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.