensure that each mobile within coverage area provides same signal level to base station receiver • CDMA is dual mode like TDMA. • The system can move a call from digital to analog when the call enters the coverage area of a cell that does not have CDMA capability. The opposite does not work.

8. Forward channels Application Pilot System mon. Synchronization Sync. Paging Signaling Traffic Voice/data Reverse channels Access Signaling Traffic Voice/data Review, IS-95 CDMA Channels • Types of channels

9. Review, IS-95 CDMA Channels , Cont... • Traffic channel on the forward direction has three components • user data • power Control (puncturing convolutional code) • signaling message • Traffic channel on the reverse direction has two components • user data • signaling message

10. Review, Handoff in CDMA • Two types of handoffs • hard handoff • Soft handoff, requires synchronization • Hard handoff is needed when the call is moved from one frequency to another and when the mobile moves the coverage area of another MSC

11. Review, Mobility Management in CDMA • Five type of registration • Periodic • Power up • Power down • Zone change • Distance. When the distance between the current base station and the previously registered base station exceeds a certain limit. Requires GPS in all base stations

12. Review, Performance Metrics • Performance metrics are defined to measure the behavior of network objectively • Availability • Retainability • Integrity • Delays: Dial tone delay, post dialing delay, through connection delay

13. Review, Performance Metrics • Two types of performance metrics • Customer perceived. • Operator Defined.

14. Review, Performance Metrics • Specific metrics defined for different signaling systems and nodes • Standards and reference models are defined • Mostly on the wire line side, government Control

15. Review, Performance Metrics • One of the challenges of a network performance is to predict the capacity of the system (also called dimensioning resources) • Erlang is the unit used when dealing with traffic. One Erlang is one call held for one hour.

16. Review, Performance Metrics • Erlang-B formula provided blocking probability, I.e. the probability of an incoming call can not find an idle device. C= Number of devices A= Offered traffic in Erlang C A C! Pr{blocking}= C k A k! K=0

17. k A k! K=0 C-1 C A + C! ( ) A C 1- Review, Performance Metrics • Delay within switching network is of concern, timers that are defined in intermediate switches. • Erlang C formula is used to calculate the probability of delay in a wait system. C A Pr [ Delay >0 ] =

18. Review, Performance Metrics • Performance metrics in wireless and wire line are similar, but not identical • Accessibility Ability to make and receive calls • Retainability Ability to maintain a call • Voice quality Voice quality during the call

19. Review, Performance Metrics • How do we calculate these metrics? • No common standards are defined. Only operator defined standards. • Measurements are collected from network elements • Formulas are developed per vendor product • Performance metrics can be • Theoretically calculated • Measured using counters produced at each node • Verified by drive testing

20. Review, Performance Metrics • Theoretical estimation • Normally used during the network design. • Queuing theory, traffic forecast, statistical estimates and product specification are the corner stone of this work • Result are good for this phase but are not always correct

21. Review, Performance Metrics, Cont… • Calculating metrics using counters • All switching nodes produce counters. • These counters represent events in the network • Using these counters metrics are defined. • Each vendor has its own counters, thus the formula for deriving performance metric varies for each vendor. • Counters are produced on different levels, MSC level, BSC level, cell level, etc.

22. Review, Performance Metrics, Cont… • Example of counters are • MSC level # of page attempts • BSC level # of intra-BSC handoff • Cell level # page responses • VLR level # of visiting mobiles • HLR level Length of mobile activity

23. Review, More on Counters • Typical counters in MSC/BSC • # of page request from HLR • # of page response after one attempt • # of pager response after second attempt • # of page with no response

24. Review, More on Counters • Typical counters in BSC/MSC • # of measured RXQUAL=n • # of dropped calls • # of call attempts • Duration of call (average)

25. Review, Verification, Drive Test • A commonly used method • Expensive and time consuming • Good tool for trouble shooting • Mobile handset is connected to a computer. All communication between the mobile handset and the base station is recorded. For example, layer three messages, layer 2 messages, measures signal strength, quality, etc.

26. Review, Verification, Drive Test • Normally, the test calls are done towards a test number that sends a tone for verification of voice quality • The quality of the test equipment influences the result • Reliability of the test software is a key.

27. Review, Primer to switching systems • Two basic types of Switching • Circuit switched • Packet switched

28. Review, Circuit Switched Connection • Connection has three phase: setup, transmission, disconnection. • Bandwidth is reserved end-to-end for duration of connection • Congestion and delay in the setup phase • Only propagation delay during transmission • Well suited for real-time, Continuous traffic, e.g., speech

29. Review, Circuit Switched Connection • Traffic can be concentrated for better use of resources Channel 1 1 1 : : : : K M Channel N Concentrator

30. Review, Packet Switch Connection • Information is packetized, i.e. segmented and padded with header and trailer information. • Contents of header and trailer information is determined by the protocol governing the packet switched network, origination and destination of the information and other services invoked.

31. Review, Packet Switch Connection, Cont… • No resources/trunks are reserved. • All network resources are shared by all users. • Delay is variable based on the load level in the network.

32. Review, Packet Switch Connection • Well suited for non-real-time, bursty traffic • 2 types of packet switching, connectionless and connection oriented: • Connectionless: each packet is routed independently • Packets can arrive out of order • Example: Internet protocol (IP)

33. Network Performance Discussion • What is the target of performance monitoring, or the level • Connection type • Function/feature • Counters • Last weeks assignment !

34. Primer: Aloha • Aloha is a wireless network designed in Hawaii and thus the name Aloha • It was experimented in many way to find a good solution for wireless communication • The system consisted of wireless devices communicating together using a communication sattelite

35. Primer: Aloha • Aloha • Stations starts sending when they have something to send • Pure Aloha, no contention resolution, relies on timed-out acks, max throughput approximately 18% • Slotted Aloha, no contention resolution, relies on timed-out acks, only can start sending in the beginning of a slot, max through put approximately 36%

36. Primer: Pure ALOHA • 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 • 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

37. Primer: Pure ALOHA • 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 - 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

38. Primer: Pure ALOHA 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 • This is maximum at G = 0.5, where S = 1/2e = 0.184 (frames per interval T) • Pure ALOHA achieves low throughput

39. Primer: 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) • 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

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

41. Primer: Slotted ALOHA • 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

42. Primer: Slotted ALOHA • 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 • Average number of retransmissions per frame increases • Average delay (from first transmission attempt to successful transmission) increases

43. Primer: (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%

44. Primer: CSMA • 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. Primer: CSMA (Cont) • Carrier Sense Multiple Access-Collision Detection (CSMA-CD) • Send when carrier is free, listen to detect collision. • CSMA-CA is the method of choice • Carrier Sense Multiple Access-Collision Avoidance (CSMA-CA) • Uses two messages before transmission, Request-To-Send (RTS) and Clear-To-Send (CTS) . • Method of choice for wireless LAN

46. Primer: CSMA/CD (cont) • 3 alternating states: (1) transmission (2) contention (3) idle time frame frame frame contention: series of time slots for collisions idle transmission

47. Primer: 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 A begins transmit A detects collision after 2D station A signal time station B B begins transmit just before signal reaches B

48. Mobile Data • 2 main options for wireless packet data: • High speed wireless LANs (eg, 802.11) • Low speed wide area services • Mobitex/RAM Mobile Data • CDPD (cellular digital packet data) • GPRS (general packet radio service) • ARDIS (advanced radio data information services)

49. Mobile Data , Cont... • ERMES (European Radio Message System) was standardized by ETSI early 1980. • Originated by Swedish Telecom (now Telia Mobitel) as private mobile alarm system for field personnel • Development Continued by MOA (Mobitex Operators Association) and Ericsson Mobile Communications • http://www.ericsson.com/wireless/products/mobsys/mobitex/mobitex.shtml)

50. Mobitex/RAM Mobile Data • Mobitex - widely accepted de facto standard for wireless packet data • Developed by Swedish Telecom (now Telia Mobitel) as private mobile alarm system for field personnel • Development Continued by MOA (Mobitex Operators Association) and Ericsson • 1986 Commercial operation in Sweden • Now widely deployed in Europe, US, Australia