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Challenges with Exchange Server 2000
Microsoft® Exchange Server 2000 has several limitations that make performance over a WAN or dial-up connection less than desirable at times. This can make it impractical to deploy Exchange 2000 servers in a centralized manner.
One of the primary limitations with Exchange 2000 is that the underlying communications architecture, called the Messaging Application Programming Interface (MAPI), is extremely “chatty”. When an email is transferred, MAPI transfers information in small (8 KB or 16 KB) blocks, with each block requiring an ACKnowledgement.
To make matter worse, all emails and content are stored on Exchange 2000 servers. If a user pages through a list of emails, the information for each email is downloaded to the Outlook client, causing the user interface to sit suspended until the download is complete. (Exchange 2000 users will recall that it is often impossible to interrupt a transfer once it was started). To compound this, once a user logs off, Outlook begins a synch process pulling a copy of the message locally for later viewing (offline). The same message is sent twice – once for viewing and once for folder synchronization.
The problem is further exacerbated by the fact that the Outlook client sends the entire message to keep the ‘Sent Items’ folder in synch between the Exchange Server and Outlook client. As a result, a single email can cross the network 4 times using MAPI.
Exchange 2003 - Improvements and Limitations
Exchange 2003 introduced a new mode called “Cached”. With Cached mode, the first time an email is downloaded for viewing, it is stored locally. Using a feature called quickflagging, marking messages as read, replying, and editing requires only a small amount of data to be pushed up to the server to keep the mailboxes synchronized. Operations on that same client (such as re-reading the same email or paging up/down) now operate on the local cached version. This eliminates the user from having to sit in a suspended state while these operations proceeded, as was the case with Outlook 2000.
Since many common user operations now do not require server interaction, Exchange 2003 is substantially better for operations over a WAN. This has the added benefit of improving Exchange 2003 server scalability. As a result, most Enterprises have transitioned or are transitioning to Exchange 2003.
But Exchange 2003 comes with a tradeoff. By default, when an email is sent it is “pushed” instantly, as opposed to a pulling mechanism used by Outlook 2000. This can create spikes in network utilization. For example, if a user sitting in headquarters sends a 1 Mb email to 100 users, 100Mbytes of traffic will immediately hit the network. In a network that is not properly engineered, traffic spikes like this can cause performance ‘brown-outs’, potentially starving other applications. This has a particularly adverse effect on real-time applications, like VoIP and streaming media.
While Exchange 2003 does improve the performance of Microsoft email across a WAN, it does not negate the need for a robust application acceleration solution. To centralize MS Exchange servers, enterprises still require advanced tools to eliminate repetitive data, maximize WAN bandwidth, and ensure that all enterprise applications are treated appropriately.
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