Other than the UTRAN interfaces, the figure 6.2 also illustrated the UTRAN Radio Interface protocols. The network element which controls the whole radio network is RNC. Therefore, UE & RNC need to communicate very often in UMTS. This communication happens using the radio protocols. Physical realization of this signalling transfer happens using the Uu Interface ( UE Node B) and Iub Interface (Node B RNC).
Figure 6.2: Overview of all UTRAN Interfaces and Protocols
6.2
QoS and Bearer
Source : 3GPP TS 23.107 ; Quality of Service (QoS) concept and architecture
End-to-End Service: End-to-end service means the service as perceived by the end user.
For example, the end-to-end service from one Terminal Equipment (TE) to another TE, or from laptop to web server. In order to provide a certain QoS to a user, there must be a bearer with well-defined characteristics and functionality.
End-to-end service is like a chain of several smaller links (or bearers) and it is a well-known fact that a chain is never stronger than the weakest list. Therefore, the weakest bearer in the chain will define the QoS of end-to-end service.
End-to-end service = UMTS bearer “ + ” External Bearer.
External bearer is beyond the scope of UMTS technology. Therefore, the operator has to rely on the QoS provided by the external bearer. If the external bearer is between GMSC
Figure 6.3: UMTS QoS Architecture and Bearer Concept (3GPP TS 23.107)
and external PSTN exchange, then these links can be the PCM lines which have excellent QoS with guaranteed bit rate. On the other hand, if these external bearers are between GGSN and some web server, then the external bearer is implemented on the IP link. The QoS in IP is a configurable thing. But we will not discuss it here and restrict ourself to the UMTS bearer.
The UMTS bearer can be understood as a chain of three smaller bearers.
UMTS Bearer = [Radio Bearer] “ + ” [Iu Bearer] “ + ” [Core Network Bearer].
where , [Radio Bearer “ + ” Iu Bearer] is often called as Radio
Access Bearer (RAB).
Radio Access Bearer can be considered as a service provided by lower layers to higher layers. Using RAB, the information is transferred between UE and core network (MSC or SGSN). In order to have a RAB, UE must have a radio bearer and Iu bearer. Radio bearers are managed by RNC. Therefore, while RAB setup, core network requests RNC and after successful response from RNC, the RAB is established.
Please note! RB Reconfiguration and RAB Reconfiguration sound very similar and quite often people mix them up.
6.2. QOS AND BEARER 177
We should remember that Radio Bearer (RB) Reconfiguration is a local sig- nalling procedure between UE and RNC, whereas, RAB Reconfiguration hap- pens with the involvement of core network. RB reconfiguration happens very often and can be seen from L3 radio messages, but to analyze the signalling of RAB reconfiguration we must use the signllinng traces on Iu-CS or Iu-PS interface.
Please refer to section 6.1 of TS 23.107 for more details.
6.2.1
UMTS QoS Classes
The QoS is simply a phrase. For implementation, we define it using a list of parameters. One of these parameters is the Traffic Class. According to 3GPP TS 23.107, all the services can be classified into 4 groups:
• Conversational class • Streaming class • Interactive class • Background class
The delay sensitivity of traffic is the main criteria for this classification. Conversational class traffic is affected very badly the bearer is lost for few hundred ms where as the bearer background class will not be affected so badly even if the bearer is unavailable for few seconds.
Other than this classification, we can also group the services in two groups: Real-Time (RT) and Non-Real-Time (NRT) services. Conversational and Streaming classes are mainly used for carrying real-time traffic flows whereas the Interactive and Background traffic classes are suitable for carrying Non-Real-Time traffic.
Conversational Class
The most well-known use of this scheme is telephony speech (e.g. GSM). But with Internet and multimedia, a number of new applications will require this scheme, for example, voice over IP and video conferencing tools. Real time conversation is always performed between peers (or groups) of live (human) end-users. This is the only scheme where the required characteristics are strictly given by human perception. Real time conversation - fundamental characteristics for QoS:
• Preserve time relation (variation) between information entities of the stream; • Conversational pattern (stringent and low delay).
Streaming Class
When the user is looking at (listening to) real time video (audio), the scheme of real time streams applies. The real time data flow is always aiming at a live (human) destination. It is a one way transport.
Real time streams - fundamental characteristics for QoS:
• Preserve time relation (variation) between information entities of the stream.
Interactive Class
When the end-user, that is either a machine or a human, is on line requesting data from remote equipment (e.g. a server), this scheme applies. Examples of human interaction with the remote equipment are: web browsing, data base retrieval, server access. Interactive traffic - fundamental characteristics for QoS:
• Request response pattern; • Preserve payload content.
Background Class
When the end-user, that typically is a computer, sends and receives data files in the background, this scheme applies. Examples are background delivery of E-mails, SMS, download of databases and reception of measurement records. Background traffic - fun- damental characteristics for QoS:
• The destination is not expecting the data within a certain time; • Preserve payload content.