Circuit Switched Data
 |
In communications, Circuit Switched Data (CSD) is the original form of
Technical
CSD uses a single radio
Prior to CSD, data transmission over mobile phone systems was done by using a modem, either built into the phone or attached to it. Such systems were limited by the quality of the audio signal to 2.4 kbit/s or less. With the introduction of digital transmission in TDMA-based systems like GSM, CSD provided almost direct access to the underlying digital signal, allowing for higher speeds. At the same time, the speech-oriented
A CSD call functions in a very similar way to a normal
At the MSC, it is possible to use a modem to convert to an
High Speed Circuit Switched Data (HSCSD)
High Speed Circuit Switched Data (HSCSD) is an enhancement to CSD designed to provide higher data rates by means of more efficient channel coding and/or multiple (up to 4) time slots. It requires the time slots being used to be fully reserved to a single user. A transfer rate of up to 57.6 kbit/s (i.e., 4 × 14.4 kbit/s) can be reached, or even 115 kbit/s if a network allows combining 8 slots instead of just 4. It is possible that either at the beginning of the call, or at some point during a call, it will not be possible for the user's full request to be satisfied since the network is often configured to allow normal voice calls to take precedence over additional time slots for HSCSD users.
An innovation in HSCSD is to allow different error correction methods to be used for data transfer. The original error correction used in GSM was designed to work at the limits of coverage and in the worst case that GSM will handle. This means that a large part of the GSM transmission capacity is taken up with error correction codes. HSCSD provides different levels of possible error correction which can be used according to the quality of the radio link. This means that in the best conditions 14.4 kbit/s can be put through a single time slot that under CSD would only carry 9.6 kbit/s, i.e. a 50% improvement in throughput.
The user is typically charged for HSCSD at a rate higher than a normal phone call (e.g., by the number of time slots allocated) for the total period of time that the user has a connection active. This makes HSCSD relatively expensive in many GSM networks and is one of the reasons that packet-switched General Packet Radio Service (GPRS), which typically has lower pricing (based on amount of data transferred rather than the duration of the connection), has become more common than HSCSD.
Apart from the fact that the full allocated bandwidth of the connection is available to the HSCSD user, HSCSD also has an advantage in GSM systems in terms of lower average radio interface latency than GPRS. This is because the user of an HSCSD connection does not have to wait for permission from the network to send a packet.
HSCSD is also an option in
Related
GSM data transmission has advanced since the introduction of CSD:
- General Packet Radio Service (GPRS) provides more efficient packet-based data transmission directly from the mobile phone at speeds roughly twice those of HSCSD.
- GSM core network.
See also
- Circuit switching
- Global System for Mobile Communications(GSM)
- General Packet Radio Service (GPRS)
- Mobile phone
- Time-division multiple access (TDMA)
- List of interface bit rates
| 0G radio telephones (1946) | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| 1G (1979) |
| ||||||||
| 2G (1991) |
| ||||||||
| 2G transitional (2.5G, 2.75G, 2.9G) |
| ||||||||
| 3G (1998) IMT-2000 (2001) |
| ||||||||
| 3G transitional (3.5G, 3.75G, 3.9G) |
| ||||||||
| 4G (2009) IMT Advanced (2013) |
| ||||||||
| 5G (2018) IMT-2020 (2021) |
| ||||||||
| Related articles | |||||||||
