Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 21
4.3.6. The Data Link Layer (Layer 2)
The data link layer provides communication between two or more connected systems. It performs
frame formatting, error checking, addressing, and other functions to provide accurate data
transmission between systems. It also governs access of users to the communication medium,
such as the radio channel.
4.3.7. The Physical Layer (Layer 1)
The bottom layer provides a physical connection for the transmission of data between data link
layer entities. It performs electrical encoding and decoding of the data (or bits) for transmission
over the medium in use (that is, the radio channel).
4.3.8. Where CDPD Fits Into the Protocol Stack
CDPD provides services and protocols to the Network Layer (Layer 3) and below. Protocols in
layers 4 through 7 are external to the CDPD network.
The CDPD network is a multi-protocol connectionless network, providing network services in
any of several different network protocols. CDPD networks support the Internet Protocol (IP)
and ISO’s Connectionless Network Protocol (CLNP). Other network layer protocols may be
offered in the future. The only requirement is that both communicating end systems using the
CDPD network use the same network layer protocol, since the CDPD network itself does not
provide protocol translation.
Connection-oriented services may be provided by end-to-end protocols operating above the
network layer (such as TCP/IP). Individual CDPD service providers do not directly provide or
operate the Transport Layer or higher services—that is the responsibility of the communicating
end systems.
4.3.9. CDPD Communications Subprofiles
The CDPD network specifications define a number of subprofiles as building blocks that may be
selected and combined to define a particular CDPD network element. These subprofiles define
specific multi-layer protocol requirements for a CDPD network element or CDPD network
service. CDPD application subprofiles specify the Layer 5, 6, and 7 requirements for each
application service.
The CDPD network achieves interoperability across all CDPD service providers by requiring the
support of lower layer subprofiles to assure interoperable data transfer for Layer 4 and Layer 3.
These include the mandatory support of lower layer services provided by Transmission Control
Protocol (TCP) over Internet Protocol (IP).
The subnetwork subprofiles refer to the data link and physical layers, which are within the
CDPD network and are therefore transparent to the CDPD subscriber. An example of a virtual
terminal subprofile is represented in the following illustration. Some of the technologies used
include X.25 and Frame Relay.