Just as in wired networks, the interworking unit (IWU) provides the protocol manipulation to connect networks with different protocols together. The IWUs act as access points between wireless stations and the Web. They address issues such as:
· Correct delivery of data to its destination.
· Congestion control.
· Differences in maximum PDU sizes.
To connect a wireless network that is using the 802.11 protocol to the Internet, IWUs are needed at access points. Access points are nodes that allow traffic flow in and out of the Wireless network. Alternatively, IWUs (IP Routers) control the traffic in and out of the Internet; thus routing wireless packets into and out of the Internet as shown below:
The 802.11 protocol can support data rates of 20 Mbps, thus making it an attractive wireless protocol for Internet connectivity. Companies such as Proxim that have been involved with the development of 802.11, are migrating rapidly to the new standard.
Internet Mobile Host Protocol
An important part of wireless connectivity is mobility. Mobile computers must be able to move between adjacent cells or across multiple network domains without disturbing the application level process. Mobile users and mobile protocols must not make any changes to the existing TCP/IP Internet protocol to insure connectivity and usability of the Internet as it exists today.
A mobile host is the Internet Mobile Host Protocol (IMHP) entity that roams through the Internet. Each mobile host has a home agent on its home network. Each home agent maintains a list known as a home list. The home list is a list of mobile hosts that the home network will serve and it also maintains the location of each mobile host as the network becomes aware of their locations. As mobile hosts roam from one network to the next, they have to register with foreign agents on new subnets as they try to connect to that network. Foreign agents are much like a home agent except they interact with visiting home agents from other networks. Each foreign agent maintains a list known as the visitor list, which identifies the mobile hosts that are currently registered with it. The combination of the foreign agents address for a particular home agent (care-of-address) along with its home address is known as a binding. A binding defines where to send packets for a particular home agent at any given time. (Perkins, Myles, and Johnson, 1994)
The registration protocol which is part of the IMHP management protocol notifies all the concerned parties of the new mobile host's location. Those include the previous foreign agents and the host's home agent. It is the responsibility of the IMHP management protocol to keep a forwarding pointer from the previous foreign agents until all information about the new location has been updated with the new network and the home network. Time stamps are used to keep visitor lists current and to delete the home agents that have left the network. Figure below shows the registration process for a home agent through a foreign agent and the notification process.
Any node may function as a cache agent by caching the bindings of one or more mobile hosts. All of these cache agents are under the umbrella of the IMHP management protocol which is running on all IMHP agents as long as they are not on their home networks. The IMHP management protocol manages the cache agents in a distributed fashion. This will allow packets to travel to their destinations without having to be routed to a home networks first. Cache agents actively attempt to reconform bindings in their location caches using the IMHP management protocol, and also periodically notifications are send out by the protocol to update caches when agents move in and out of networks.
IP Tunneling
IMHP entities direct and send packets to a mobile host's current location using a tunneling technique. Tunneling in IMHP management protocol takes the form of encapsulation. The protocol will add 8 bytes to each packet sent to a mobile host if the sender has a location cache entry for the destination mobile host, otherwise it adds 12 bytes to each packet. The tunneling header is inserted into the packet immediately following the existing IP header. In the IP header, the protocol number is set to indicate the IMHP encapsulation tunneling protocol, and the destination address is set to the mobile host's care-of-address, and finally the source address is set to the IP address of the encapsulating agent. (Perkins, Myles, and Johnson, 1994)
This tunneling procedure will inssure packet delivery throughout the Internet as it exists today, since the intermediate routers will see a normal IP packet. It is only the IMHP network that can recognize the packets by seeing the protocol number and deliver them to their final destination.