1. The Remote Desktop Protocol
The Remote Desktop
Protocol (RDP) is the protocol that drives Terminal Services. RDP is
based on and is an extension of the T.120 protocol family of standards.
It is a multichannel-capable protocol that allows for separate virtual
channels for carrying device communication and presentation data from
the server, as well as encrypted client mouse and keyboard data. RDP
provides a very extensible base from which to build many additional
capabilities, supporting up to 64,000 separate channels for data
transmission as well as provisions for multipoint transmission.
Figure 1 shows the structure of RDP and
its functionality from a high-level perspective.
The new Terminal
Services client software included in Windows Server 2003 (Remote Desktop
Connection, or RDC) uses RDP 5.2, and many local resources are
available within the remote session: the client drives, smart cards,
audio card, serial ports, printers (including network), and clipboard.
Additionally, you can select color depth from 256 colors (8-bit) to True
Color (24-bit) and resolution from 640×480 up to 1600×1200.
RDP basically takes
instructions from a terminal server host machine on screen images and
draws them onto a client's screen, refreshing that image about 20 times
every second if there's activity on the client side. (To save bandwidth,
if no activity is detected on the client side, it cuts the refresh rate
in half.) It then notes any keyboard and mouse activity (among other
things) and relays those signals to the terminal server host machine for
processing. This two-way exchange of information is wrapped into what's
called a session,
which consists of the programs running on the host machine and the
information being sent over RDP between the terminal server and the
client machine.
2. Requirements for Terminal Services
Because most of the
processing power that was traditionally on the desktop has moved to the
server in a Terminal Services scenario, it follows that the machine
hosting Terminal Services for your users should be significantly beefier
than you might otherwise be used to. Perhaps the two most critical
points as far as hardware requirements are CPU and RAM, followed by the
network interface and links.
2.1. CPU Requirements
CPU requirements
can be difficult to measure because individual users require different
slices of processor time at different intervals. Two main factors
determine CPU usage: intensity of the applications that users are
running, and the number of simultaneous users. Table 1 gives a rough estimate for CPU requirements based on number of users and application intensity.
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When Microsoft
introduced its revamped version of Terminal Services in Windows 2000,
many predicted the demise of Citrix's venerable MetaFrame product.
Citrix basically invented the idea of multiple sessions on one Windows
host, and RDP is loosely based on Citrix's protocol, Independent
Computing Architecture (ICA). MetaFrame has traditionally sat atop
Terminal Services, using its basic functionality as a foundation and
adding useful features on top. But many people have asked what with all
the improvements to RDP, including bandwidth reduction, client-side
caching, device mapping, sound redirection, and increased color depth,
why a corporation would continue to buy Citrix's flagship product?
A feature known
as Seamless Windows one-to-many connections is available with MetaFrame,
but not with raw Terminal Services. For the client, this means
applications you see are displayed without having to scroll or use
full-screen mode. On the server, though, it's even better: all sessions
on the same server that use Seamless Windows are operating out of the
same physical instance of the program, saving on hardware resources.
Another hot item is
application publishing, which uses a Citrix concept called Program
Neighborhood to direct a user to applications via a menu of program
options. The user doesn't have to know which servers run those
applications with Citrix because he can just choose a program from the
menu and be connected directly to it. To use native Terminal Services
and RDP, the user would need to find the appropriate server and connect
to it.
Additional core features
of MetaFrame distinguish it from native Terminal Services. And as
always, Citrix releases new features in its Service Releases. Larger
organizations that use Terminal Services connections extensively might
want to investigate how MetaFrame might improve their efficiency.
However, most organizations, especially small businesses, will find that
the initially free Terminal Services components of Windows Server 2003
(you pay only for licensing and not the initial software purchase) will
suffice for their needs.
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Table 1. Rough estimates of CPU requirements for Terminal Services
| Simultaneous users | Intensity of application(s) | Recommended processor |
|---|
| 20-25 | Low | Pentium III 750 MHz+ |
| | High | Dual Pentium III 750 MHz+ or Pentium IV Xeon any speed |
| 35-40 | Low | Pentium III or IV, 1 GHz+ |
| | High | Dual Pentium IV Xeon 2 GHz+ |
| 50-55 | Low | Dual Pentium IV Xeon 2 GHz+ |
| | High | Not recommended |
2.2. Amount of RAM
Here are some hard and fast facts about RAM usage with Terminal Services
that you might find helpful:
Each user takes up roughly 20 MB of the Terminal Services host machine's RAM just by logging in.
A
typical load of two to three Office applications per user will add
approximately another 25 MB of RAM to the server's RAM usage.
Of course, additional
applications on top of that consume more RAM, and power users typically
will not run only Office applications, but rather, more powerful
applications that require more hardware resources.
2.3. Network Interface Card
The Network Interface
Card (NIC) is managed by Windows and should not require any
configuration for use with Terminal Services. You should focus more on
the available bandwidth and average latency on the network to which the
card is connected, and not necessarily on the card itself.
Terminal Services
does a surprisingly nice job of adjusting the bandwidth usage of the RDP
client to the conditions of the link to which it's connected with the
host. You can expect that most RDP connections will take up between 2
kbps and 7 kbps depending on the depth of color requested by the client,
the amount of graphics being transmitted, whether sound is included,
and other options.
2.4. Disk Space
The actual Terminal
Services components inside Windows Server 2003 do not require any
additional disk space on top of what is consumed by the normal system
files. Around 15 MB is taken up for the file share that stores the
client installation files for RDP. In addition, users store about half a
megabyte of data for their Terminal Services profile information when
they first log on to a server. (Remember that Terminal Services users'
profiles are automatically roaming because their sessions follow them to
whatever workstation is running the RDP client.) Other than this, not
much disk space is required to support RDP.
2.5. Sizing for Scaling
Let's say, for instance,
that you've built a server to run TS and now want to see how well it
scales. Microsoft has released a group of scaling scripts and utilities
that can be downloaded from:
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http://www.microsoft.com/windows2000/techinfo/administration/terminal/loadscripts.asp
The single download
includes scripts that should be modified to fit your environment as
necessary, and an Excel spreadsheet that will guide you through
fine-tuning certain Registry settings to achieve best performance on a
machine dedicated solely to hosting TS applications.
