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DRNC
Node B
UE SRNC

Figure 6.42 Radio network control plane protocol stack


control (RRC) protocol. RRC is the signalling protocol operating between the user and the
RNC, and is responsible for controlling the lower RLC, MAC and physical layers. At the
MAC layer, the signalling control and user data streams are combined together to present
a transport channel to the physical layer. Notice that the protocol stack is very similar
to the previous user plane stack and that the transport of this control information to the
SRNC is also carried over the frame protocol (FP) and ATM adaptation layer 2 (AAL2).
For UMTS, the physical layer is considered in two stages: across the air interface,
Uu, it consists of the WCDMA mechanisms, and across the Iub interface, it consists
of the ATM transport and its respective underlying physical layers, such as SDH/¬bre,
etc. Figure 6.43, shows this architectural model. Across this transport network, an FP is
added in between the ATM and upper layer service data unit (SDU). The purpose of this
is to carry additional information, such as quality achieved on the air interface and user
identi¬cation on common channels.
The following subsections will assume that data is passing down through the protocol
stacks and will discuss each in that order. Layer 2 consists of the MAC, RLC, PDCP and
6.12 RADIO INTERFACE PROTOCOL ARCHITECTURE 311


Control User
Plane Plane

example service




Layer 3
Radio Resource voice cell
Control eMail call broadcast

radio bearers
signalling
radio bearers
PDCP

BMC




Layer 2
Radio Link Control

Logical Channels

Media Access Control

Transport Channels




Layer 1
Physical Layer




Figure 6.43 Radio interface protocols


BMC protocols, and is responsible for the mapping of data onto layer 1 across the transport
channels. To understand this mapping, some general concepts about data transport must
be de¬ned. All transport channels are considered to be unidirectional and a UE can have
simultaneously one or more transport channels in the downlink, and in the uplink.



6.12.1 Broadcast/multicast control (BMC)
The BMC protocol is used for scheduling and transmission of cell broadcast information
to the UE. It is only used in the downlink direction, using the CTCH logical channel
transported over the FACH. For R99, only the broadcast function is supported, with mul-
ticasting to be introduced in later releases. It is designed to send text broadcast messages
over a certain geographical area, containing relevant information. Its service is similar
in concept to the teletext system for television. Messages are limited to 1200 bytes, and
currently SMS cell broadcast is the only supported service. There is work in progress at
the 3GPP to expand the scope of this service to deliver richer content such as multimedia.
The introduction of multicasting would also enhance the service considerably, opening
up the possibility of provision of services such as selective advertising to a certain group
of subscribers in a particular area.
The protocol stack for cell broadcast is shown in Figure 6.44 below. An IP-based
cell broadcast centre (CBC) is located in the operator network. This is connected to the
312 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM



CBS CBS

BMC
BMC SABP SABP

RLC
RLC TCP TCP
Iu-BC
MAC
MAC IP IP

AAL5 AAL5
L1
L1
ATM ATM
UE RNC CBC

Figure 6.44 Cell broadcast service


RNC via the Iu-BC and broadcast messages are transported by the service area broadcast
protocol (SABP). At the RNC, these are transferred to the BMC protocol and distributed
on the FACH to all cells in the broadcast area.



6.12.2 Packet data convergence protocol (PDCP)

The PDCP maps the network-level protocol such as IP (v4 and v6) or PPP onto the under-
lying network. It is only used on the packet switched network and only for user data. The
main role of PDCP is to support header compression of upper-layer headers, for example
TCP/IP and RTP/UDP/IP compression. Headers associated with IP are only relevant at
the UE and RNC, and therefore only add overhead across the Uu and Iub/Iur interfaces.
Consider a TCP/IP packet; the header accounts for 40 bytes. Sequences of IP packets
tend to have a considerable amount of static information in headers. By way of example,
consider sending a ¬le, which is broken up into many segments. Each segment will have a
header containing the same IP source and destination address. Header compression takes
advantage of this to substantially reduce the headers. A TCP/IP header can typically be
reduced to 3“4 bytes. Currently, 3GPP de¬nes only one header compression technique,
which is ˜IP Header Compression™ as de¬ned in RFC 2507. Each packet switched RAB
is associated with its own PDCP.



6.12.3 Radio link control (RLC)
The RLC layer provides three types of data transfer to the higher layers.

Transparent data transfer
This exchanges packets with the higher layers without adding any protocol information.
The encryption of user data for transparent mode is performed at the MAC layer. The
functions provided by transparent mode, and the protocol data unit (PDU) format, are
6.12 RADIO INTERFACE PROTOCOL ARCHITECTURE 313


byte 1
Mode Functions provided
Segmentation and reassembly
Data
Transparent User data transfer
(TR)
byte n
Discard of errored SDU

Figure 6.45 RLC transparent mode PDU


shown in Figure 6.45. Since no overhead is added, the format of the PDU is very simple,
consisting only of the data from the upper layer.
For all types, error detection is by means of the physical layer CRC check, details of
which are passed by the physical layer to the MAC and then on to the RLC.

Unacknowledged data transfer
This passes packets onward without ensuring delivery; packets are passed without ac-
knowledgement. The RLC can segment data into an appropriate size for transmission, and
reassemble it at the far end. Should the data be too short, it can be padded or concatenated
to a valid length. The functions offered and the format of the unacknowledged mode PDU
are shown in Figure 6.46. It contains a 7-bit sequence number to allow the segments to
be reassembled. An extension bit (E) of 1 indicates that the next ¬eld will be a length
indicator (LI). If E = 0, then data follows.
The LI points to where the SDU received by the RLC layer from the upper layer
¬nishes within this PDU. It can be either 7 or 15 bits long, depending on the maximum
length of the PDU, as de¬ned in the upper-layer signalling. If the RLC size is greater
than 125 bytes, the 15-bit LI is used. Generally, the RLC PDU size will be the transport
block size (see later) less the MAC layer overhead. The LI ¬eld has some reserved values
which are used to inform the receiver of such occurrences as the ¬rst part of an SDU, the
last part of an SDU and padding. These are needed for the segmentation, concatenation
and reassembly functions. For the 7-bit LI, these are shown in Table 6.10.
By way of example, consider that an RLC SDU needed to be segmented into three
RLC PDUs. They would be arranged as shown in Figure 6.47. The ¬rst PDU indicates in
the LI that the payload is the start of an SDU. The second has no LI as it is a continuation
of the data, and the third has two LI ¬elds. The ¬rst shows where the SDU ¬nishes in

Mode Functions provided E byte 1
SEQ No.
Segmentation and reassembly E
LI
User data transfer
...




Discard of errored SDU
Unacknowledged E
LI
Concatenation
Mode (UM)
Padding
Data
Ciphering
Sequence number checking
byte n
PAD

Figure 6.46 RLC unacknowledged mode data PDU
314 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


Table 6.10 Length indicator ¬eld for an RLC UM PDU
Length indicator Description
0x00 The last received PDU did not have a length indicator but contained the
last part of an SDU
0x7C The data in the payload is the start of an SDU
0x7F The remainder of the PDU is padding


RLC PDUs
RLC SDU


1 Seq. no plus E=1
0x31
0 Indicates start of SDU
0x7C


data




0 Seq. no plus E=0
0x32
No LI since
continuation of SDU
data
data




1 Seq. no plus E=1
0x33
Indicates no. of
0x1E 1 bytes to end of SDU
0x7F 0 Indicates rest of
payload is padding
data

Padding

Figure 6.47 RLC UM data segmentation

the payload (in this case 0x1E bytes down), and the second indicates that the rest of
the payload is padding. This would be as opposed to the ¬rst part of the next SDU for
concatenation.

Acknowledged data transfer
This passes packets onward and guarantees delivery of the packets. It ensures that dupli-
cate packets are discarded and that packets in error are retransmitted using an automatic
repeat request (ARQ) mechanism. If delivery is not possible then an error message is send
to the sender to notify it that there was a problem. Both in-sequence and out-of-sequence
delivery modes are supported, since in many cases the upper-layer protocols are capable
of restoring the correct order of the packets if required. It should be noted that there is
a single RLC connection for each radio bearer. The functions provided by acknowledged
6.12 RADIO INTERFACE PROTOCOL ARCHITECTURE 315

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