Issues in Mobile Computing

Issues in Mobile Computing 1. The data link layer: It is relatively easy to convince students that, with the increasing popularity of mobile computation and communication systems, the demand for the limited number of wireless-communication channels is also increasing. So, efficient channel utilization is of paramount importance. To illustrate the Importance that service providers attach to bandwidth availability, I talk about the high price they pay for it in FCC (Federal Communication Commission) bandwidth auctions. Even though the course will cover mobile computing applications later, it is important to state early on that applications' needs for channels differ significantly. Most non-real-time packet-based applications do not require a dedicated channel. However, real-time applications that expect a certain quality of service (QoS) would be best served by having some dedicated bandwidth. This topic provides an easy transition to discuss reservation-based solutions such as • • Channel allocation for cellular networks and Contention-based approaches for wireless LANS. Channel allocation This solution is appropriate when connections between the mobile node and the base stations will be long-lived. Proposed channel allocation techniques take one of two approaches: centralized or distributed. If centralized solutions are simpler and always result in channel utilization that is at least as high as that for distributed solutions. Centralized solutions have a single point of failure and might not scale well. FCA (Fixed Channel Assignment) assigns a fixed number of channels to each cell in a cellular network. It can result in poor channel utilization, especially if the spatial distribution of demand is non-uniform. A third alternative is hybrid channel allocation, which divides the set of channels into a fixed and a dynamic set. This solution divides channels in the fixed set into subsets of nominal channels associated with each cell and allocates these channels using FCA algorithms. Channels in the dynamic set are available to all cells and are allocated using DCA (Dynamic Channel Assignment) algorithms when a cell's nominal channels are all in use. Contention-based approaches Early on, the students need to understand the hidden terminal problem. Those who have taken networking courses have studied carrier sense multiple access with collision detection (CSMA/CD) protocols, which rely on the transmitter for collision detection. However, in wireless LANS, an interferer can be in the intended receiver's communication range but the transmitter doesn't sense the interferer's transmissions. In this situation, the transmitter won't sense a collision even though the receiver experiences interference, and CSMA/CD will not yield desirable performance. Therefore, protocols such as IEEE 802.11 employ collision avoidance. Wireless LANS can also operate in contention-free mode when the base station regulates access to the channel. So, unlike Ethernet, where control is distributed, protocols such as IEEE 802. 11 support both distributed and centralized operation. 2. Location management and Mobile IP: Location management in mobile networks doesn't fit neatly into a layer of the protocol stack. However, we cover it before Mobile IP and routing because: call establishment in cellular networks and packet routing in mobile networks must deal with a similar problem, and the solutions are similar in both contexts. Location management in cellular telephony employs home location registers and visitor location registers. When a mobile phone is in a foreign region, it registers with the VLR serving the foreign region. The VLR convey this registration information to the mobile phone's HLR. Subsequently, when a call for the mobile phone is routed to its home region, the HLR forwards the call to the VLR. Mobile IP, the protocol to deliver IP packets to mobile nodes, has a similar approach. Home agents and foreign agents correspond to the HLRS and VLRS. Unlike mobile-telephone networks, Mobile IP doesn’t' support real-time handoff of an ongoing call between cells.