An important goal for wireless communications has been to make the application layer transparent to the underlying protocol (TCP/IP) in order to have more acceptability by the Web users. To understand the kind of standards developed for wireless networks, it helps to see the affected layers in an OSI (Open System Interconnect) model. The bottom two layers are the ones of interest to us. At the very bottom is the Physical layer. This layer defines the electrical characteristics of the actual connection between network nodes. For wired networks, it covers topics like voltage levels and type of cabling. But for wireless networks, it addresses areas such as frequencies used and modulation techniques, including spread-spectrum technologies.
The next layer up is the Data Link Layer. It deals with how the network is shared between nodes. The Data Link Layer defines rules such as who can talk on the network, how long they can occupy network resources. This layer can be further divided into two separate layers (shown below).
· The Medium Access Control (MAC) layer.
· The Logical Link Control (LLC) layer.
The first five layers of the OSI model remains unchanged; hence, TCP and IP can be implemented in their respective layers.
The wireless network interface manages the use of air through the operation of a communications protocol. For synchronization, wireless networks employ a carrier sense protocol similar to the common Ethernet standard. This protocol enables a group of wireless computers to share the same frequency and space.
The lack of standards has been a significant issue with wireless networking. In response to this problem, the Institute for Electrical and Electronic Engineers (IEEE) has been involved in the development of wireless LAN standards for the last seven years. This effort is nearly complete, and the final standard (IEEE 802.11) will be ready by May of 1997.
As with other 802 standards such as Ethernet and token ring, the primary service of the 802.11 standard is to deliver MSDUs (MAC Service Data Units) between LLC (Logical Link Control) connections to the network. In other words, the 802.11 standard will define a method of transferring data frames between network adapters without wires. In addition, the 802.11 standard will include:
· Support of asynchronous and time-bounded delivery service
· Continuity of service within extended areas
· Accommodation of transmission rates between 1 and 20 Mbps
· Support of most market applications
· Multicast service
· Network management services, Registration and authentication services
The IEEE 802.11 standard supports operation in two separate modes, a distributed coordination (DCF) and a centralized point-coordination mode (PCF). The IEEE 802.11 MAC is called DFWMAC (Distributed Foundation Wireless MAC), and the access mechanism is based upon the principal of CSMA/CA (Collision Sense Medium Access with Collision Avoidance), which is another adaptation of CSMA/CD used by Ethernet networks.
Under CSMA/CD, when a station has data to send, it first listens to determine whether any other station on the network is occupying the medium. If the channel is busy, the station will wait until it becomes idle before transmitting data. Since it is possible for two stations to listen at the same time and discover an idle channel, it is also possible that two stations could then transmit at the same time. When this occurs a collision will take place, and then a jamming signal is sent throughout the network in order to notify all stations of the collision. The stations will then wait for a random period of time before re-transmitting their respective frames.
CSMA/CA is a modified version of the CSMA/CD access system. Under the CSMA/CA technique, as before stations are listening to the medium at all times. A station that is ready to transmit a frame will sense the medium, if the medium is busy, it will wait for an additional predetermined time period of DIFS (DCF Interframe Space) length and then, based upon a random calculation, picks a time slot within a contention window to transmit its frame. If there were no other transmissions before this time slot has arrived, it will start transmitting its frame. On the other hand if there were transmissions by other stations during this back-off time period, the station will freeze its counter and will pick-up the count where it left off after the other station has completed its frame transmission. The collisions can now occur only when two or more stations select the same time slot to transmit. These stations will have to reenter the contention procedure to select new time slots to transmit the collided frames. The figure below illustrates DFWMAC access scheme.
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.
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.