Page 240 - From GMS to LTE

226 From GSM to LTE-Advanced Pro and 5G

the ID of the current serving MSC so that incoming circuit‐switched calls and SMS
●
messages can be routed correctly;
the ID of the SGSN or MME, which is used in case the user’s HSS profile is updated
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to push the changes to those network elements.

4.2.7 Billing, Prepaid and Quality of Service
The network nodes and interfaces described in the previous sections are the main com-
ponents required to offer wireless connectivity to the user. In addition, several other
supporting network components and interfaces are usually deployed in practice to
complement the network with additional services.
To charge mobile subscribers for their use of the system, billing records are created,
for example, on the MME. These are collected and sent to a charging system, which
once a month generates an invoice that is then sent to the customer. This is also referred
to as offline billing or postpaid billing.
Another billing method is online charging, which lets subscribers buy vouchers for
certain services or for a certain amount of data to be transferred within a certain time.
This is also referred to as prepaid billing and originally became popular for circuit‐
switched voice calls. In UMTS and LTE, network operators can offer prepaid billing and
usage tracking in real‐time or near real‐time, which requires interaction with core net-
work components such as the MME, S‐GW or PDN‐GW. As such services are not
standardized, user interaction depends on the particular implementation. A popular
implementation is to offer ‘landing pages’ that are automatically displayed to the user
once they connect to the network for session‐based billing or once the amount of data
has been used up. Through the landing page, the user can subsequently buy additional
credit or enter an ID from a voucher that was previously bought in a shop.
Usage tracking for billing purposes is often also used for postpaid subscribers who are
charged on a monthly basis and who have subscribed to an Internet option, which is
throttled to a very low speed once a subscribed data bucket has been used up. This
requires a function in the network that can monitor the data usage of the subscriber and
a device that can limit their datarate once they have used up their monthly data volume.
Some network operators also offer the option to postpaid subscribers to buy additional
data volume which is then invoiced via their monthly bill. Typically, such systems are
located behind the SGi interface.
For real‐time applications such as Voice over Internet Protocol over LTE (VoLTE),
ensuring a constant delay and a minimal bandwidth on all interfaces within the LTE
network are crucial during times of high traffic load. This can be done via a stand-
ardized QoS node, the Policy and Charging Rules Function (PCRF). Applications can
use the standardized Rx interface to request a certain QoS profile for a data flow. The
PCRF then translates this request and sends commands to the PDN‐GW and the
S‐GW, which in turn enforce the QoS request in the core and access network.
The PCRF is part of the 3GPP IMS specifications and was originally intended for use
by IMS services. In practice, it can also be used by other network operator‐deployed
services, e.g. as another way to rate limit Internet connectivity depending on the
tariff of the subscriber. It is important to note that only network operator services
can access the PCRF. Internet‐based services have no means of requesting any kind
of QoS from the LTE network.