This section provides an overview of a popular packet-switched communication medium called Frame Relay. This section will also describe Frame Re lay operation, devices, congestion control, Local Management Interface (LMI) and frame formats.
Packet-switching technology, as it pertains to Frame Relay, gives multiple networks the capability to share a WAN medium and available bandwidth. Frame Relay ge nerally costs less than point-to-point leased lines. Direct leased lines involve a cost that is based on the distance between endpoints, whereas Frame Relay subscribers incur a cost based on desired bandwidth allocation. A Frame Relay subscriber will share a router, Data Service Unit (DSU), and backbone bandwidth with other subscribers, thereby reducing usage costs. If subscribers require dedicated bandwidth, called a committed information rate (CIR), they pay more to have guaranteed bandwidth during busy time slots.
Operation, Devices, Data-Link Connection Identifiers, and Virtual Circuits
Devices that participate in a Frame Relay WAN include data terminal equipment (DTE) and data circuit-terminating equipment (DCE). Customer-owned equipment such as routers and network stations are examples of DTE devices. Provider-owned equipment provides switching and clocking services, and is contained in the DCE device category. Figure 3.16 illustrates an example of a Frame Relay WAN.
Data- link communication between devices is connected with an identifier and implemented as a Frame Relay virtual circuit. A virtual circuit is defined as the logical connection between two DTE devices through a Frame Relay WAN. These circuits support bidirectional communication; the identifiers from one end to another are termed data-link connection identifiers (DLCIs). Each frame that passes through a Frame Relay WAN contains the unique numbers that identify the owners of the virtual circuit to be routed to the proper destinations. Virtual circuits can pass through any number of DCE devices. As a result, there are many paths between a sending and receiving device over Frame Relay. For the purposes of this overview, Figure 3.16 illustrates only three packet switches within the Frame Relay WAN. In practice, there may be 10 or 20 routers assimilating a multitude of potential courses from one end to another.
There are two types of virtual circuits in Frame Relay, switched virtual circuits (SVCs) and permanent virtual circuits (PVCs), defined as fo llows:
Figure 3.16 Frame Relay WAN.
• Switched Virtual Circuits (SVCs). Periodic, temporary communication sessions for infrequent data transfers. A SVC connection requires four steps:
1. Call setup between DTE devices.
2. Data transfer over temporary virtual circuit. 3. Defined idle period before termination. 4. Switched virtual circuit termination.
SVCs can be compared to ISDN communication sessions, and as such, use the same signaling protocols.
• Permanent Virtual Circuits (PVCs). Permanent communication sessions for frequent data transfers between DTE devices over Frame Relay. A PVC connection requires only two steps:
1. Data transfer over permanent virtual circuit. 2. Idle period between data transfer sessions.
PVCs are currently the more popular communication connections in Frame Relay WANs. Congestion Notification and Error Checking
Frame Relay employs two mechanisms for congestion notification: forward-explicit congestion notification (FECN) and backward-explicit congestion notification (BECN). From a single bit in a Frame Relay header, FECN and BECN help control bandwidth degradation by reporting congestion areas. As data transfers from one DTE device to another, and congestion is experienced, a DCE device such as a switch, will set the FECN bit to 1. Upon arrival, the destination DTE device will be notified of congestion, and process this information to higher- level protocols to initiate flow control. If the data sent back to the originating sending device contains a BECN bit, notification is sent that a particular path through the network is congested.
During the data transfer process from source to destination, Frame Relay utilizes the common cyclic redundancy check (CRC) mechanism to verify data integrity, as explained in the Ethernet section earlier in this chapter.
Local Management Interface
The main function of Frame Relay’s local management interface (LMI) is to manage DLCIs. As DTE devices poll the network, LMI reports when a PVC is active or inactive. When a DTE device becomes active in a Frame Relay WAN, LMI determines which DLCIs available to the DTE device are active. LMI status messages, between DTE and DCE devices, provide the necessary synchronization for communication.
The LMI frame format consists of nine fields as illustrated in Figure 3.17, and defined in the following list:
• Flag. Specifies the beginning of the frame.
• LMI DLCI. Specifies that the frame is a LMI frame, rathe r than a standard Frame Relay frame.
Figure 3.18 Frame Relay frame format.
• Protocol Discriminator (PD). Always includes a value, marking frame as an LMI frame. • Call Reference. Contains zeros, as field is not used at this time.
• Message Type. Specifies the following message types: • Status-inquiry message. Allows devices to request a status. • Status message. Supplies response to status- inquiry message.
• Variable Information Elements (VIE). Specifies two individual information elements: • IE identifier. Identifies information element (IE).
• IE length. Specifies the length of the IE.
• Frame Check Sequence (FCS). Verifies data integrity. • Flag. Specifies the end of the frame.
Frame Relay Frame Format
The following descriptions explain the standard Frame Relay frame format and the fields therein (shown in Figure 3.18):
• Flag. Specifies the beginning of the frame.
• Address. Specifies the 10-bit DLCI value, 3-bit congestion control notification, and FECN and BECN bits.
• Data. Contains encapsulated upper- layer data.
• Frame Check Sequence (FCS). Verifies data integrity. • Flag. Specifies the end of the frame.