Review, Aloha

1. 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%

2. Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks Review Session Instructor: Jila Seraj email: jseraj@engr.smu.edu http://www.engr.smu.edu/~jseraj/ tel: 214-505-6303

3. 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 - 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

4. 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

5. 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

6. 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%

7. 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

8. 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

9. 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.

10. 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

11. 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

12. 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.

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

14. 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.

15. 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

16. 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.

17. 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

18. 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

19. 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

20. Review, CDPD - Network Architecture , Cont... • Mobile end system (MES): may be handheld PDA or laptop or 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

21. 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)

22. 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

23. 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)

24. 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

25. 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

26. 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

27. 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.

28. 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.

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

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

31. GPRS Interfaces

32. GPRS – Data Connection phases • GPRS attach/detach • PDP context activation • Data Transmission

33. Access Point Access Point Review, Wireless LAN Architecture Ad Hoc Laptop Laptop Server DS Pager Laptop PDA Laptop

34. Review, Wireless LAN Protocol • LAN protocol consists of 3 layers • LLC = Logical Link Control layer • MAC = Medium Access Control layer • PHY = Physical layer, which is divided into two sub layers: • PLCP= Physical Layer Convergence Protocol sub-layer • PMD = Physical Medium Dependent sub-layer

35. Review, Wireless LAN Protocol, Cont.. • There are many type of LAN protocols: • LBT • Integrated CSMA-CA and TDMA • Polling

36. Review, Power Management • Strong need to minimize power usage for wireless modem • Wireless stations have three states • Sleep • Awake • Transmit

37. Review, Power Management, Cont… • 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 only when it is sending data, receiving data or listening to TDIM.

38. Review, 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

39. Review, Bridge Functions • Speed conversion between different devices, results in buffering. • Frame format adaptation between different incompatible LANs • Adding or deleting fields in the frame to convert between different LAN standards • Creating and maintaining the routing table

40. Review, Mobility Management, Cont.. • A node can associate when it enters the coverage area of an AP • A node can disassociate when power down or leaving the service area • It shall re-associate when it handoffs to another AP. • AP bridge function keeps track of all nodes associated with it.

41. Review, WLAN Addressing • In wireline LAN, each node has an IP address that is associated with its physical location • When a device can move from one location to another, the association between the physical location and IP address no longer holds • The solution is presented in mobile IP

42. Review, 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.

43. Review, Mobility Management in WLAN • Mobile IP principles are used to take care of mobility in the wireless LAN. • Every wireless device has an address in its Home LAN, and gets a c/o address in the Visiting LAN.

44. Review, Frame type and subtypes • Three type of frames • Management • Control • Asynchronous data • Each type has subtypes • Control • RTS • CTS • ACK

45. Review, 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)

46. Review, Frame type and subtypes, Cont.. • Management… • TIM (Traffic Indication Map) indicates traffic to a dozing node • dissociation • Authentication

47. Review, Mobile IP Principals • Constraints of mobile IP are • Mobility should be at network layer • No impact on higher levels • No impact on the nodes not directly involved in the mobile IP function • Uninterrupted operation for mobile devices

48. Review, Mobile IP Principals, Cont… • The principle is very simple, use c/o addressing • For each mobile device, we associate a Home IP address associated with a Home Network. • The new LAN is called the Visiting Network • The software that takes care of mobility in each server (router) is called agent.

49. Review, Mobile IP Principals, Cont… • Two types of agents, Home Agent and Visiting Agent. • Whenever the mobile device connects to a new network, a c/o address is given to it by the Visiting Agent. • This c/o address is reported to the Home Agent. • All packets addressed to the mobile device are addressed to its Home Address, and thus sent to its Home Network.

50. Review, Mobile IP Principals, Cont… • Upon reception of the packet, the Home Agent recognized the address belonging to a mobile device. • Home Agents looks up the c/o address in its table. • The packet is then wrapped in a new packet with the c/o address on it, called encapsulation • C/o address causes the packet to be forwarded to the Visiting Agent.