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 7: Mobile Data, Part II 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 • More GPRS • LAN Primer • IP Primer

3. Review, 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 satellite

4. Review, 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%

5. - 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 Review, Pure ALOHA Throughput • 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

6. Review, 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

7. Review, 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 (e.g., 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

8. Review, 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

9. Review, 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

10. 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

11. Review, 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%

12. Review, 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

13. Review, 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

14. Review, Mobile Data • 2 main options for wireless packet data: • High speed wireless LANs (e.g., 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)

15. Review, 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)

16. Review, 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

17. Review, Mobitex , Cont... • 1986 Commercial operation in Sweden • Now widely deployed in Europe, US, Australia • In US, RAM Mobile Data, a joint venture between RAM broadcasting and Cingular. • http://www.cingular.com/business/mobitex_map

18. Review, Mobitex, Major features, Cont... • Major features • Seamless roaming • Store and forward of messages • Dependability above 99.99% • Interoperability and many connectivity options • Capacity to support millions of subscribers • Security against eavesdropping

19. Review, Mobitex, Major features, Cont... • Major features • Packet switching occurs at lowest level of system hierarchy - relieves backbone traffic • Packet multicasting (to multiple recipients) is handled by network • Closed User Group (CUG) feature • Frequency depends of the country, 900 MHZ in US and 450 in most others.

20. Review, Mobitex - Architecture NCC NCC: network Control center Main exchange Local switch covers a service area, each with 10-30 frequency pairs Regional switch Regional switch Local switch Local switch Base stations use 1-4 frequencies each 8 kb/s FEP

21. Review, Mobitex - Architecture , Cont... • Network Control Center (NCC), provides network management functions • Main Exchange and Regional Switch have basically the same function, but they reside on different level of network hierarchy. • Packet switching • Protocol handling (X.25 and HDLC) • Subscriber data for nodes below • Multiple connection to other switches • Alternate routing

22. Review, Mobitex - Architecture , Cont... • Local Switches, similar to regional switches. Also handles • Communication with base stations • Connection to host computers via FEP (Front-End-Processor) • FEP provides • Protocol conversion to hosts supporting X.25, TCP/IP, and SNA • Convert connectionless protocol to connection oriented protocol.

23. Applications MPAK MASC RS232 Applications MPAK X.25 X.21 MPAK MASC ROSI RS232 GMSK MPAK HDLC X.25 X.21 X.21 MPAK ROSI HDLC GMSK X.21 Review, Mobitex, protocol architecture 4-7 3 2 1 Server Local switch Base Station Mobile Radio modem

24. Review, Mobitex - Network Layer • Network layer packet = MPAK (Mobitex PAcKet) • User data, segmented into packets of maximum 512 bytes • Alert messages (high priority) • Network layer signaling, e.g., login/logout requests

25. Review, Mobitex - Network Layer , Cont... • MPAK header Contains • Identification of application that generated packet • Class (significance) • Type (priority) • Whether can be stored in recipient’s mailbox (temporary storage) if cannot be delivered immediately

26. Review, Mobitex - Data Link Layer • Data link layer protocol is MASC (Mobitex Asynchronous Communication) • MPAK delivers user packets plus addressing and network data to MASC • Data link layer functions • Selection of most suitable base station • Retransmissions of frames lost on the radio channel (stop-and-wait ARQ) • Channel access procedure - variation of slotted ALOHA

27. Review, Mobitex - Data Link Layer , Cont... • Base station initiates a Contention cycle by sending a FREE frame downlink • Mobile stations can Contend for number of free timeslots by choosing a random slot and transmitting during that slot • If mobile has more data than fits in a time slot, it can start by sending a short access request message • Base station grants access to requesting mobile stations after a free cycle

28. Review, Mobitex - Data Link Layer , Cont... • ROSI (Radio Signaling), takes care of transmission towards Mobitex infrastructure • GMSK (Gaussian Minimum Shift Keying) • X.21 and X.25 are the packet data communication protocols used for many years. Good for connectionless short bursts of data.

29. Review, Mobitex - Radio Interface , Cont... • Data link also handles channel access procedure - variation of slotted ALOHA • Base station broadcasts a FREE frame indicating a free cycle, including number and length of time slots in the free cycle • Mobile terminal chooses a random timeslot in next free cycle and transmits its frame then

30. Review, Mobitex - common functions • Requires subscription • individual • groups of terminals • host computer • groups of host computers • Security • Password based • ESN • CUG (Closed User Group)

31. Review, Mobitex - Mobility • Mobiles monitor and evaluate signals from other base stations • At power-up, mobile tries to register with the last base station in its memory, if possible • Base station provides necessary information, such as acceptable signal strength, neighbor list,etc periodically.

32. Review, CDPD • Cellular digital packet data (CDPD): connectionless packet-switched data designed to work with an analog cellular system (e.g., AMPS) • Originated by IBM as packet-switching overlay to analog cellular system, early 1990s developed by CDPD Forum, now developed by Wireless Data Forum • Overlay system uses unused bandwidth in cellular system and existing AMPS functions and capabilities

33. Review, CDPD , Cont... • CDPD is a value added system. Other users do not need to be aware of its presence in the network. This has implications: • CDPD transmission must not interfere with transmission of other services • No dedicated bandwidth, uses only idle time between users, channel-hop • No dedicated Control channel, all Control is in-band.

34. Review, CDPD , Cont... • CDPD is transparent to voice system • To avoid collisions with voice calls, CDPD uses channel hopping when antenna detects a power ramp-up (indicating initiation of voice traffic) • Base station closes current transmission channel within 40 msec and new idle channel is chosen to hop to

35. Review, CDPD , Cont... • CDPD is transparent to voice system • New channel may or may not be announced before old channel closed • If not announced, mobile terminal must hunt around set of potential CDPD channels to find new one

36. Review, CDPD - Network Architecture Internet or other networks IS Intermediate systems = generic packet switches in backbone network IS IS Mobile data intermediate systems = packet switches with mobility management capabilities MD-IS MD-IS Mobile data base station = base station

37. Review, CDPD - Network Architecture , Cont... • Mobile end system (MES): may be handheld PDA to laptop to terminal • Stationary or mobile, but treated as potentially mobile • Network Continually tracks location to ensure that packets are delivered even if physical location changes • May sleep - messages are then queued in network

38. Review, CDPD - Network Architecture , Cont... • Mobile data base station (MDBS): mobile data link relay • Supports CDPD MAC and data link protocols across radio interface • Handles radio channel allocation, interoperation of channels between CDPD and voice calls, tracks busy/idle status of channels • Often co-located with AMPS base stations (shares AMPS antenna)

39. Review, CDPD - Network Architecture , Cont... • Mobile data intermediate system (MD-IS) • Mobility management: location tracking, registration, authentication, encryption • Exchange location information by CDPD-specific mobile network location protocol (MNLP) • “Mobile home function” (MHF) in home network maintains current location info for a mobile end system and forwards packets

40. Review, CDPD - Network Architecture , Cont... • Mobile data intermediate system (MD-IS) • “Mobile serving function” (MSF) in visited network maintains info for visiting mobile end systems in its area (through registration process) • Accounting and billing (based on usage)

41. Review, CDPD , Cont... • CDPD network layer • Internet protocol (IP and mobile IP) and connectionless network protocol (CLNP, OSI’s equivalent of IP) are supported • Backbone network of intermediate systems (IS) provides connectionless packet routing • IS can be off-the-shelf IP or CLNP routers

42. 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

43. 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

44. GPRS , Cont... • SGSN = Serving GPRS Support Node • Ciphering • Authentication, IMEI check • Mobility Management • Logical Link Management towards mobile station • Packet routing and transfer • Connection to HLR, MSC, BSC and SMS-MC

45. 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.

46. 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.

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

48. GPRS Features • Packet data can use up to 8 timeslots to provide 115.2 kb/s • Suitable for bursty data such as web browsing

49. GPRS Interfaces

50. 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.