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the IP address and GTP tunnel end point identi¬er (TEID) of the SGSN, and therefore the


BTS/UE RNC CN



RAB Assignment Request

Radio link and radio bearer
establishment procedures
RAB Assignment Response

Bearer establishment (CS only)




Figure 6.110 RAB assignment procedure
6.19 UMTS TERRESTRIAL RADIO ACCESS NETWORK (UTRAN) 385


Table 6.36 RAB assignment request elements
Information element Description
RAB ID Unique identi¬er for the RAB across the Iu interface
RAB parameters See Table 6.37
User plane info Mode Indicates either transparent or support for prede¬ned SDUs
Versions Indicates the UP version, currently 1
Transport layer info Address Although not interpreted by RANAP, in practice for the
CS-CN this will be the AAL2 ATM address of the CN
element, and for PS-CN, the IP address
Association This will either be a binding ID for use with AAL2
signalling for CS-CN connection, or a GTP TEID for the
PS-CN

Iu-PS does not require any ALCAP signalling since this is enough information to create
the GTP tunnel.
The RAB parameters are listed in Table 6.37.
For the residual bit error rate, the delivery of erroneous SDUs parameter is speci¬ed.
Its values can be:

• Yes: error detection is used and SDUs with errors are delivered
• No: error detection is used and SDUs with errors are discarded
• No-error-detection-consideration: SDUs are delivered, and no error detection is per-
formed.

The use of this for frame classi¬cation is described in Section 6.18.4.2.
Table 6.37 RAB parameters
Parameter Description
Traf¬c class This will be one of conversational, streaming, interactive and
background
RAB asymmetry indicator Indicates whether this is symmetric or asymmetric,
bidirectional or unidirectional (and whether uplink or
downlink)
Maximum bit rate A maximum rate for this RAB in bits/second in the range
1 bps to 16 Mbps
Guaranteed bit rate For conversational and streaming classes, a minimum
guaranteed bit rate is speci¬ed
Delivery order Indicates if the SDUs need to be delivered in order or not
Maximum SDU size Indicates how big, in bits, an SDU can be
SDU parameters See Table 6.38
Transfer delay For conversational and streaming classes, indicates the
maximum tolerable transfer delay (in ms)
Traf¬c handling priority For the interactive class, speci¬es a priority for the SDU. The
priority range is 1 to 14 with 1 being highest priority. A
value of 15 indicates no priority
Source statistics descriptor For conversational and streaming classes, indicates whether the
RAB contains speech or unknown traf¬c
386 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


Table 6.38 SDU parameters
Parameter Description
SDU error ratio The fraction of SDUs that may be either lost or contain errors. It
is expressed in terms of a mantissa and exponent where the
rate is given by mantissa — 10’exponent
Residual bit error ratea Indicates the tolerated BER for each sub¬‚ow, expressed as
above. It also provides an indication of how errors should be
dealt with, as described in the text
SDU format informationa Provides details of the size, in bits, of each sub¬‚ow combination
possible
a
Expressed for each sub¬‚ow.


As an example, Figure 6.111 shows an abridged trace sample of a RANAP RAB
assignment request from the CS-CN to the RNC. In this example, a circuit switched call
is being established and a RAB for AMR 12.2 kbps (max bit rate: 12 200). For 12.2 kbps
rate, the SDU size is 244 bits, and there are three sub¬‚ows. The RAB has a drop rate
of 7 — 10’3 SDUs (i.e. BLER), and the sub¬‚ows have the format shown in Table 6.39.
The message also speci¬es a 20-byte NSAP address (see Section 7.13), which identi¬es
the core network element originating the signalling message. This and the binding ID are
used in the AAL2 signalling to establish the actual transport layer connection.
On receipt of an assignment request for a circuit switched connection, the RNC will
invoke the transport layer signalling (AAL2 signalling) to establish the AAL2 transport
bearer for the RAB. In the case of a packet connection, all the necessary information is
within the assignment request. To establish, the RNC will also apply its internal arbitration
to determine if it can meet the request, and what it is in a position to allocate. Once
successfully completed, the RNC will reply with a RAB assignment response message.
This generally only contains the RAB ID to relate it to the request message.
The RAB assignment request is also used by the CN to modify and release a RAB. In
the case of a release, the message will contain the RAB ID of the RAB to be released,
and a cause for the release, such as UE release, or successful relocation. The RNC may
also release a RAB using the RAB release message. Again, this will indicate the particular
RAB to release and the cause.
If there are no longer any RABs established across the Iu interface for a particular user,
the core may initiate the Iu release command. On receipt, the RNC will clear the resources
allocated for this connection and reply with an Iu release complete (Figure 6.112). Only
the CN can release the Iu connection. However, the RNC can request this release through
the Iu release request. If the CN agrees, it will then invoke the Iu release.


6.19.6.3 RANAP paging message
When the core network wishes to page the user, it will send a paging message on the
Iu interface to the RNC covering the user™s location/routing area. If the location/routing
area covers more than one RNC, the core will repeat the paging messages to the relevant
RNCs. The paging message will contain at least the CN domain identi¬er (CS or PS) and
the permanent NAS UE identity, which is the IMSI (or TMSI/P-TMSI) number. The RNC
6.19 UMTS TERRESTRIAL RADIO ACCESS NETWORK (UTRAN) 387


RANAP-PDU
RAB-AssignmentRequest
protocolIEs
RAB-SetupOrModifyList
- rAB-ID: '00000001'B
rAB-Parameters
- trafficClass: conversational
- rAB-AsymmetryIndicator: symmetric-bidirectional
maxBitrate
- MaxBitrate: 12200
guaranteedBitRate
- GuaranteedBitrate: 12200
- deliveryOrder: delivery-order-requested
- maxSDU-Size: 244
sDU-Parameters
sDU-ErrorRatio
- mantissa: 7
- exponent: 3
residualBitErrorRatio
- mantissa: 1
- exponent: 6
- deliveryOfErroneousSDU: yes
sDU-FormatInformationParameters
- subflowSDU-Size: 81
- subflowSDU-Size: 39
residualBitErrorRatio
- mantissa: 1
- exponent: 3
- deliveryOfErroneousSDU: no-error-detection-consideration
sDU-FormatInformationParameters
- subflowSDU-Size: 103
- subflowSDU-Size: 0
residualBitErrorRatio
- mantissa: 5
- exponent: 3
- deliveryOfErroneousSDU: no-error-detection-consideration
sDU-FormatInformationParameters
- subflowSDU-Size: 60
- subflowSDU-Size: 0
- transferDelay: 80
- sourceStatisticsDescriptor: speech
userPlaneInformation
- userPlaneMode: support-mode-for-predefined-SDU-sizes
- uP-ModeVersions: '0000000000000001'B
transportLayerInformation
- transportLayerAddress:
'45 00 00 00 00
00 65 96 8F 00
00 00 00 00 00
00 00 00 00 00' H
iuTransportAssociation
- bindingID: '0100954F'H


Figure 6.111 RANAP assignment request trace. Reproduced by permission of NetHawk Oyj

Table 6.39 RAB sub¬‚ows
Sub¬‚ow 1 Sub¬‚ow 2 Sub¬‚ow 3 Rate (kbps)
No. of bits 81 103 60 12.2
39 0 0 1.8
10’6 10’3 5 — 10’3
BER
388 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM



RNC CN



Iu Release Command


Iu Release Complete




Figure 6.112 Iu release

will decide how to page the user across the UTRAN depending on the RRC connection
status of the UE.


6.19.6.4 Relocation
The relocation procedures are invoked when it is necessary to perform an SRNC reloca-
tion, which is moving the UE connection to the core network from one RNC to another
as a result of user movement. This situation arises when a user is no longer connected
to any BTSs that are under the control of its SRNC. The relocation procedure for this is
shown in Figure 6.113.
First, the RNC informs the CN that a relocation is necessary using the relocation
required message. This will contain identi¬ers for the source (SRNC) and target RNCs.
On receipt, the CN will send a relocation request, which is much the same as a RAB setup
request, to the target RNC, which will then perform transport layer setup and respond with
the relocation request acknowledge. The CN will then send a relocation command to the
source RNC to release its RABs. The source and target RNC will use RNSAP signalling
between them to commit this relocation, and, once successful, the target will respond
with the relocation complete message. The CN will then call the Iu release procedure as
described above.
This relocation procedure is also used for inter-system handover from UTRAN to GSM.
In this case, the relocation required message will contain GSM-related parameters.


6.19.6.5 RANAP security control
Another important feature is the security mode procedures. In this, security information
is passed from the core network to the RNC to protect both data and signalling messages
across the UTRAN. The CN issues a security mode command, which contains ciphering,
and integrity information. This information is a list of available algorithms being used
for ciphering and integrity, plus the keys for each that have been generated during the
UE-CN authentication procedure. The RNC will choose an algorithm to use for each, and
inform the CN of this choice using the security mode complete message (Figure 6.114).
Subsequent data and signalling messages in UTRAN are now protected.
6.19 UMTS TERRESTRIAL RADIO ACCESS NETWORK (UTRAN) 389



Target
SRNC MSC/SGSN
RNC


Relocation Required

Relocation Request


transport bearer setup (MSC only)

Relocation Request Ack

Relocation Command
RNSAP: Relocation Commit
Relocation Complete

Iu Release Command


transport bearer release (MSC only)

Iu Release Complete



Figure 6.113 Serving RNC relocation



RNC CN

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