Page 324 - From GMS to LTE

310 From GSM to LTE-Advanced Pro and 5G

dedicated and non‐standardized hardware and a CSCF runs on another expensive non‐
standardized server and both run only at a fraction of their total capacity. In practice the
concept is somewhat more complex due to logical and physical redundancy concepts
needed to make sure there are as few outages as possible. This increases the number of
CSCF and MME instances running simultaneously. However, the concept of mixing
and matching virtualized network functions on the same hardware allows scaling and
can also be used for much more complex scenarios perhaps with even more benefits
compared to the simple scenario just described.

4.18.9 Virtualizing Routers
In addition to network functions that are purely concerned with signaling, such as
MMEs and CSCFs, networks contain many physical routers that analyze incoming IP
packets and make decisions as to which interface they should be forwarded on and if
they should be modified before being sent out again. A practical example is the LTE
Serving Gateway (SGW) and the Packet Data Network Gateway (PDN‐GW), which are
instructed by the MME to establish, maintain and modify tunnels between a moving
subscriber and the Internet to hide the user’s mobility from the Internet. To make rout-
ers as fast as possible, parts of this decision‐making process are not implemented in
software but as part of dedicated hardware (ASICs). Thus, virtualizing routing equip-
ment is a challenge because routing can no longer be performed in hardware but must
be done in software running in a virtual machine. This means that apart from making
the routing decision process as efficient as possible, it is also important that forwarding
IP packets from a physical network interface to a virtual machine and then sending
them out again altered or unaltered over another virtual network interface to another
physical network interface must incur as little overhead as possible. In practice, several
companies are working on such solutions. Intel, for example, offers its Data Plane
Development Kit (DPDK) and Single‐Root IO‐Virtualization (SR‐IOV) solutions to
address the issue.

4.18.10 Software‐Defined Networking
Software‐Defined Networking (SDN) is a term that is often used in combination
with Network Function Virtualization but is an entirely different topic. Getting IP
packets from one side of the Internet to the other requires routers. Each router
between the source and destination of an IP packet looks at the packet header and
makes a decision as to which outgoing network interface to forward it to. This pro-
cess starts in the DSL/Wi‐Fi/cable‐router, which looks at each IP packet sent from a
computer in the home network and decides whether to forward it over the DSL link
to the network. Routers in the wide area network usually have more than one net-
work interface, so here, the routing decision, i.e. to which network port a packet
should be forwarded, is more complex. This is done by using routing tables that
contain IP address ranges and corresponding outgoing network interfaces. Routing
tables are not static but change dynamically, e.g. when network interfaces suddenly
become unavailable, e.g. due to a fault or because new routes to a destination become
available. Even more often, routing tables change because subnets on other parts of
the Internet are added and deleted. There are a number of network protocols, such
as BGP (Border Gateway Protocol), that are used by routers to exchange information