KB14139 - Effects of latency on BlackBerry Enterprise Server for Microsoft Exchange

Si · #1 (**permalink**) 02-16-2009, 04:55 AM

KB14139 - Effects of latency on BlackBerry Enterprise Server for Microsoft Exchange

Environment

BlackBerry® Enterprise Server for Microsoft® Exchange

Overview

This article provides an overview of the impact that latency may have on the performance of the BlackBerry Enterprise Server for Microsoft Exchange, and how latency can negatively affect the BlackBerry Enterprise Server connection to the messaging environment.
As organizations grow, so must their networks. Because it is impossible to maintain global office connectivity through Local Area Network (LAN) connections, remote offices are typically connected by Wide Area Network (WAN) connections. With BlackBerry Enterprise Server for Microsoft Exchange, the BlackBerry Enterprise Server connects to the Microsoft Exchange Server using the Messaging Application Programming Interface (MAPI) protocol. MAPI was designed to function on LAN connections, not WAN connections. LAN connections are faster and have low latency, whereas WAN connections are less reliable and have higher latency. To help address network latency issues, it is important to understand the role of MAPI in network performance.
BlackBerry Enterprise Server for Microsoft Exchange utilizes the MAPI notification system, in which a MAPI Advise session is registered for BlackBerry smartphone users. When a new item arrives in a BlackBerry smartphone user’s mailbox or another change occurs, the Microsoft Exchange Server sends a User Datagram Protocol (UDP) notification to the BlackBerry Enterprise Server. Once the notification is received, the BlackBerry Enterprise Server starts to process the change. High latency, packet loss, network congestion, and multiple hop counts between end points (i.e., the BlackBerry Enterprise Server and the Microsoft Exchange Server) increase the likelihood that UDP packets do not arrive at their destination. In a BlackBerry Enterprise Server environment, this means that the BlackBerry Enterprise Server does not receive the change notifications that the Microsoft Exchange Server sends.
Applications that use UDP must have application-level error handling processes, as opposed to protocols with error correction functionality, such as TCP/IP. The BlackBerry Enterprise Server provides error correction by rescanning mailboxes periodically to determine if they contain any changed or new items for which notifications were not received. Regardless of whether the UDP notification arrives, the BlackBerry Enterprise Server performs mailbox rescans in order to process all new items or changes. Rescans are performed at intervals of 15 to 30 minutes; therefore, if a changed item exists but no UDP notification for it arrives, the BlackBerry Enterprise Server might begin processing the item 15 to 30 minutes later.
When latency between the BlackBerry Enterprise Server and the Microsoft Exchange Server increases, the number of factors that may impede the successful delivery of UDP notifications increases as well. Increased latency lengthens the time required for data transfer--which uses TCP/IP--between applications, for instance, the higher the latency between the BlackBerry Enterprise Server and the Microsoft Exchange Server, the longer it takes for data to travel between them. The increased data transfer time is relational to the latency of the WAN connection, and not necessarily its bandwidth. This is because TCP acknowledgments take longer to be sent if network latency exists, which delays the occurrence of the next data transfer.
If latency exists between the BlackBerry Enterprise Server and the Microsoft Exchange Server, it is expected that latency exists in the messaging environment. To help achieve optimal BlackBerry Enterprise Server performance, refer to the recommendations made in the Capacity Planning and Performance Tuning for Environments Using the BlackBerry Enterprise Solution document.

Additional Information

Note: Refer to Request For Comment (RFC) 768 for a detailed explanation of UDP. RFC768 is available on the Internet.