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UMTS (Insight into Uu access stratum) - 12

Insight into Uu access stratum [Under UMTS]
» Radio interface layer - 25.301 «
» MAC - 25.321 «
» RLC - 25.322 «
» PDCP - 25.323 «
» BMC - 25.324 «
» RRC - 25.331 «
» Phy layer services - 25.302 «

GSM systems always stressed on channel based picture of radio access. First, logical channels were visualised to do certain logically separated functions e.g. Broadcast Control CHannel (BCCH) for broadcasting system information (radio parameters, network information etc.), Signaling Dedicated CHannel (SDCH) for dedicated signaling connection etc. And then later these logical channels were mapped to physical channels. Continuing on same lines, UMTS introduced a concept of "transport channels". Immediate question to ask would be "why do we need transport channels ?".

Let us take an example of traffic channel. As we know, traffic channel is used for transmission of user data like voice data for voice call, packet data for internet connection. Logical channel required for user data transmission in both services is still "Traffic channel". But for transporting these data, we need different types of transport: dedicated type of transport for voice data traffic, shared/packet type of transport for packet data. UMTS did these split and introduced abstraction of transport channels. MAC - who is in charge of medium access - is the right entity to provide this abstraction i.e. to map logical channel requirements into transport channel requirements. Physical layer now has better abstraction to work on, that is of transport channels. Having separations between logical, transport, and physical representations makes things bit difficult to understand as whole, but it helps in future if we want to change channel mapping at each layer. Say we want to introduce a new service which require different type of transport for user data transmission than the existing ones. Logical channel that is to be used for user data transmission can still be "traffic channel". MAC will map it to appropriate transport channel. PHY will provide appropriate physical channel for new transport channel (possibly it will multiplex it in existing physical channel) without affecting MAC as far as it satisfies MAC requirements for the transport channels.

What does RLC do ? RLC does not change logical channel abstraction. It provides "link modes" with which channel is to be operated. The modes are Transparent mode, Ack mode, and Unack mode. All modes need not be applicable to all logical channels. For example, system information sent over BCCH need not be touched by RLC, so only transparent mode is required for BCCH. But for traffic channel, all modes are possible (e.g. transparent mode for voice data, ack mode for internet packet data, unack mode for streaming data required for streaming services).

RLC/MAC/PHY have been assigned certain jobs, the job orders for which come from RRC. The responsibility of configuration and control of RLC/MAC/PHY is with RRC. For example, which transport MAC should, what radio parameter values PHY to use, these instructions and values come from RRC. Control (shown in diagram from earlier article) existed even for GSM, but was first put in specification by UMTS.

GSM communication is a two step process: First is signaling phase (wherein signaling connection is established and signaling message exchange happen) and next phase is transmission phase (wherein as per signaling message exchange transmission connection is established and transmission start). Same is the case for UMTS.

For upper layers in control plane, RRC provide an abstraction of signaling connection called "RRC connection". Upper layer use this connection for their signaling message exchange (refer article on signaling briefly). RRC itself uses special predefined radio bearers called signaling radio bearers (SRBs) for signaling message exchange. SRBs are created during RRC connection establishment. Note that unlike radio bearers in user plane, SRBs are predefined i.e. each SRB is meant for certain purpose (based on messages - NAS or RRC, high priority or low etc.).

Bearer management of radio bearers is done by RRC. At certain point in signaling (refer diagram from article on signaling briefly), RNC initiates radio bearer setup. RRC does the duty of configuring PDCP/RLC/MAC/PHY for the radio bearer (as per approve QoS) at this point.

As mentioned above, RRC also controls user plane entities, PDCP and BMC. PDCP is meant for packet switched data. BMC is for broadcast data in downlink direction. If you notice in diagram from earlier article, one of the radio bearers directly goes to RLC and is not connected to PDCP or BMC. This is meant for circuit switched data (the radio bearer would possibly be operated in transparent or unack mode).

In next article, we will look at the big channel mapping diagram.

References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.

Copyright © Samir Amberkar 2010§

Radio access layers briefly « UMTS Index » Channel mapping diagram