In the .NET Framework, most of the time, the name of an assembly matches the namespace of the classes in that assembly.

Since WCF is kind of an add-on to the .NET 2.0 Framework this is not quite true for the assembly System.ServiceModel.Web.dll

Below a screenshot of what .NET Reflector shows to be inside this assembly.

As you can see this assembly extends a number of namespaces like System.Runtime.Serialization and System.Collections.ObjectModel.

The Json serialization classes are also in this assembly.