WHAT IS CDPD?

CDPD in a Nutshell

CDPD stands for Cellular Digital Packet Data. In the most simple terms, CDPD consists of a laptop computer with a cellular modem transmitting data over the existing cellular telephone network, commonly referred to as AMPS (Advanced Mobile Phone System). CDPD uses the same transmission channels as the cellular telephone network for short transmissions of small data packets. How CDPD shares these channels with cellular phone users depends on the type of CDPD network employed. 
There are two types of CDPD networks: dedicated channel and channel hopping. In a dedicated channel network, certain channels are set aside solely for use by the CDPD network. In a channel hopping network, all the channels are shared by CDPD and the cellular telephone network. A cellular phone call has preemptive priority over any CDPD call, meaning that if a person wants to place a call on a cell phone over a channel that happens to be in use by CDPD, the CDPD calls will be booted off that channel. The person making the phone call will never know that anyone else was on his or her channel, because the CDPD users vacate the channel within 40 ms of detecting voice activity. The users then move, or "hop" to another available channel and continue their sessions. If there is no channel available to the CDPD users, they are dropped.

Each channel on a CDPD network can be accessed by up to 30 CDPD users at a time. The users take turns transmitting on the channel stream, just like on an Ethernet. If more than one person tries to send something at the same time, a "collision" occurs and all users involved try to transmit again a short time later. 

The Parts of a CDPD Network

MES

The MES consists of a personal communications device, such as a laptop computer connected to a cellular data modem, or a point of sale terminal. The MES may operate as a normal computer, used for downloading files or checking email, etc, or it may serve a specialized purpose, such as credit card verification. The MES both sends data to the network via the MDBS and receives data from the network via the MDBS at a rate of 19.2 kbps. (Because of overhead, the actual data rate is approximately 9.6 kbps.) Before transmitting the data, the MES packetizes it and also encrypts it for security. 

MDBS

The MDBS lies within a cell site. It uses the same antenna as the voice network to receive and send data between the MES and the MDIS. The MDBS is mainly responsible for channel matters. While CDPD calls are taking place, the MDBS monitors the channels to detect the existence of competing voice calls, and also monitors all unused channels to find suitable "hoppping" channels. A hop occurs when a CDPD call must give up its channel for a voice call. Hops also occur when a user moves from one service area to another, such as from one base station's coverage area to another's. 

MDIS

The MDIS acts as the liaison between the cellular portion and the fixed, or land-line, portion of the network. It accepts packets sent from the MDBS and converts them to IP format for transmission over the fixed network. Similarly, it accepts packets from the fixed network and encrypts and further processes them for transmission over the airlink via SNDCP (Subnetwork Dependent Connection Protocol). The MDIS also handles registration (connection establishment) for the MES's in its service area. MDIS's can be divided into 2 categories: home and serving. A home MDIS is an MES's primary MDIS, the one it normally registers to. A serving MDIS is an MDIS in another coverage area which an MES registers to if it is within that area. Home MDIS's are responsible for forwarding packets to the appropriate serving MDIS's of all of its MES's. 

IS and FES

The intermediate systems and fixed-end systems comprise the fixed network. These are off-the-shelf products which are "invisible" to the mobile network. IS's are routers which direct the packets to the correct location. FES is a general lump term used to describe the different possible destinations of the packets. FES's include external computer networks (such as campus networks), administrative servers, and network accounting servers, as well as many other examples.