Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 16
4. Infrastructure: CDPD Network Architecture
4.1. Physical: Components of the CDPD Network
As mentioned in section 3.2.6, a cellular carrier can construct a CDPD network with a few new
components added to the existing AMPS cellular phone infrastructure. Such a process costs less
than creating a completely separate wireless data infrastructure. However, it is far from
inexpensive, since not only does it require new interface elements between the CDPD network and
the Internet, but also additional equipment installed at each cell site. Users desiring CDPD
connectivity also require CDPD-capable modems, such as those designed by Sierra Wireless (see
section 6).
A CDPD service provider network can be constructed from four of the five basic building blocks
described below (that is, the M-ES, MDBS, MD-IS, and IS). The F-ES is the destination system,
which can be outside the CDPD network. This network construction takes into account such
things as:
• Providing coverage over a large geographic area. This ranges from coverage within a city to
nationwide and international coverage.
• Matching the airlink capacity available with the demand placed on it by users requiring
service within the CDPD coverage areas.
• The need to provide access to private and commercial external networks.
Networks are often constructed by interconnecting several networks into an internetwork (see
section 3.1). The networks that make up the internetwork often have their own administration and
routing policies and are called administrative domains. The CDPD network is actually an
internetwork composed of multiple administrative domains, with each administrative domain
operated by a service provider. This administrative domain is referred to as the CDPD service
provider network.
4.1.1. End Systems (M-ES and F-ES)
The purpose of the CDPD network is to allow data to be transmitted to and received from End
Systems (ESs) that are attached to the network. An end system in the CDPD sense is a host
running a user application and having a unique identity. This unique identity is provided in the end
system through at least one globally unique Network Entity Identifier (NEI—see section 5.1.1).
The CDPD subscriber obtains an NEI from the service provider when they activate their CDPD
modem on the service provider network. This is equivalent to activating a cellular telephone with
a cellular service provider when signing up for service on that voice network.
In the terminology of CDPD networks, the CDPD subscriber device—often, for instance, a PC
Card CDPD modem inserted into a notebook computer—is known as the Mobile End System, or
M-ES. It connects to the network via the airlink, and is always part of the CDPD network. By
contrast, a host computer connected into the CDPD network with a traditional wired connection,
such as through the Internet, is known as a Fixed End System, or F-ES. An F-ES can be
anywhere, either inside the CDPD network or outside of it.
The CDPD network makes a distinction between Mobile End Systems and Fixed End Systems for
the purposes of mobility management. M-ESs can change their Subnetwork Point of
Attachment (SNPA) dynamically while receiving network services, whereas F-ESs do not. Also,
M-ESs must support a wireless connection to the CDPD network, and F-ESs generally must
support a wired connection to the CDPD network or some connected external network. Because
of the need to manage the mobility of M-ESs throughout the CDPD network, the connection
between the CDPD network and the M-ES requires two network elements that are unique to the
CDPD network: the MDBS and the MD-IS.