Page 321 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 307

operating system requests the current time from a network‐based time server or from
the local real‐time hardware clock, there is a large gap between this time and the system’s
software clock. This is because when the snapshot is restored, the system’s software
clock still contains the value of the time when the snapshot was taken.

4.18.5 Cloning a Virtual Machine
Another interesting feature of virtualization is the ability to easily clone a virtual
machine, i.e. to make an exact copy of it. This is done by copying the hard disk image
and tying it to a new virtual machine container. The file that contains the hard disk
contents together with a description of the properties of the virtual machine, such as
the kind of hardware that was simulated, can also be copied to a different computer and
used with hypervisor software there. If the other computer uses the same type of
processor, the operating system running in the virtual machine will never notice the
difference. Only if a different CPU is used (e.g. a faster CPU with more capabilities) can
the guest operating system actually notice that something has changed. This is because
the hypervisor does not simulate the CPU but grants the guest operating system access
to the physical CPU until the point where the guest wants to execute a machine instruc-
tion that communicates with the outside world, as described above. From the guest
operating system’s point of view, this looks as though the CPU was changed on the
motherboard.

4.18.6 Virtualization in Data Centers in the Cloud
Before discussing Network Function Virtualization (NFV) and Software‐Defined
Networking (SDN) there is one more topic to look at: virtualization in cloud computing.
One aspect of cloud computing is the large server farms operated by companies such as
Amazon, Rackspace, Microsoft, etc. that offer use of virtualized servers to other com-
panies and private individuals instead of equipment physically located on a company’s
premises or at a user’s home. Such servers are immensely popular for running anything
from simple web sites to large‐scale video streaming portals. This is because companies
and individuals using such cloud‐based servers get a high‐speed connection to the
Internet they might not have from where they are located and all the processing power,
memory and storage space they need and can afford without buying any equipment.
Leaving privacy and security issues out of the discussion at this point, using and operat-
ing such a server is no different from interacting with a local physical server. Most sev-
ers are not administrated via a graphical user interface but via a command line console
such as ssh (secure shell). As a consequence it does not matter whether a system admin-
istrator connects to a local Ubuntu server over the local network or to an Ubuntu server
running in the cloud over the Internet; it looks and feels the same. Most of these cloud‐
based servers are not running directly on the physical hardware but in a virtual machine.
This is because, even more so than on the desktop, server‐optimized processors and
motherboards have become so powerful that they can run many virtual machines
simultaneously. Modern x86 server CPUs have 8 to 16 cores and have direct access
from dozens to hundreds of gigabytes of main memory. As a consequence it is common
to see such servers running 10 or more virtual machines simultaneously. As on the
desktop, many applications require processing power only very infrequently. If many of
such virtual servers are put on the same physical machine, CPU capacity can be used