Now that we have outlined the network components and the conceptual routing architecture, let us examine the protocol architecture. In Figure 4.3, a diagram of the protocol stacks used in CDPD communications is presented.
The M-ES is an end system or host, and therefore must contain the complete 7 layer protocol stack. At the physical layer, the M-ES uses a digital radio modulation scheme called Gaussian Minimum Shift Keying (GMSK)4.4 , which is a filtered variant of Frequency Shift Keying (FSK) modulation. The next sublayer is the medium access control mechanism, which provides support for channel sharing by multiple M-ESs. CDPD uses Digital Sense Multiple Access (DSMA) for this purpose; DSMA is related to and resembles the collision sense multiple access (CSMA) scheme of Ethernet.
The data link is managed by a link access protocol named Mobile Data Link Protocol (MDLP). This protocol is derived from the standard Link Access Protocol on the D channel (LAPD) from ISDN. Above the data link, the Subnetwork Dependent Convergence Protocol (SNDCP) provides the functionality necessary to interwork with the supported network protocols. Above this, the standard Internet protocol (IP) and Connectionless Network Protocol (CLNP) suites operate at Layer 3.
The MDBS in the CDPD network functions as a data link relay. As such, it cooperates with the M-ES over the airlink by performing the GMSK modulation function and by managing the DSMA medium access control mechanism. The MDBS retrieves MDLP data link frames and relays them between the serving MD-IS and the M-ES.
The serving MD-IS interoperates with the MDBS via conventional data networking infrastructure for carriage of the MDLP frames. The serving MD-IS is the peer entity to the M-ES data link; the M-ES and the serving MD-IS operate the end-points of the MDLP data link connection. Above the data link, the serving MD-IS performs the peer SNDCP function. The network packets are then passed up to the network layer routing function. At Layer 3, the home and serving MD-ISs function much the same as special mobility-aware routers.
From this discussion and from Figure 4.3, it should be clear that the CDPD network provides a network layer routing service. All protocol data units at the network layer and above are carried and delivered without alteration. CDPD provides a special media-radio-but does not define a gateway architecture.