[FAB-2290] add channel documentation

add documentation for channel definition

Change-Id: I6b76b7d0513aba7074599b521c31ba56b988e5ea
Signed-off-by: Christopher Ferris <chrisfer@us.ibm.com>
Signed-off-by: Yacov Manevich <yacovm@il.ibm.com>

docs/source/channels.rst

@@ -1,4 +1,36 @@
 Channels
 --------
 
-...coming soon
+A Hyperledger Fabric **channel** is a private "subnet" of communication between
+two or more specific network members, for the purpose of conducting private and
+confidential transactions. A channel is defined by members (organizations),
+anchor peers per member, the shared ledger, chaincode application(s) and the ordering service
+node(s). Each transaction on the network is executed on a channel, where each
+party must be authenticated and authorized to transact on that channel.
+Each peer that joins a channel, has its own identity given by a membership services provider (MSP),
+which authenticates each peer to its channel peers and services.
+
+To create a new channel, the client SDK calls configuration system chaincode
+and references properties such as **anchor peer**s, and members (organizations).
+This request creates a **genesis block** for the channel ledger, which stores configuration
+information about the channel policies, members and anchor peers. When adding a
+new member to an existing channel, either this genesis block, or if applicable,
+a more recent reconfiguration block, is shared with the new member.
+
+The election of a **leading peer** for each member on a channel determines which
+peer communicates with the ordering service on behalf of the member. If no
+leader is identified, an algorithm can be used to identify the leader. The consensus
+service orders transactions and delivers them, in a block, to each leading peer,
+which then distributes the block to its member peers, and across the channel,
+using the **gossip** protocol.
+
+Although any one anchor peer can belong to multiple channels, and therefore
+maintain multiple ledgers, no ledger data can pass from one channel to another.
+This separation of ledgers, by channel, is defined and implemented by
+configuration chaincode, the identity membership service and the gossip data
+dissemination protocol. The dissemination of data, which includes information on
+transactions, ledger state and channel membership, is restricted to peers with
+verifiable membership on the channel. This isolation of peers and ledger data,
+by channel, allows network members that require private and confidential
+transactions to coexist with business competitors and other restricted members,
+on the same blockchain network.