Avaya Configuring Line Services Bedienungsanleitung PDF herunterladen (Seite 33)

Overview of Line Protocols

1-9

FDDI Ring Operation

An FDDI ring consists of nodes in a ring architecture. There are two classes of

nodes:

stations

(a node with no master ports), and

concentrators

(a node with

master ports).

The FDDI standards deﬁne two types of stations: Single Attachment Stations

(SAS) and Dual Attachment Stations (DAS). The SAS connects to only one ring;

it cannot wrap the ring in case of a fault. The DAS connects to both the primary

and secondary rings. The Bay Networks router is an example of a DAS.

FDDI initializes the ring and transmits data as follows:

1. The nodes on the ring establish connections with their neighbors.

The Connection Management (CMT) portion of SMT controls this process, as

described in “FDDI SMT,” later in this chapter.

2. The nodes negotiate the target token-rotation time (TTRT), using the claim

token process.

The TTRT is the value that the MAC sublayer uses to time its operations. The

claim token process determines which node initializes the ring (generates the

token). The node with the lowest bid for the TTRT wins the right to generate

the token.

3. After a node has initialized the ring, the ring begins to operate in steady state.

In steady state, the nodes exchange frames using the timed-token protocol

(TTP). The TTP deﬁnes how the TTRT is set, the length of time a node can

hold the token, and how a node initializes the ring. The ring remains in steady

state until a new claim token process occurs (for example, when a new node

joins the ring).

4. The nodes pass the token from one node to another on the FDDI ring.

5. A node on the ring captures the token when it wants to transmit data, and then

transmits data to its downstream neighbor.

6. Each node reads and repeats frames as it receives them. If a node detects an

error in a frame, the node sets an error indicator.

7. A frame circulates on the ring until it reaches the node that ﬁrst transmitted it.

That node removes the frame from the ring.

8. When the ﬁrst node has sent all of its frames, or exceeded the available

transmission time, it releases the token back to the ring.

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Avaya Configuring Line Services Bedienungsanleitung Seite 33