Page 161 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 147

For the exchange of control and configuration messages on the Iub interface, the
Node‐B Application Part (NBAP) is used between the RNC and the Node‐B. It has the
following tasks:
cell configuration;
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common channel management;
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dedicated channel management such as the establishment of a new connection to a
●
subscriber;
forwarding of signal and interference measurement values of common and dedicated
●
channels to the RNC;
control of the compressed mode, which is further explained in Section 3.7.1.
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User data is exchanged between the RNC and Node‐Bs via the Frame Protocol (FP),
which has been standardized for dedicated channels in 3GPP 25.427 [6]. The FP is respon-
sible for the correct transmission and reception of user data over the Iub interface and
transports user data frames in a format that the Node‐B can directly transform into a Uu
(air interface) frame. This is done by evaluating the Traffic Format Identifier (TFI) which
is part of every FP frame. The TFI, among other things, instructs the Node‐B to use a
certain frame length (e.g. 10 milliseconds) and which channel coding algorithm to apply.
The FP is also used for synchronization of the user data connection between the RNC
and the Node‐B. This is especially important for data transfer in the downlink direction,
as the Node‐B has to send an air interface frame every 10, 20, 40 or 80 milliseconds to
the mobile device. In order not to waste resources on the air interface and to minimize the
delay it is necessary that all Iub frames arrive at the Node‐B in time. To ensure this, the
RNC and Node‐B exchange synchronization information at the setup of each connection
and also when synchronization of a channel has been lost.
Finally, FP frames are also used to forward quality estimates from the Node‐B to the
RNC. These help the RNC during the soft handover state of a dedicated connection to
decide which Node‐B has delivered the best data frame for the connection. This topic is
further discussed in Section 3.7.1.

3.5.2 The RNC, Iu, Iub and Iur Interfaces, RANAP and RNSAP
The heart of the UMTS radio network is the RNC. As can be seen in Figures 3.18
and 3.19, all interfaces of the radio network are terminated by the RNC.
In the direction of the mobile subscriber the Iub interface is used to connect several
hundred Node‐Bs to an RNC. During the first years after the initial deployment of
UMTS networks, most Node‐Bs were connected to the RNC via 2 Mbit/s E‐1 connec-
tions either via fixed‐line or microwave links. The number of links used per Node‐B
mainly depended on the number of sectors and of frequencies used. Today, most sites
are connected to the RNC via high‐speed IP‐based links over fiber or Ethernet micro-
wave, with backhaul speeds of several hundred megabits, if required.
An initial disadvantage of IP‐based links was that the transport protocol used on
them, for example, Ethernet, was not synchronous and therefore could not be used by
the base stations to synchronize themselves with the rest of the network. As a conse-
quence, protocol extensions had to be developed to enable recovery of a very precise
clock signal from such links before they could be relied on as the only means to connect
Node‐Bs to the RNCs.