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2.3.1 HSCSD
The Nokia D211 supports non-transparent data. It also supports High Speed Circuit Switched
Data (HSCSD) in non-transparent mode. With HSCSD a mobile can use several time slots at
the same time. This makes the data transfer rate higher. Time slot usage is presented as the
number of down links and up link slots. The Nokia D211 data transfer modes are presented
in Table 1.
Table 1. The Nokia D211 data transfer modes.
Data transfer mode
Non-transparent data
The Nokia D211 supports multi slot class 6 (Max Rx time slots 3, Max Tx time slots 2, sum
4). One time slot can be from 9.6 kbit/s to 14.4 kbit/s (network dependent).
In HSCSD allocated time slots are reserved only for the user. Other technologies, such as
General Packet Radio Service (GPRS), will complement and enhance the value of HSCSD.
There are applications for both circuit-switched and packet-switched world. For instance, if
the user needs to make PSTN/ISDN connection (towards corporate modem pools), only
HSCSD can be used. HSCSD is ideal for real-time applications and GPRS for "always
connected" applications. For example, HSCSD connections provide "active" e-mail and file
transfer, while GPRS runs in the background for wireless data networking.
3+1 operation mode (43.2 + 14.4 kbit/s) is optimal for receiving e-mail, WWW browsing,
video streaming, etc. 2+2 operation mode (28.8 + 28.8 kbit/s) is optimal for symmetric
applications such as voice and video calls (e.g. Netmeeting) and e-mail message sending.
GSM CSD
Modulation
CSD-3(T/NT) GMSK
CSD-4(T/NT) GMSK
2.3.2 GPRS
The nature of packet data is very different from the circuit switched data. GSM was originally
designed for circuit switched traffic and thus introduction of packet data to GSM meant quite
significant additions to the GSM network architecture (Figure 4). GPRS interworks with public
data networks like the Internet (TCP/IP protocol) and X.25 networks. The architecture of
GPRS infrastructure is based on GPRS support nodes, Serving GPRS Support Node
(SGSN) and Gateway GPRS Support Node (GGSN), which receive and transmit packets
between MSs and terminals connected to public data networks. The support nodes also
forward packets to those MSs that are not located in the home GSN area. The operation is
similar to the operation of routers in data networks (e.g. the Internet), except that GPRS
nodes keep track of the locations of MSs.
Mode
Asynchronous data
Asynchronous data
Asynchronous data HSCSD
Code
Data rate / time
rate
slot (kbit/s)
0.53
9.6
0.64
14.4
Data rate
9600 bps
14400 bps
43200 bps (Multi slot class 6, 3+1)
Two slots
Three slots
(kbit/s)
(kbit/s)
19.2
28.8
28.8
43.2
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