Services [Under UMTS]
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UMTS services/capabilities - 22.105 «
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QoS architecture - 23.107 «
UMTS makes the distinction between bearer services and teleservices. Teleservices is more of "end-to-end" service. Terminal equipment functions, network functions, interworking functions all come under Teleservices. Bearer services are related to lower layer capabilities and often described in terms particular characteristics or capabilities of lower layers. Access functions, Transport functions comes under Bearer services.
Examples:
1) Voice call is standardised UMTS teleservice - "Spech" - which require (circuit switched) bearer service having characteristics like "Real time (constant delay, say < 100ms)", "Point to point Bidirectional", "Constant bit rate (say 9.6kpbs)". (here constant delay and constant bit rate indicate "transparent operation".
2) Internet surfing is standardised UMTS teleservice - "Internet Access" - which requires (packet switched) bearer service having characteristics like "Non-real time (variable delay)", "Point to point Bidirectional Asymmetric", "Variable bit rate (with maximum value)".
To maintain compatibility with GGE, circuit switched and packet switched bearer services are maintained in UMTS. Access stratum works on the basis of bearer characteristics (QoS parameters); core network takes decision whether the service will be done through CS domain or PS domain.
Various QoS parameters are:
- Traffic class (conversational, streaming, interactive,
background)
- Maximum bitrate (kbps)
- Guaranteed bitrate
(kbps)
- Delivery order (in-sequence SDU delivery or not - y/n)
-
Maximum SDU size (octets)
- SDU format information (bits)
-
SDU error ratio
- Residual bit error ratio
- Delivery of
erroneous SDUs (y/n/-)
- Transfer delay (ms)
- Traffic
handling priority
- Allocation/Retention Priority
- Source
statistics descriptor (‘speech’/’unknown’)
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Signalling Indication (y/n)
Note about AMR speech
AMR stand for Adaptive Multi-Rate. In a voice call (or in general, any conversations), participants are do not talk at the same time, but rather alternate. Also, there are silence periods in the conversation. AMR speech codec makes use of these properties to convert speech to much lower transmission data, resulting in saved bandwidth. This mean transmission bit rate is not constant but varies (drops to zero when voice is not detected). Overall speech quality can be tuned as per transmission error rate. AMR gives overall lower average bit rate, reducing interference.
Release 5 introduced Wideband AMR - which has more sampling frequency (giving better voice quality) for the same average bit rate.
We will now have a look at protocol stacks for circuit switched bearer services.
References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.
| Copyright © Samir Amberkar 2010 | § |
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