Global Site

Microsoft Exchange Server system architects usually base their infrastructure design on some rules of thumb. For example, you should have at least 128Kbps of network bandwidth available if you put two servers in the same site and at least 64Kbps of network bandwidth available before you use an Exchange site connector. However, sometimes these rules keep you from getting the most out of your system. When you understand how Exchange Server works and what goes on between the Message Transfer Agent (MTA) and the Directory Service (DS), you can stretch these rules.

Let's look at a company that broke the rules—or at least stretched them—to build a global messaging system. With offices spread over three continents, this company has achieved consistent worldwide message delivery times of less than 10 seconds.

The Challenge
NatWest GFM, a trading and investment bank in the UK-based NatWest Group, needed a reliable, scalable, and—above all—fast messaging environment. The existing messaging infrastructure had evolved haphazardly and offered global message delivery times of about 4 hours. The company's management wanted a new infrastructure that would deliver standard messages in less than 10 minutes.

The company has offices in Tokyo, Singapore, Hong Kong, London, New York, Madrid, Beijing, Taiwan, and Shanghai, and company personnel travel around the world. In the company's old system, WAN connection speeds between offices varied from 1Mbps between major offices to only 2Kbps committed information rate (CIR) between minor offices.

Advantages of One Site
After some preliminary research, NatWest GFM believed that it could put all its servers in one site. The company came to this conclusion for several reasons.

First, a single site is the fastest design possible because of its inherent fully meshed structure—every server communicates directly with every other server. When you have multiple sites, communication takes place between sites through one or more bridgeheads. This situation means you have one or more additional links in each chain from sender to recipient. Each link in the chain represents a point of potential failure.

Second, London- and New York-based administrators could easily administer the entire organization with a single-site arrangement. Some of the remote Asian locations could offer only limited support for Exchange Server systems. Although you can centrally administer a multiple-site environment if you have appropriate permissions in place, a single site makes this process slightly easier because administrators don't need to manually connect to each site to make changes. One site also promotes the idea of internal unification within the IS infrastructure. Administrators of different sites tend to make changes that might not be in place at other sites. A single site enforces common standards such as a common Internet email address format or Global Address List (GAL) display format.

Third, single sites accommodate mobile users well. The bank's personnel routinely work abroad for several weeks or months at a time. Because Exchange Server doesn't offer a standard capability for moving mailboxes between sites, you have few options to provide messaging facilities to company personnel on different continents. With a single site, you can move mailboxes from one server to another easily.

Fourth, servers work together better in one site than they do in multiple sites. In one site, public folders, custom forms, calendar information, custom directory attributes, and event scripts work better and spare you the difficulties of getting them to work across site boundaries.

The theory behind the one-site solution was sound, but the project team had to prove the new design would work before proceeding.

The Technical Assessment
After NatWest GFM decided on a one-site strategy, those of us in the design team needed to determine whether implementing a single site was possible. At design time in mid-1998, few other organizations had global sites. Key prerequisites of a one-site strategy include the following:

• A permanent and reliable connection must exist between all servers in the site.
• Physical network connectivity must exist between all Exchange servers.
• Every Exchange server must have name resolution to every other Exchange server in the site.
• Every client must have name resolution to every Exchange server in the site.
• Every Exchange server must use the same service account.
• To avoid remote procedure call (RPC) errors, the WAN must have sufficient capacity to let servers respond to RPC requests within 30 seconds.

Although NatWest GFM had a very good WAN, some of the links were inadequate. For example, the frame relay WAN link between Tokyo and Singapore was only 24Kbps CIR in one direction and 16Kbps in the other. Also, the WAN link between some of the Asian offices and New York was just 2Kbps, though studies had shown that the link consistently experienced bursts of up to 24Kbps. With frame relay networks, the network provider lets you use more bandwidth than you've paid for as long as no one else is using it, but that privilege can disappear at any time. You can't base a design on bandwidth rates that aren't guaranteed. The bank decided to upgrade to 48Kbps CIR in both directions for Tokyo and Singapore and force all Asia-New York traffic through London.

One reason for the upgrade was to ensure that the Exchange servers had sufficient capacity to prevent RPC errors. When an Exchange server issues an RPC request, it expects an acknowledgment within 30 seconds. If the network is too congested or a fault occurs, Exchange Server reports errors and communications might fail.

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