In 1992, AT&T Wireless Services developed cellular digital packet data (CDPD) protocol, a data-only protocol that (re-)uses the AMPS or IS-136 network. Packets (typically some 1.5 kilobytes) use vacant cellular channels – either an assigned channel or between calls.

CDPD does not communicate with the underlying network (but does utilize knowledge of this networks channel assignment algorithms to predict when channels will be available for CDPD’s use).

Mobile Data Base Stations – do channel sniffing to find idle channels. It is essentially an implementation of Mobile*IP.

Motivation for CDPD:

Most traditional cellular systems (such as AMPS) are unsuited for packet data

• Long call setup times – many seconds (vs. CDPD with from under 1 to 4 sec)
• Modem handshaking required – this modem training can take more time than the data
transfer time!
• Analog providers already have AMPS allocation
• Re-use AMPS channels to provide data service.
•Must not interfere with existing analog service (i.e., operator’s bread and butter)
• no new spectrum license needed – but you get to make more money with the
spectrum you already have (IFF you can share the spectrum wisely)


•Low speed data: Paging, short message, e-mail, …(achieve 10-12kbps)
• Broadcast and multicast (for example, for fleet management)
• "always on-line" packet data service
• Transparent to existing AMPS service, but shares spectrum with it

CDPD Entities

Mobile End System (M-ES)

• Subscriber unit – interfaces with the radio at 19.2 kbps
• Subscriber Identity Module (SIM) – used to identify subscriber
• Mobile Application Subsystem – actually provides the functionality
(could be a PDA, Laptop, embedded processor, …)

Mobile Data Base Station (MDBS)

• controls the radio: radio channel allocation, channel usage, …
• one modem/transceiver per radio channel pair (up & down link)
• generally co-located with the AMPS base stations (so they can share antenna, site, …)

Mobile Data-Intermediate System (MD-IS)

• Frame relay switch + packet router.
• Buffers packets intended to M-ES it knows about (with TEI assigned).
• Supports user mobility by a mobile location protocol.

Roaming Management

•Each M-ES has a unique Network Equipment Identifier (NEI) which is associated with a
home MD-IS (Mobile Home serving Function (MHF) {a Mobile IP Home Agent}.
•Home MD-IS keeps location directory of the MD-IS currently serving each of its mobiles.
•Each MD-IS keeps a registration directory listing currently visiting mobile (Mobile
Serving Function (MSF)) {a Mobile IP Foreign Agent}.
•When a M-ES moves, the home MD-IS explicitly cancels the registration at the former
•Packet routing is handles just as in Mobile IP.

CDPD Support

1.CDPD supports both:
• ISO connectionless network protocol
• IP
2. CDPD has explicit provisions for Multicast and enables mobiles to register for a
multicast NEI – this must include a Group Member Identifier (GMID) which is unique
with in the group.


•No direct M-ES to M-ES communication
• Radius of a CDPD cell is limited to <10 miles (i.e. < 17km)
• Each M-ES can only send two packets back to back – to avoid hogging the channel

Operation of MBDS & M – ES

MDBS broadcasts a list of available channels. When M-ES finds link quality has dropped below a threshold, it checks the channels from the MDBS’s that it can hear; if there is a better channel it initiates a link transfer – by switching to the new channel and registering with the new MDBS

MD-IS maintains a registration directory

• Contains a list of Temporary Equipment Identifiers (TEI)
• Associated with each TEI is a element inactivity timer (T203)

• Associated with each radio channel stream is a TEI notification timer (T204) – when
this timer goes off MD-IS broadcasts a list of TEI’s with data buffered for them
{mobiles with nothing to send can sleep until the next TEI notification frame}

• When a mobile wakes up and hears there is data for it, it send a Receiver Ready (RR)