User's Guide

Teletrac, Inc. - Prism TM Information and Installation Guide
1622-0300 B1 3/18/04 15
packets can originate from mobile devices and travel to external networks, such as the Internet or
privately connected intranets. IP packets from external networks will reach mobile devices, even
when moving. GPRS doesn't care what protocols operate above IP. This indifference enables all
standard Internet protocols to operate, including TCP, UDP, HTTP, Secure Sockets Layer (SSL),
and IPSec.
GPRS uses two essential new infrastructure elements, the Serving GPRS Support Node (SGSN)
and Gateway GPRS Support Node (GGSN). The SGSN, which connects to base-station
controllers, tracks the mobile station's location and sends data packets to and from the mobile
station. It forwards packets using a tunneling protocol to the GGSN, which acts as a gateway to
external networks, such as the Internet or private intranets. An operator will have multiple SGSNs
for different service areas, but needs only one GGSN for each external network it interconnects
with. The GGSN assigns IP addresses to mobile stations, and IP packets from external networks
route to the GGSN, which tunnels them to the appropriate SGSN for delivery to the mobile
station.
Architecture and protocols are fine, but how do users actually connect to the network and send
data, and how does the network keep track of users as they move around? When users turn on
the GPRS device (GPRS PC Card modem) in a GPRS coverage area, the device first registers
with the network and then requests a Packet Data Protocol (PDP) context. The PDP context
activates an IP address for the device, generally a dynamic address assigned by the GGSN. At
this stage the device can send and receive data.
To actually send a packet of data, the device makes requests using a packet random-access
channel. Channels are logical data paths consisting of predefined time slots in select GPRS radio
channels, and are the primary mechanism in the MAC layer. The network responds by assigning
a data-traffic channel for a temporary period sufficient to send the data packet. GPRS networks
use 200KHz radio channels, with each channel divided into eight time slots. Each time slot can
support 13Kbits/sec of throughput in today's networks (though options exist to increase data rates
to over 20Kbits/sec), and so actual user throughput will depend on the number of time slots a
user's device can handle and the particular service options from the carrier.
To support mobility, the GPRS device informs the SGSN when it's within a new base station's
coverage range. If the user travels out of one SGSN's coverage to another, then the old SGSN
and the new SGSN must collaborate and inform the GGSN of the user's new location. Users will
also be able to roam into networks operated by other GPRS carriers.
THE OVERALL TELETRAC PICTURE
Now that you have an understanding of GPS and GPRS, let’s talk about how Teletrac uses these
systems in order to provide location and messaging services to our customers. In Unit One we
talked about the components
that make up the Prism TM,
now let’s talk about how those
components work together.
As shown in the diagram, a
computer running eClient
connects to the Teletrac NCC
via the Internet. The NCC is
where all the customer
databases are stored and
where customer location
requests are processed. From
eClient
eClient
GPRS Site
CDPD Site
Radio Tower
GPS Satellite
Teletrac NCC
Internet
Internet
The three types of location systems Teletrac uses
VLUplus or VLU-G
RF VLU