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Digital sense multiple access is the protocol used by the mobile unit to connect to the local MDBS. DSMA is similar to the CSMA protocol used in Ethernet. DSMA is a technique for multiple mobile units to share a single cellular frequency, much as CSMA allows multiple computers to share a single cable. The key difference between the two, apart from the data rate, is that CSMA requires the stations on the cable to act as peers contending for access to the cable in order to transmit, whereas in DSMA the MDBS acts as a referee, telling a mobile unit when its transmissions have been garbled.

A pair of protocols permit communications between the mobile unit and the backbone router. The MDLP uses MAC framing and sequence control to provide basic error detection and recovery procedures; the SNDCP provides segmentation and head compression.

In addition to segmentation and header compression for transmission efficiency, other important features of S-NDCP include encryption and mobile unit authentication. While the cellular network provides a certain amount of protection against eavesdropping because of its channel-hopping techniques, the applications expected to be used on the CDPD network require definite security — competing businesses must have the confidence that their information cannot be seen by competitors. SNDCP encryption uses the exchange of secret keys between the mobile unit and the backbone router to ensure that there can be no violation of security when transmitting over the airwaves. The authentication procedure guards against unauthorized use of a network address.

TRANSPARENT OPERATION

Complete mobility is one of the key goals of CDPD networks. Because applications software must be able to operate over the network, the network itself must make any required operational changes transparently.

For example, the mobile units must automatically identify themselves to the network using the MNRP protocol, which recognizes the network addresses of mobile units whenever subscribers power on their computers or move to a new cell.

Data sent to a mobile unit is always sent through its home backbone router — another example of transparent operation. The home backbone router maintains an up-to-date table of the locations of the mobile units it is responsible for, thus making it possible to send connectionless data transmissions to a roaming mobile unit at any time. The home backbone router sends the data to the current serving backbone router. This scheme ensures that data reaches an end system regardless of its location, while keeping internal routing table updates to a minimum.

A connectionless service is one in which a physical connection need not be established in order to transmit data because the network is always available. In this scheme, each block of data is treated independently and contains the full destination host address. Each packet may traverse the network over a different path. A connection-oriented service, on the other hand, requires a destination address in the first packet only. Subsequent packets follow the path that has been established.

SENDING DATA FROM A MOBILE UNIT

Registration Procedure

Before a mobile unit can begin transmission, it enters into a dialogue, called the registration procedure, with the backbone router serving the area in which it is currently located. This dialogue identifies the mobile unit’s OSI network layer address to the CDPD network. The serving backbone router tells the home backbone router responsible for that mobile unit that it is requesting service. The home backbone router authenticates the mobile unit, checking such things as the user’s access rights and billing status. The registration procedure must be performed whenever the mobile unit is first powered on, or roams to a new serving backbone router.

Once the registration and authentication procedures are completed, the mobile unit begins sending data. The mobile unit is now on what appears to be a LAN connecting all such units operating within the cell of the telephone network. The LAN is really a single set of transmit and receive frequencies shared by the mobile units that access this cellular LAN using the digital sense multiple access technique.

The cells, or DSMA LANs, are interconnected by the backbone routers in much the same way that routers connect Ethernet or token ring LANs. The serving backbone router examines the data sent by the mobile units, looking for the destination address. By comparing the destination address with those in its tables, the backbone router can send the data to the appropriate destination by the best path available (see Exhibit 4). The user can now log on to the portable computer, access shared services such as CompuServe, or send information directly to other roaming mobile units. When sending data from a mobile unit to other computers, the CDPD network must only ensure that the user is allowed to transmit. Once the user is authenticated, data is sent in a manner similar to the way it is sent in current LAN internetworks.


Exhibit 4.  Potential Data Paths.


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