Introduction to Wireless Networks and Mobile Computing

1. Introduction to Wireless Networks and Mobile Computing D. Manivannan

2. Overview • Background and motivation • Mobile computing systems: Models • Issues involved in supporting mobility of hosts on the Internet • Proposed solutions/standards addressing the issues for supporting mobility on the Internet • Topics to be covered in the course

3. What is a Mobile Computing System? • It is a distributed system in which some of the hosts can move from one location to another location • The communication network connecting the hosts may consist exclusively of wireless links (e.g., mobile ad hoc networks) or both wired and wireless links (e.g., cellular networks). • Movement of a host is transparent to the applications running on the host -- similar to cellular telephones

4. Mobile Computing Systems: Models • The Adhoc Model (MANETs) • There is no support for mobile hosts from fixed infrastructure. • This model has applications in the battlefield environment, and in environments where we need to set up a temporary communication infrastructure such as emergency relief operations such as during earthquake, etc.

5. The Adhoc Model

6. Mobile Computing Systems: Models… • The Cellular Model • The geographical area is divided into regions, called cells • Mobile hosts present in a cell are serviced by a mobile support station in the cell for communication with other hosts • The mobile support stations are generally connected by a wireline network

7. The Cellular Model

8. Characteristics of Mobile Computing Systems • Wireless connections • Are of lower quality than their wired counterpart • Have lower bandwidth • Have high bandwidth variability • Have high error rates • Have more spurious disconnections • Susceptible to eavesdropping

9. Characteristics of Mobile Computing Systems… • Mobile hosts • Have the risk of physical damage, unauthorized access, loss and theft. • Have limited energy (battery power) • Have smaller storage capacity • Have less CPU power than their desktop counterparts • Protocols designed for the mobile environments should take all these constraints into consideration

10. Some Issues in Cellular Networks • Efficient, dynamic, distributed, fault-tolerant channel allocation schemes • Efficient, dynamic, distributed, fault-tolerant location management schemes • Security issues

11. Channel Allocation in the Cellular Model 4, 5, 6 7, 8, 9 7, 8, 9 1, 2, 3 4, 5, 6 4, 5, 6 7, 8, 9

12. Location Management

13. Providing Support for Mobility of Hosts on the Internet • Mobile IP • Mobile TCP

14. The Internet Protocol (IP) • IP is responsible for connecting the hosts on today’s Internet • IP is a network layer protocol, responsible for routing the packets on the internet • IP routes packets from source to destination by allowing intermediate routers to forward packets from incoming network interface to appropriate outgoing interface according to routing tables • The routing table typically maintains next hop information for each destination IP address, according to the number of the network to which the IP address belongs • Thus, the IP address carries with it information about the point of attachment of the host

15. IP Works Similar to Postal Service • If I mail a letter, the postal service personal in the intermediate locations don’t have to look into the complete address on the letter to forward the letter to its destination • Only the destination post office has to look into the complete address to deliver it to the receiver. • Letters mailed need not be received in the same order, letters mailed to the same destination need not follow the same route, and letters could be lost in transit • Post office will forward my letters to my new address if I am on vacation or have moved to a new location • But IP will not forward packets if the host moves to a new location – no mechanisms for change of address notification.

16. IP Doesn’t Provide Support for Mobility on the Internet

17. The Problem… • Each host on the Internet has a fixed IP address • Applications running on the hosts use this fixed address and port numbers to communicate with each other • So, if the applications that are currently running should not be disrupted, the host has to keep the same address when it moves • Since IP routes packets based on the network address, to ensure correct delivery of packets to the mobile node’s current point of attachment, IP routing must be modified.

18. Solution - Mobile IP • A Mobile host will have two IP addresses • A home address which is static and does not change • A care-of address which changes at each new point of attachment • At the home network, there exists a home agent which is responsible for forwarding the packets received for the mobile host which has moved to a foreign network. • To accomplish this, when a mobile node moves, it registers its new care-of address with the home agent

19. Problems with this Solution

20. TCP • TCP provides a reliable transport layer using the unreliable service of IP. TCP uses timeouts, acknowledgements, retransmissions, … to accomplish this. FTP server FTP client TCP TCP Router IP IP IP T.R. Driver Ethernet Dr. E.Dr. T.R.Dr.

21. TCP Performance in the Presence of Wireless Links • TCP has been tuned over time to cope with congestion in the network • TCP assumes that dropped packet means congestion in the network. i.e., TCP responds to all packet losses by invoking congestion control and avoidance algorithms • In a wireless environment, packet loss does not necessarily mean congestion because packets could be lost in wireless links • So, the current implementations of TCP also need to be modified to cope with mobility on the Internet

22. TCP Performance...

23. TCP Needs to be Modified Too!! • Snoop Protocol - a proposed solution: • It introduces a snoop agent, at the base station. • The agent monitors every packet that passes through the TCP connection in both directions and maintains a cache of TCP segments sent across the link that have not been acknowledged • The snoop agent retransmits packets from cache upon the determination of packet loss.

24. Snoop Protocol..

25. A Different Approach to Mobile Computing • Worldwide Web is the world’s largest distributed database system • No concurrency control mechanisms are enforced for the consistency of this database • Yet it is the most popular database ever built • It is always available and is easy to use • Many would like to access it anytime and anywhere for paying bills, checking email, shopping, banking, online trading, getting location dependent information like hotels, restaurants, etc

26. A Different Approach to Mobile Computing… • Many companies are vying to tap this huge customer base • Use the existing wireless infrastructure to provide access to Internet • Micro-browsers on cell phones is an alternative solution to provide anytime/anywhere access to the Web from anywhere, anytime. • WAP – Wireless Application Protocol is an open international standard for achieving this even though other alternatives such as NTT DoCoMo’s (Nippon Telephone and Telegraph ‘anyplace you go’) i-mode exist in Japan which has its presence in Europe, Asia and US as well. • WAP enables access to Internet from mobile phones or PDAs

27. WAP… • WAP is a suit of protocols • WDP (WAP datagram protocol) makes every data network look somewhat similar to UPD to the upper layers • WTLS provides public-key base cryptography similar to TLS • WTP (WAP transport layer) provides transport layer support (similar to TCP)

28. Web Server HTTP Accessing the Web through WAP-Enabled Devices

29. Wireless Networking Technologies • Bluetooth: Result of an effort to develop an integrated voice/data home wireless network, started in 1998. Along with Nokia, IBM, Intel and Toshiba, Erikson founded Bluetooth. (Suitable for WBANs and WPANs) • ZigBee is another standard for WPANs • IEEE 802.11: Suitable for Wireless LANs

30. Tentative list of topics to be covered • Cellular Networks • Channel Allocation • Location Management • Supporting mobility on the Internet • Mobile IP • Mobile TCP • Ad hoc networks • Routing • Group Communication • Security • Topology control • Quality of Service (QoS) • Mesh Networks, Sensor Networks • Available technologies for wireless networks • Bluetooth • ZigBee • Wireless LAN Standard 802.11 … • Wireless PAN Standard 802.15.. • Mobile Databases, fault-tolerance • Mobile agents

31. Expected Outcome ? • Would have exposed to various issues/problems in mobile computing systems • Become familiar with solutions to some of the problems pertaining to mobile computing • May be able to propose better solutions to some of these problems