Page 322 - From GMS to LTE

308 From GSM to LTE-Advanced Pro and 5G

very efficiently as the CPUs are never idle but are always put to good use by some of the
virtual machines at any point in time.
Virtual machines can also move between different physical servers while they are
running. This is convenient, for example, in cases when a physical server becomes over-
loaded due to several virtual machines suddenly increasing their workload. When that
happens, less CPU capacity is available per virtual machine, and capacity may have been
guaranteed by the cloud provider. Moving a running virtual machine from one physical
item of hardware to another is done by copying the contents of the RAM currently used
by the virtual machine on one physical server to a virtual machine instance on another.
As the virtual machine is still running while its RAM is being copied, some parts of the
RAM that were already copied will be changed so the hypervisor has to keep track of
this and recopy those areas. At some point the virtual machine is stopped and the
remaining RAM that is still different is copied to the virtual machine on the target
server. Once that is done, the state of the virtual machine, such as CPU registers and the
state of the simulated hardware, is also copied. At the end of this procedure there is an
exact copy of the virtual machine on the target server. The hypervisor then resumes the
operating system in the cloned virtual machine, which then continues to execute from
exactly the point where it was stopped on the original server. Obviously it is important
to keep the cut‐over time as short as possible. In practice, values in the order of a frac-
tion of a second can be reached. Moving virtual machines from one physical server to
another can also be used in other load‐balancing scenarios and also for moving all vir-
tual machines running on a physical server to another so that the machine can be pow-
ered down for maintenance or replacement.

4.18.7 Managing Virtual Machines in the Cloud
Another aspect that needs to be discussed before moving forward is how to manage
virtual resources. On a desktop or notebook PC, hypervisors such as Virtualbox come
with their own graphical administration interface for starting, stopping, creating and
configuring virtual machines. A somewhat different approach is required when using
virtual resources in a remote data center. Amazon Web Services, Google, Microsoft,
Rackspace and many others offer a web‐based administration console for the virtual
machines they offer. Getting started is as simple as registering for an account and select-
ing a preconfigured virtual machine image with a base operating system (such as
Ubuntu Linux, Windows, etc.) with a certain amount of RAM and storage. Once this is
done, a single click launches the instance and the server is ready for the administrator
to install the software they would like to use. While Amazon and others use a proprie-
tary web interface, others such as Rackspace use OpenStack, an open‐source alterna-
tive. OpenStack is also ideal for companies to manage virtual resources in their own
physical data centers.

4.18.8 Network Function Virtualization
Having discussed the different properties of virtualization this section now focuses on
Network Function Virtualization (NFV) and starts with a practical example. Voice over
LTE (VoLTE) requires a number of logical network elements, referred to as Call Session
Control Functions (CSCF), that are part of the IP Multimedia Subsystem (IMS). These
network functions are usually shipped together with server hardware from network