Page 165 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 151

of the big limitations of GSM/GPRS in most deployed networks today has been resolved.
To transmit several data streams for a user simultaneously, the RNC has to be able to
modify a radio bearer at any time. If a circuit‐switched voice call is added to an already
existing packet‐switched data connection, the RNC modifies the RAB to accommodate
both streams. It is of course also possible to add a packet‐switched data session to an
ongoing circuit‐switched voice call.
Another option for packet‐switched data transfer over the air interface is to send
data to the user via the FACH. For data in the uplink direction, the RACH is used.
This is an interesting concept as the primary role of those channels is to carry
signaling information for radio bearer establishments. As the capacity of those
channels is quite limited and also has to be shared, the use of these common chan-
nels only makes sense for small amounts of data or as a fallback in case a user has
not transmitted any data over a dedicated connection or the high‐speed shared
channel for some time. Another disadvantage of using common channels is that the
mobile device is responsible for the mobility management and therefore, no seam-
less handover to other cells is possible (see Section 3.7.1). Therefore whenever the
network detects that the amount of data transferred to or from a mobile device has
increased again, a dedicated or high‐speed shared connection is quickly
reestablished.
Independent of whether a dedicated, common or shared channel is assigned at the
request of the SGSN during PDP context activation, the bandwidth of the established
connection depends on a number of factors. Important factors are, for example, the
current load of a cell and the reception conditions of the mobile device at its current
location. Furthermore, the number of available spreading codes and the distance of the
mobile device from the Node‐B are also important factors.
The mobile device can also influence the assignment of radio resources during estab-
lishment of a PDP context. By using optional parameters of the ‘at + cgdcont’ command
(see Section 2.9) the application can ask the network to establish a connection for a
certain Quality of Service (QoS) level. The QoS describes properties for a new connec-
tion such as the minimal acceptable datarate or the maximum delay time allowed, which
the network has to guarantee throughout the duration of the connection. It is also pos-
sible to use different APNs to let the network automatically assign the correct QoS
settings to a connection. The HLR therefore stores a QoS profile for each user that
defines which APNs a user is allowed to use and which QoS level a user is allowed to
request for a new connection (see Figure 3.20).
The assignment of resources on the air interface can also be influenced by the service
level assigned to a user. By this process network operators can, for example, allocate
higher maximum datarates to users who pay more for their subscription.
The Iur interface completes the overview of the UTRAN interfaces for this chapter.
This interface connects RNCs with each other to support the soft handover proce-
dure between Node‐Bs that are connected to different RNCs. Further information
about this topic can be found in Section 3.7.1. Furthermore, the Iur interface allows
keeping up a packet‐switched connection that is currently in the Cell‐FACH,
Cell‐PCH or URA‐PCH state if the functionality is used in the network. More infor-
mation about the different connection states can be found in Section 3.5.4. The
protocol responsible for these tasks is called the Radio Network Subsystem
Application Part (RNSAP).