Wireless Mobility Introduction
Whether its vehicle tracking or teleoperation or even just connecting people on the move, mobile wireless communication enables a wide variety of applications. A key aspect of wireless mobility is mobility management. Mobility management deals with the problem of trying to connect two nodes, when one or both of those nodes change their point of connection into the larger network over time. This review will first go over some applications of mobile wireless communications and then discuss some of the technical aspects of mobile IP. Finally, some of the building blocks necessary in the achievement of wireless mobility will be discussed.
Commercial Applications
Some of the more commonly known applications of wireless mobility have been personal mobile devices like cell phones and PDAs as well as wireless enabled laptop devices. An interesting integration of wireless and cellular technologies is the hotspot @ home service provided by T-mobile.[1] It allows the user to use wireless at home instead of utilizing the mobile service allowing for cheaper calls. More interestingly the UMA enabled phone used in this service, allows a seamless handover from WiFi to the mobile network without disrupting any ongoing calls during the transfer. [2] Applications of wireless mobility have also immerged in a number of other areas.
One such application is remote monitoring and tracking of vehicles, one example being On-Star from GM, which uses existing wireless infrastructure like CDMA mobile phone networks for communication.[3] Similar research has also been performed by C@RE, a research company based in Pakistan, which, among a number of other
technologies like VHF modulation, also utilizes GPRS and SMS over GSM networks for communication. [4]
GPRS can also be used for teleoperation. A number of engineers from the University of Deusto, have performed experiments where a fleet of Microbots were remotely controlled over GPRS. GPRS was also used to feed back sensor data from the robots.[5] These are only a few of the many current and future applications of mobile wireless communication.
Underlying Technology
As already described, one of the key aspects of wireless mobility is mobility management; how to keep track of the address of, and connect to, a node that changes its point of connection into the network over time. Consider the example of IP. In IP version 4, the IP address of a node uniquely identifies its point of connection into the network. When a mobile node’s point of connection changes, its IP address also needs to change in order to allow packets to be routed to it correctly. One way of getting around this is tunneling, an idea also used in GPRS. [6]
In mobile IP, a mobile node has two addresses associated with it, its permanent address in what is called it home network and foreign address, associated with its current point of connection, which in turn is the foreign network. Tunneling also requires the assistance of a router in the home network called the home agent and a router in the foreign network called the foreign agent. When a packet with the node’s permanent address as the destination is received by the home agent, the packet is encapsulated inside a second IP packet and retransmitted. This IP packet however has the foreign agent’s address, called the care of address, as the destination address. When the foreign agent receives the packet and recognizes the destination of the encapsulated packet as belonging to the foreign node, it sends the packet that was encapsulated to the mobile node, which now sees only the original packet. This allows mobility to appear transparent to the transport and application layers. [7]
When ever the mobile node connects to a new foreign network, it must first register with a foreign agent, which in turn registers with the home agent, which enables tunneling to work in the first place. The node finds the foreign agent, by advertisement broadcast by the foreign agent, either on its own, or in response to a solicitation broadcast by the mobile node. [8]
GPRS (General Purpose Packet Switching), a packet switching service implemented on GSM also uses the concept of tunneling for routing packets between SGSN, nodes which directly connects to the radio network in which the mobile device resides, and GGSN, nodes which connect to the external IP networks. [9]
This section only considered one aspect of mobile networking. The following section goes over aspects of technology used in the other layers of the OSI model.
Building Blocks of Wireless Mobility
The OSI model allows various level aspects of communication to be abstracted and made independent from each other. First consider the physical and link layers. In the case of 802.11 wireless protocol, a number of different modulation techniques including OFDM and CCK have been used in different versions of the protocol. From a link layer perspective, CSMA/CA is used for medium access with a number of provisions for getting around the hidden nodes problem, such as CS and RS messages. These protocols enable data transfer rates ranging from 2Mbits/sec to 248Mbits/sec and maximum distances from 20 to 70 meters.[10] These tasks are normally performed by wireless routers and adapter hardware. Mobility at a micro level, where there is handoff from one AP to another AP in the same subnet, doesn’t really require much modification to the existing protocol. Cell phones however use significantly different technologies since they have different requirements and operate under different conditions, like guarantee of service requirement and low bandwidth connections, among other things.
IP or the internet protocol takes care of the network layer tasks. Whether in routers, embedded devices or wireless enabled computers, IP services are normally implemented in software. Most handheld devices these days usually run some form of an embedded operating system like XP embedded or CE, which take care of IP and also provide transport layer services like TCP and UDP to higher level applications. As already mentioned, mobility at this macro level also requires services from the network, specifically those of home agents and foreign agent. This is only some of the
technologies and protocols that exist at all the OSI layers that enable mobile wireless communication.
References
[1] “T-Mobile HotSpot @Home – General Information”, [online article] Available HTTP:
http://support.t-mobile.com/knowbase/root/public/tm23449.htm#12 [2] Nokia 6086 User Guide, Issue 1
[3] “OnStar” [wiki entry] Available HTTP:
http://en.wikipedia.org/wiki/On_star
[4] Muhammad Raza ur Rehman, “SIG - Secure Intercommunication Gateway” [personal webpage] Available HTTP:
http://razaurrehman.googlepages.com/sig
[5] Diego López de Ipiña1, Iñaki Vázquez2, Jonathan Ruiz de Garibay3 and DavidSainz,
“An architecture for sentient GPRS-enabled MicroBots”, IEEE CIRA, 2005, pp.145-150, 27-30 June 2005.
[6] Sami Ala-Luukko, “Mobility Management in IETF and GPRS Specifications” [Online Report] (May 2000), Available HTTP:
http://www.tml.tkk.fi/Opinnot/Tik-110.551/2000/papers/management_in_IETF/iwork.htm
[7] James F. Kurose and Keith W. Ross Computer Networking: A Top-Down Approach 3rd Edition New York: Pearson, 2005
[8] James F. Kurose and Keith W. Ross Computer Networking: A Top-Down Approach 3rd Edition New York: Pearson, 2005
[9] Sami Ala-Luukko, “Mobility Management in IETF and GPRS Specifications” [Online Report] (May 2000), Available HTTP:
http://www.tml.tkk.fi/Opinnot/Tik-110.551/2000/papers/management_in_IETF/iwork.htm [10] “IEEE 802.11” [wiki entry]
Available HTTP:
