Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 28
When receiving data on the forward (receive) channel from the MDBS, it must:
• Receive packet data from the network
• Request retransmission of packets depending on error correction performance
• Decrypt the packets
• Disassemble the packets to extract the application data
• Pass the serial data to the end-user’s application equipment
5.3. Subnetwork-Dependent Convergence Protocol (SNDCP)
In a CDPD network, the Subnetwork Dependent Convergence Protocol (SNDCP) provides
compression, encryption, and segmenting for data transferred over the network. It operates
between the Internet-standard IP and the next layer, the MDLP (section 5.4), and between the
M-ES and its serving MD-IS. In other words, SNDCP takes standard Internet packets, compresses
their header information, segments them for transfer over the CDPD network, and encrypts the
segments.
5.4. Mobile Data Link Protocol (MDLP)
The Mobile Data Link Protocol (MDLP) is CDPD’s link layer protocol, also operating between
the M-ES and the MD-IS. It provides the interface between SNDCP and the MAC layer (see
section 5.5), and enables framing, the data link connection, sequence control, and flow control.
MDLP controls the throughput of a connection, and divides the segments provided by SNDCP
into frames. It also manages CDPD sleep mode (see section 5.7).
MDLP establishes procedures for frame delivery, and detects and recovers from frame loss.
The CDPD network establishes and maintains a number of values using MDLP, including:
• Assigning the Temporary Equipment Identifier (TEI) for each connected M-ES (see
section 5.1.2)
• Setting the maximum number of frames that can be transmitted or received in a single block,
known as the window size
• Defining wait-for-acknowledgement periods and other administrative values
• Measuring information and statistics about the number and various types of data frames
transferred
5.5. Medium Access Control (MAC)
The MDBS supports a Medium Access Control (MAC) mechanism to coordinate the
transmissions from many M-ESs on a single radio channel. While one M-ES is transmitting, the
MAC mechanism prevents other M-ESs from doing so simultaneously. MAC is the CDPD airlink
protocol that provides Forward Error Correction (FEC) and controls the sharing of the airlink
resource between multiple users. MAC helps manage and prevent congestion on the CDPD
network.
MAC operates over the last leg of the CDPD connection: the airlink between the MDBS and the
M-ES subscriber device. It controls and manages channel access, synchronizes data
communications, provides error correction, manages the DSMA-CD collision detection and packet
retry process (see sections 3.2.8 and 5.5.2), and divides frames into blocks for transfer over the
airlink.
Using MAC, the CDPD carrier establishes a number of parameters, including:
• How many times to attempt retransmissions
• The maximum number of blocks to transmit in a single burst
• Values for the M-ES to use when generating a random back-off time when packet
retransmission is necessary