Radio access layers briefly [Under
UMTS]
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Radio interface layer - 25.301 «
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MAC - 25.321 «
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RLC - 25.322 «
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PDCP - 25.323 «
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BMC - 25.324 «
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RRC - 25.331 «
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Phy layer services - 25.302 «
Let us have a quick look at the way various Radio access functions have been organised in layers.
Below diagram shows the various entities of access layers in UE (RNS access layers will be similar).
Phy layer access is based on WCDMA. It uses Scrambling codes and Channelisation codes. Scrambling codes (PN sequences) separate out UEs and Node Bs. Channelisation codes separate out channels. Channelisation codes are orthogonal in nature and spreads the signal with 3.84 Mcps chip rate. Scrambling codes are applied after Channelisation codes. Scrambling codes are useful in Asynchronous operation. Location of UE changes and also location differs for each UE, so Asynchronous operation is more suitable. Modulation techniques used are QPSK and 16QAM for HS (high speed) operations. We will see more about it in later articles on Phy layer.
Phy layer provide an abstraction of "Transport channels" to MAC, which in turn provide an abstraction of "Logical channels" to RLC. If you recall, GSM too have Logical channels and Physical channels (e..g. FACCH and TCH are different logical channels, but actually same physical channel). MAC layer has entities to handle broadcast channels (MAC-b), high speed channels (MAC-hs, added in R5), control and shared channels (MAC c-sh), and dedicated channels (MAC-d).
RLC along with PDCP and BMC provide an abstraction of Signaling radio bearers (SRB) and Radio bearers (RB) to L3. In User plane, RLC provide generic modes of transmission (like Transparent, Ack, and Unack). Unlike GPRS, RLC and MAC are better separated..
PDCP (Packet Data Convergence Protocol) is equivalent to SNDCP for GPRS, taking care of functions compression/decompression etc.
BMC (Broadcast/Multicast Control protocol) takes care of broadcast and multicast services. BMC messages could be cell broadcast SMS or multimedia messages. Till R6, only cell broadcast SMS messages are supported. R6 makes a major addition with MBMS (Multimedia Broadcast Multicast Services).
RRC bridge the gap between Layer 2 and NAS entities. It gives an abstraction of RRC "connection", required for NAS entities to operate. Overall control of AS entities (cell selection, configuration of radio bearers, etc.) comes from RRC. RRC has functional entities for broadcast (BCFE), paging/notification (PNFE), dedicated control (DCFE), shared control (SCFE).
Above diagram also shows the control connections ending on RRC; these are used for configuring the layers etc.
References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.
| Copyright © Samir Amberkar 2010 | § § § |
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