Channel is actually a bridge between a client and a service.  Messages can travel between clients and services through channels.  There are four main types of channels:

·         Simplex Input

·         Simplex Output

·         Request-Reply

·         Duplex

But how a channel can be created?  The first time when a message is going to be sent (only at this time) the client tries to create a channel on a service endpoint (you will read about it later in the next section) based on the type of channel that is required and an address.  If the service is listening on that address then a channel will be created.  Here there is a point: you do not have any channel until the first message is sent.  When communication between client and service ends, the channel will be destroyed.

A channel is a queue of some features that make up its properties, such as security, session or messaging pattern. There are different types of each feature available, but you must choose a compatible combination to be able to create a channel.  Here I describe these features.

Each channel type supports one, two or all (three) messaging patterns so you should be aware of channel characteristics to be able to use it for your aims.

Different transports can be used in WCF.  It is also possible to write a new transport but HTTP, TCP and MSMQ are the most common transports.  These transports have some characteristics so you have to choose a correct transport for your application.  For example, some transports provide better security features.

In addition to the security level that comes with transports, you can use SOAP message security and its authentication or authorization and other security mechanisms to improve security in your application.

It is possible to encode messages in a WCF application.  You can use pure text format or binary format to encode messages.  There are some built-in encoders, but you can write new encoders or extend them.

Reliable Sessions let you to have reliable messaging and it helps you to keep communication alive whenever something is lost.   Reliable Sessions also let you to have sessions.  Sessions keep states on services during several actions.

Interoperability of a channel makes it possible to open doors to other platforms and technologies.  Four types of interoperability are available in WCF.  Basic Profile is very simple and you saw it in traditional web services.  .NET interoperability needs .NET on the client and service, but provides some helpful features such as binary encoding.  MSMQ interoperability is a way to interoperate with other deployments of MSMQ.  The fourth and last interoperability that is very common is WS-*.  This interoperability opens doors for other platforms and can use WS-* protocols.  These protocols provide many important features such as security, transactional operations, reliability and service.