Frame Relay is a CCITT(Consultative Committee for International Telegraph and Telephone. International organization responsible for the development of communications standards. Now called the ITU-T. See ITU-T.) and American National Standards Institute (ANSI) standard that defines the process for sending data over a public data network (PDN). It is a data-link technology that is streamlined to provide high performance and efficiency. It operates at the Physical an Data Link Layers of the OSI model but it relies on upper-layer protocols such as TCP for error correction.
Note that Frame Relay defines the interconnection process between your customer premises equipment (CPE) (also known as data terminal equipment - DTE), such as a router, and the service provider’s local access switching equipment (known as data communications equipment - DCE).
Frame Relay provides a means for multiplexing many logical data conversations (referred to as virtual circuits) by assigning each pair of DTEs connection identifiers. The service provider’s switching equipment constructs a table mapping connection identifiers to outbound ports. When a frame is received, the switching device analyzes the connection identifier and delivers the frame to the associated outbound port. The complete path to the destination is established prior to the sending of the first frame.
Frame Relay Terminology
Following are some terms that are used frequently when discussing Frame Relay:
Local access rate—The clock speed (port speed) of the connection (local loop) to the Frame Relay cloud. It is the rate at which data travels into or out of the network.
Data-link connection identifier (DLCI)—A number that identifies the logical circuit between the source and destination device. The FR switch maps the DLCIs between each pair of routers to create a PVC.
Local Management Interface (LMI)—A signaling standard between the CPE device and the FR switch that is responsible for managing the connection and maintaining status between the devices. LMIs may include support for a keepalive mechanism, which verifies that data is flowing; a multicast mechanism which can provide the network server with its local DLCI; multicast addressing, providing a few DLCIs to be used as multicast (multiple destination) addresses, and the ability to give DLCIs global (whole Frame Relay network) significance, rather than just local significance (DLCI used only to the local switch); and a status mechanism, which provides an ongoing status on the DLCIs known to the switch.
There are several LMI types and routers need to be told which LMI type is being used. Three types of LMIs are supported:
cisco—LMI type defined jointly by Cisco, StrataCom, Northern Telecom, and DEC
ansi—Annex D defined by ANSI standard T1.617
q933a—ITU-T Q.933 Annex A
Frame Relay Terminology (cont.)
Committed information rate (CIR)—The rate, in bits per second, that the Frame Relay switch agrees to transfer data.
Committed Burst (Bc)—The maximum number of bits that the switch agrees to transfer during any Committed Rate Measurement Interval (Tc).
Excess Burst—The maximum number of uncommitted bits that the Frame Relay switch will attempt to transfer beyond the CIR. Excess Burst is dependent on the service offerings available by your vendor, but is typically limited to the port speed of the local access loop.
Forward explicit congestion notification (FECN)—When a Frame Relay switch recognizes congestion in the network, it sends an FECN packet to the destination device indicating that congestion has occurred.
Backward explicit congestion notification (BECN)—When a Frame Relay switch recognizes congestion in the network, it sends a BECN packet to the source router instructing the router to reduce the rate at which it is sending packets.
Discard Eligibility (DE) Indicator—When the router detects network congestion, the FR switch will drop packets with the DE bit set first. The DE bit is set on the oversubscribed traffic; that is, the traffic that was received after the CIR was met.
Frame Relay Data Link Connection Identifier
Frame Relay virtual circuits are identified by data link connection identifiers (DLCIs). DLCI values are typically assigned by the Frame Relay service provider (for example, the telephone company).
Frame Relay DLCIs have local significance. That is, the values themselves are not unique in the Frame Relay WAN. Two DTE devices connected by virtual circuit might use a different DLCI value to refer to the same connection.
Frame Relay Frame Format
The Frame Relay frame is shown n the graphic. The flags field indicate the beginning and end of the frame. Following the leading flags field are two bytes of address information. Ten bits of these two bytes make up the actual circuit ID (called the DLCI, for data link connection identifier).
The 10-bit DLCI value is the heart of the Frame Relay header. It identifies the logical connection that is multiplexed into the physical channel. 3 of the remaining bits in the field provide congestion control. The forward explicit congestion notification (FECN) bit is set by the Frame Relay network in a frame to tell the DTE receiving that frame that congestion was experienced in the path from source to destination. The backward explicit congestion notification (BECN) bit is set by the Frame Relay network in frames traveling in the opposite direction from frames encountering a congested path. The notion behind both of these bits is that the FECN or BECN indication can be promoted to a higher-level protocol that can take flow control action as appropriate.
The discard eligibility (DE) bit is set by the DTE to tell the Frame Relay network that a frame has lower importance than other frames and should be discarded before other frames if the network becomes short of resources. Thus, it represents a very simple priority mechanism. This bit is usually set only when the network is congested.
Frame Relay Frame Field Descriptions
Flags - Delimits the beginning and the end of the Frame Relay frame.
Address - Contains the following information:
DLCI Value - Indicates the data link connection identifier (DLCI) value. Consists of the first 10 bits of the Address field.
Extended Address (EA) - Indicates the length of the Address field. While Frame Relay addresses are currently all 2 bytes long, the EA bits allow for the possible extension of address lengths in the future. The 8th bit of each byte of the Address field is used to indicate the EA.
C/R - Bit that follows the most significant DLCI byte in the Address field. The C/R bit is not currently defined.
Congestion Control - The three bits that control the Frame Relay congestion notification mechanisms. These are the FECN, BECN, and DE bits, which are the last 3 bits in the Address field.
Data - Variable-length field that contains encapsulated upper-layer data.
FCS - Frame Check Sequence (FCS), used to ensure the integrity of transmitted data.
Frame Relay Addressing
Assume two PVCs, one between Atlanta and Los Angeles, and one between San Jose and Pittsburgh. Los Angeles uses DLCI 12 to refer to its PVC with Atlanta, while Atlanta refers to the same PVC as DLCI 82. Similarly, San Jose uses DLCI 12 to refer to its PVC with Pittsburgh. The network uses internal proprietary mechanisms to keep the two locally significant PVC identifiers distinct.
Frame Relay Operation - LMI
Frame Relay signaling reports the status of PVCs. the original Frame Relay signaling specification is called Link Management Interface (LMI - also known as Local Management Interface). LMI was proposed by the Frame Relay Forum. Subsequently, the American National Standards Institute (ANSI) and the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) have standardized slightly different versions of LMI. (ITU-T was formerly called CCITT)
The main purpose for the LMI process is:
PVC status - What is the operational status of the various PVCs that the router knows about.
Transmission of keepalive packets to insure that the PVC stays up and does not shut down due to inactivity.
The router must be programmed to choose which LMI type encapsulation will be used.
Options are:
IETF Encapsulation Type
Cisco Encapsulation Type
Inverse ARP
The Inverse ARP mechanism allows the router to automatically build the Frame Relay map. The router learns the DLCIs that are in use from the switch during the initial LMI exchange. The router then sends an Inverse ARP request to each DLCI for each protocol configured on the interface if the protocol is supported. The return information from the Inverse ARP is then used to build the Frame Relay map.
Frame Relay Mapping
The router next-hop address determined from the routing table must be resolved to a Frame Relay DLCI. The resolution is done through a data structure called a Frame Relay map. This data structure may be statically configured in the router, or the Inverse ARP feature can be used for automatic setup of the map.
Frame Relay Operation - Switching
The Frame Relay switching table consists of four entries: two for incoming port and DLCI, two for outgoing port and DLCI. The DLCI could, therefore, be remapped as it passes through each switch; the fact that the port reference can be changed is why the DLCI is “locally significant."
Frame Relay Operation
Frame Relay is a Layer 2 protocol that describes how the DTE device communicates with and connects to a Frame Relay switch. A summary of how this protocol operates follows:
You order Frame Relay service from a service provider, or you create a private Frame Relay cloud.
Each router, either directly or through a CSU/DSU, connects to the Frame Relay switch.
When the CPE router is enabled, it sends a Status Inquiry message to the FR switch. The message notifies the switch of the router’s status, and asks the switch for the connection status of the other remote routers.
When the FR switch receives the request, it responds with a Status message that includes the DLCIs of the remote routers to which the local router can send data.
For each active DLCI, each router sends an Inverse ARP request packet introducing itself and asking for each remote router to identify itself by replying with its network-layer address.
Frame Relay Operation (cont.)
For each DLCI that each router receives an Inverse ARP message about, the router will create a map entry in its Frame Relay map table that includes the local DLCI and the remote router’s network-layer address, as well as the state of the connection. Note that the DLCI is the router’s locally configured DLCI, not the DLCI that the remote router is using. Three possible connection states appear in the Frame Relay map table:
Active state—Indicates that the connection is active and that routers can exchange data.
Inactive state—Indicates that local connection to FR switch is working, but the remote router’s connection to FR switch is not working.
Deleted state—Indicates that no LMI is being received from the FR switch or no service between the CPE router and FR switch is occurring.
If Inverse ARP is not working, or the remote router does not support Inverse ARP, you need to configure the routes (DLCIs and IP addresses) of the remote routers. This configuration is referred to as static maps and is discussed later in the “Configuring Frame Relay” section.
Every 60 seconds, the routers exchange Inverse ARP messages.
Every ten seconds or so (this is configurable), the CPE router sends a keepalive message to the FR switch. The purpose of the keepalive message is to verify that the FR switch is still active.
The router will change the status of each DLCI, based on the response from the FR switch
Time-division multiplexing (TDM)—Information from many sources has bandwidth allocation on a single media. Circuit switching uses signaling to determine the call route, which is a dedicated path between the sender and the receiver. By multiplexing traffic into fixed time slots, TDM avoids congested facilities and variable delays. Basic telephone service and Integrated Services Digital Network (ISDN) use TDM circuits
Frequency-division multiplexing.
Technique whereby information from multiple channels can be
allocated bandwidth on a single wire based on frequency.
X.25 - ITU-T standard that defines how connections between DTE and DCE are maintained for remote terminal access and computer communications in PDNs (Public Data Networks). X.25 specifies LAPB, a data link layer protocol, and PLP, a network layer protocol. Frame Relay has to some degree superseded X.25. See also Frame Relay, LAPB, and PLP.
