UMTS Networks : Architecture, Mobility and Services

326 UMTS Networks
reconfiguration and release of other radio bearers for user-plane traffic may be executed
by exchanging commands and status information between peer RRC entities over
signalling radio bearers. RRC connection will continue to exist until all user-plane
bearers have been released and the RRC connection between the UE and RNC is
explicitly released.
The security mechanisms applied to radio bearers are activated and deactivated
under the control of the RRC protocol. Besides activation of confidentiality protection
by ciphering, the RRC protocol can also guarantee the integrity of all higher layer
signalling messages as well as most of its own signalling messages. This property, which
is discussed more thoroughly in Chapter 9, is achieved by attaching to every RRC
PDU a 32-bit message authentication code, which is calculated and verified by the
RRC entities themselves. This ensures that the receiving RRC entity can verify that
signalling data have not been modified and that the data genuinely originated from the
claimed peer entity. In addition, the operation to change the keys used by these functions
is performed by the RRC protocol under instructions that originally come from
the CN.
UE MM at the UTRAN level is controlled by RRC signalling. Such MM functions
executed by the RRC protocol are cell update, URA update and active set update, all of
which were discussed in Chapter 5 and some examples of which will be shown in
Chapter 11.
Control and reporting of radio measurements is taken care of by the RRC protocol.
A total of seven different categories of measurements can be activated at the UE,
among them measurement of UE location. Each measurement can be controlled and
reported independently by means of further measurements.
Besides the above-mentioned tasks the RRC protocol also controls the broadcast of
system information and paging of UEs and exchange parameters for power control
purposes.
As a protocol, RRC is fairly complex: altogether it has some 40 different procedures
and more than 60 different kinds of PDU.
The RRC protocol is specified in 3GPP specification TS 25.331.
10.4.2 Radio Network User Plane
10.4.2.1 PDCP—Packet Data Convergence Protocol
As its name suggests PDCP is designed to make WCDMA radio protocols suitable for
carrying the most common user-to-user packet data protocol, TCP/IP. As shown in
Figure 10.26, PDCP entities can be found at both ends of the WCDMA radio interface:
the RNC and the UE. The key functionality of PDCP is its ability to compress the
headers of payload protocols, which—if sent without compression—would waste the
invaluable radio link capacity (e.g., without header compression the header size for an
RTP/UPD/IP header is at least 40 bytes for IPv4 and at least 60 bytes for IPv6. Since
payload size in such services as IP voice is about 20 bytes or less, the overhead without
compression is obvious.
In the 3GPP radio interface protocol model, PDCP belongs to the radio link layer