Wireless Data Services
Circuit switching is inefficient for dedicated mobile data services such as facsimile (fax), electronic mail (e-mail), and short messaging. The demand for packet data services has, until recently, been significantly less than the demand for voice services, and first generation subscriber equipment design has focused almost solely on voice-only cellular communications. The U.S. cellular industry developed the cellular digital packet data (CDPD) standard to coexist with the conventional voice only cellular system .Two other data-only mobile services called ABDIS and RMD were developed to provide packet radio connectivity throughout a network.
Cellular Digital Packet Data (CDPD)
Figure 8.2The CDPD network.
- CDPD is a data service for first and second generation U.S. cellular systems and uses a full 30 kHz AMPS channel on a shared basis. CDPD provides mobile packet data connectivity to existing data networks and other cellular systems without any additional bandwidth requirements
- CDPD directly overlays with existing cellular infrastructure and uses existing base station equipment, making it simple and inexpensive to install.
- CDPD does not use the MSC, but rather has its own traffic routing capabilities.
- CDPD occupies voice channels purely on a secondary; noninterfering basis, and packet channels are dynamically assigned (hopped) to different cellular voice channels as they become vacant, so the CDPD radio channel varies with time.
- CDPD channel is duplex
- CDPD simplex link occupies a 30 kHz RF channel, and data is sent at 19,200 bps.
- CDPD is packet-switched; a large number of modems are able to access the same channel on an as needed, packet-by-packet basis.
- CDPD supports broadcast, dispatch, electronic mail, and field monitoring applications.
- CDPD transmissions are carried out using fixed-length blocks
- Two lower layer protocols are used in CDPD
- The mobile data link protocol (MDLP) is used to convey information between data link layer entities (layer 2 devices) across the CDPD air interface
- The MDLP also provides sequence control to maintain the sequential order of frames across a data link connection, as well as error detection and flow control
- The radio resource management protocol (RRMP) is a higher, layer 3 protocol used to manage the radio channel resources of the CIJPD system and enables an M-ES to find and utilize a duplex radio channel without interfering with standard voice services(See figure 8.2)
- The RRMP handles base-station identification and configuration messages for all M-ES stations
- The RRMP also handles channel hopping commands, cell handoffs, and M-ES change of power commands.
Table 1 lists the link layer characteristics for CDPD