In this chapter a background for broadband techniques in respect to integration with the mobile network is given. The computer and telecommunication industries are converging towards one single communication technology in the fixed network. This Asynchronous Transfer Mode (ATM) was originally designed to make Broadband ISDN a reality. ATM permits transparent integration and seamless interface for local and wide area networks. B-ISDN was created conceptually as an extension of ISDN so it can provide integrated broadband services along with traditional ISDN services. B-ISDN adds a tremendous new features to ISDN. B-ISDN will support services with both constant and variable bit rates, data, voice, still and moving picture and multimedia applications. The future B-ISDN is conceived as a universal network supporting different kinds of applications and customer categories. To be able to offer B-ISDN services over ATM, different ATM Adaptation Layers (AAL) are needed depending on the service requirements. The development of B-ISDN can possibly expand as a backbone for large private networks. [Hän]
ATM is a connection oriented packet transfer mode using a fixed cell length and is based on asynchronous time division multiplexing. The ATM cell, 53 byte long, consists of a 5 byte header and 48 byte long payload. Cells are routed in the switches according to Virtual Channel Identifiers (VCI) and Virtual Path Identifiers (VPI) located in the header. VP may include a group of VCs. Switching is based on VP or VC/VP. Virtual channel (VC) preserves cell sequence integrity. Cell header is protected with error control but payload is transparently carried through. The ATM cell is depicted in figure 12.
Figure 12. ATM cell structure at the UNI. [I.361]
Signalling and user information are carried on separate virtual channels. Connections are either permanent, semi-permanent or switched. For each ATM cell an ATM switch changes VCI and VPI values in the cell header for the next switch and routes the cell to the right output port.
ATM Adaptation Layer's main purpose is to adjust service requirements from user and available services at the ATM Layer. It maps user information into ATM cells and may provide error detection. ITU has defined 4 service classes according to basic service parameters. Both parameters and service classes are described in table 3. Service classification gives base for the Adaptation Layer definition.
Table 3. Service classes for adaptation. [I.362]
AALs are defined to support each of the service classes. AAL provides a range of bearer connections which are more suitable for the support of UMTS than an ATM connection alone. ATM Adaptation Layers are:
ATM Adaptation Layers are divided into two sublayers:
Service categories have been defined for the ATM Layer to support general applicability of the ATM network. For each purpose they can be specified further by changing parameters. No explicit relation between service category and AAL exists but certain AALs are most suitable for specific service categories. Service categories defined by ATM Forum are:
Transport of ATM Cells Over Radio Interface
The payload of the ATM cells has to be transported over radio interface. This means that in the BTS and possibly in the MT the ATM cells or just payload of them must be mapped to the radio access specific frames and vice versa. Code interleaving on the radio interface makes it impossible to perform mapping directly between ATM cell and a radio frame. This automatically increases the complexity of handover procedure.
Time Transparency and Synchronisation
Delay difference between separate paths in macro diversity mode or during handover must be taken care of as well as delay jitter. Synchronisation between source and destination, and buffering data in the network could be useful here. Besides this, synchronisation is needed between connection elements in macro diversity mode. In CDMA/CODIT, the same data has to be sent at the same time from several BTSs within a time window of 100 ms. Synchronisation management could be performed at the multicasting point in the network or MT could indicate over the radio interface the BTSs to send the right radio frame. In case of ATDMA it is possible to use sequence numbers carried by the AAL header or payload to synchronize macro diversity paths. Synchronisation could be of use between BTSs for quick handovers. This might not be able to be performed by ATM but the synchronization of BTSs is not strictly necessary anyway. [MON110]
Reference Configuration for UMTS/ATM Connections
Reference configuration for ATM connections over UMTS for user and signalling information transfers is depicted in figure 13. In this scenario the VC is terminated in the Radio Access System (RAS). This is because it is most probably that only the payload of the ATM cells are transported over the radio interface in the name of saving radio resources. Due to this interworking is performed for the radio transport in the RAS and MT respectively.
Figure 13. Reference Configuration for UMTS/ATM Connections [SSD]
Transcoding is performed in the Core Network after macro diversity point. Three separate VCs are needed for one path from mobile user to fix user, namely between:
User device and MT/Interworking Unit (IWU).
IWU and Transcoder.
Transcoder and fixed user.
B-ISDN is a broadband service network which is able to integrate all data services for the time being into this one network. It exploits ATM as a transport and switching technique. The B-ISDN network uses the same logical hierarchical protocol architecture as in the OSI model. The B-ISDN protocol reference model consists of three planes: the user plane, the control plane and the management plane. The layers are from down to top: Physical Layer, ATM Layer, ATM Adaptation Layer (AAL) and the higher layer protocols. The B-ISDN protocol reference model is depicted in figure 14.
Figure 14. B-ISDN protocol reference model. [Hän]
The key feature of the B-ISDN is its suitability to support different service types. The most widely applied of them are listed in the table 4.
Table 4. User services in B-ISDN. [MON100]
Services Typical bit rates (kbit/s) Telephony 64 Telefax 64 Data 1- 2000 Videotex 1-64 Video telephony 64-384
B-ISDN User Network Interface (B-UNI)
The B-ISDN signalling transport network is an ATM-based transport network which is logically separated from the user information transport network. The protocol stack for the B-UNI is presented in figure 15.
Figure 15. The B-ISDN signalling protocol stack at the UNI. [SSD]
B-UNI with UMTS requires an extension of Q.2931 signalling protocol (Q.2931*). Q.2931* offers UMTS necessary functionalities e.g.:
support of multipoint and broadcast communication configurations.
adding and removing connections from an existing call.
interworking between different coding systems. [MON65]
ATM Protocol
ATM protocol stack is illustrated in figure 16. It is very similar to the B-ISDN protocol reference model (see figure 14). The B-ISDN signalling is transported in ATM virtual channels logically separated from user data.
Figure 16. ATM protocol stack with sublayers. [I.321]
The Physical Layer consist of two sublayers. The Physical Medium sublayer supports pure transfer medium dependent functions. The Transmission Convergence sublayer converts the ATM cell stream into Physical Medium sublayer. The ATM Layer is independent of the physical medium. It translates the cell identifier for switching, provides the user with one QoS class, to mention a few. The ATM Adaptation Layer adapts the ATM Layer service along the requirements imposed by user services as well as control and management functions.