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How
Data Services Work in PCS Digital Cellular Networks
By Rysavy
& Associates
Definition
of terms
In this section we take a detailed
look at the key interfaces involved, protocols used and a key system
within the cellular network called the interworking function (IWF).
This is the gateway between the cellular network and other networks
such as the telephone network or the Internet. Digital cellular
refers to the deployment of digital technology in existing analog
cellular networks that operate in the 800 MHz band, whereas PCS
refers to the same digital technology applied in new radio bands
at 1900 MHz (and 1800 MHz in Europe). Note that the data services
for digital cellular and PCS are exactly the same.
The key digital technologies (GSM,
TDMA, CDMA) are being deployed in both cellular (800 MHz)
and PCS (1900 MHz) bands in the US. Deployment is proceeding as
follows:
| Technology |
Classification |
Frequency
Band |
| AMPS |
Analog
Cellular |
800MHz |
| CDMA |
Digital
Cellular or PCS |
800 MHz
or1900 MHz |
| TDMA |
Digital
Cellular or PCS |
800MHz
or 1900 MHz |
| GSM |
PCS |
1900MHz |
Though there are differences in how
data services will operate between GSM, TDMA, CDMA, the types of
data services they can offer are all quite similar. This is no surprise
given that the data services are built on underlying transports
designed historically for voice communications.
Link layer
Figure 1: Link layers
Referring to the OSI model for networking,
we find that at the physical layer, PCS networks treat voice and
data identically. The physical layer takes data from the link layer
above, formats and encodes it in a way that makes it best suited
for radio communications, then transmits it by modulating a radio
carrier signal. Modulation refers to varying the carrier signal's
amplitude, phase or frequency, or combination of these.
What differs dramatically between voice
and data is at the next layer up, the link layer. Whereas voice
information must be delivered without variable delay for it to be
intelligible, data needs to be delivered without errors even if
that means variable delays. Hence PCS networks use different link
layers to accommodate these different needs. In particular, the
protocols encompass various methods to correct errors when communicating
data, including "forward error correction" where redundant
data is included with the original data and "automatic repeat
request" where blocks of data received in error are retransmitted.
Circuit-switched
data and the interworking function (IWF)
With the current circuit-switched connections,
your computer dials a phone number to establish a circuit on an
end-to-end basis before communicating, just like with a traditional
modem. The computer can then communicate data until it hangs up.
You get billed based on the duration of your connection
, whether you are roaming, and
whether you are dialing long distance ¾
just like with voice calls. Faxing
over cellular networks also uses circuit-switched connections.
Figure 2: Interworking function
To make circuit-switched data work,
the carrier must provide an interworking function (IWF) at the switching
center that serves as a gateway between the digital data signals
used in the cellular network and other networks such as the public
telephone network. Traditionally the IWF has consisted of modem
pools to allow modem-based communication across the telephone network.
Modem pools allow users to make calls to Internet service providers,
online service providers (for example, AOL*) and modems or
standard fax machines at corporate and other sites. You can think
of the combination of your cellular phone and cellular network as
a serial cable extension cord to the modems at the interworking
function. Another type of IWF is one that connects users directly
to the Internet where the Internet access is provided by the carrier
itself, or is handed over automatically over digital trunked lines
to an Internet service provider.
Because circuit-switched data is similar
to voice communications in terms of call handling and billing, cellular
carriers have been able to offer circuit-switched data service more
readily than packet-switched data, and have already done so for
a number of PCS networks.
Figure 3: Circuit-switched data in
a PCS network
Modem interworking functions are available
today for GSM networks and are in development for TDMA and CDMA
networks. Internet interworking functions should start to become
available in the 1998 timeframe.
Click
here to learn more about packet-switched data.
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