Microsoft provides tools as part of the Xbox 360 development environment that let engineers experiment with network performance in multiplayer games.  The one we use most often is a network emulator called N.E.W. - see this Wikipedia entry for a high level overview of Network Emulation 

There are several open source tools that perform a similar function.  One of the easier ones to get set up is Dummynet :

Network emulation essentially requires a PC with two Ethernet cards to act as a gateway:  One card connects to the outside world and the other connects to rest of your network via a switch.  [Under Windows, this is equivalent to Internet Connection Sharing ]

Dummynet runs on the PC and controls the flow of packets to/from network devices.  You need to have some knowledge of how to control a command-line application to get anything useful out of the tool.

The things we control for the testing of SSFII HD Remix include: 

• Bandwidth : We need to meet requirements that all applications run under 64 kbps bandwidth restriction - this is roughly the low-end limit of a DSL/Cable connection.
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• Latency : We can simulate poor latency connections by inserting a delay in the Internet uplink.  The general worst case latency situation we are looking at is 250+ms roundtrip sustained.
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• Packet Loss : Games generally need to be able to withstand a 10% random packet loss.

There is a very good article on the use of Dummynet on the personal website of Márton Anka, codefromthe70s.org

Using Márton's step-by-step instructions, you can set up a Dummynet environment on a single PC using VMWare to simulate several virtual PC's.

  - Gary