Page 312 - From GMS to LTE

298 From GSM to LTE-Advanced Pro and 5G

Once high‐speed backhaul connectivity is available at a base station site, it is usually
used by all equipment at that site. In many cases, this will be GSM, UMTS and LTE. As
LTE is purely based on IP technology, the backhaul link should preferably offer native IP
connectivity. Older UMTS base stations sometimes still require ATM connectivity,
which can be simulated over IP. The current generation of UMTS digital modules in
base stations is also capable of natively connecting NodeBs over an IP connection to the
RNC. GSM technology continues to be based on an E‐1 timeslot‐based architecture on
the Abis backhaul link. Here, virtualization of E‐1 connections can help to transparently
tunnel the backhaul link over the installed IP connection. This way, all three radio tech-
nologies can be backhauled over a single link. In practice, GSM, UMTS and LTE are
usually integrated into a single multi‐mode digital module today and only a single digi-
tal backhaul module is required, which routes the different traffic types transparently
over a single IP connection.

4.17 IPv6 in Mobile Networks

As in fixed‐line networks a major limitation in mobile networks today is the exhausted
pools of public IPv4 addresses. Most mobile networks today assign private non‐routa-
ble IPv4 addresses to their customers and use Network Address Translation (NAT) to
translate outgoing TCP and UDP connections to fewer but publicly routable IP
addresses used on the Internet. The method used is the same as in DSL, cable or fiber
deployments. One major shortcoming of this approach is that devices behind the
NAT, while being able to make outgoing connections to the Internet, are not reachable
for incoming connection requests. To operate a web server or other service that
requires to be reachable from the Internet, a TCP or UDP port mapping needs to be
configured on the DSL or cable‐router. On mobile devices this is not possible. Another
limitation is that many mobile network operators today have more customers than
the 16.7 million private IP addresses the Class A private IP address range (10.0.0.0)
can accommodate. As a consequence, mobile network operators have to assign the
same private IPv4 address to several customers, which makes communication between
devices in the mobile network impossible as well. While for smartphones this is not a
concern, it is a significant logistical inconvenience to reuse private IP addresses in the
mobile network and it poses a problem for other uses such as web servers and services
connected to the Internet over a cellular network, for Internet of Things (IoT) appli-
cations and for Machine Type Communication (MTC). As a consequence, some
mobile network operators have thus started to offer IPv6 Internet connectivity to
customers in addition to using IPv6 for their Voice over LTE (VoLTE) service, as
described in the next chapter.

4.17.1 IPv6 Prefix and Interface Identifiers
Unlike in IPv4 where IP addresses have a length of 32 bits, the network does not assign
an IPv6 address to a mobile device. In IPv6 a router advertises an IPv6 prefix and each
device receiving an advertising packet can choose its own interface identifiers, which
serve as the second part of a full 128‐bit IPv6 address. This is standard IPv6 procedure
and is not specific to mobile networks.