Frame Relay
Frame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. Frame Relay originally was designed for use across Integrated Services Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well. This chapter focuses on Frame Relay's specifications and applications in the context of WAN services.
Frame Relay is an example of a packet-switched technology. Packet-switched networks enable end stations to dynamically share the network medium and the available bandwidth. Variable-length packets are used for more efficient and flexible transfers. These packets then are switched between the various network segments until the destination is reached. Statistical multiplexing techniques control network access in a packet-switched network. The advantage of this technique is that it accommodates more flexibility and more efficient use of bandwidth. Most of today's popular LANs, such as Ethernet and Token Ring, are packet-switched networks.
Frame Relay often is described as a streamlined version of X.25, offering fewer of the robust capabilities, such as windowing and retransmission of lost data, that are offered in X.25. This is because Frame Relay typically operates over WAN facilities that offer more reliable connection services and a higher degree of reliability than the facilities available during the late 1970s and early 1980s that served as the common platforms for X.25 WANs. As mentioned earlier, Frame Relay is strictly a Layer 2 protocol suite, whereas X.25 provides services at Layer 3 (the network layer) as well. This enables Frame Relay to offer higher performance and greater transmission efficiency than X.25 and makes Frame Relay suitable for current WAN applications, such as LAN interconnection.
Initial proposals for the standardization of Frame Relay were presented to the Consultative Committee on International Telephone and Telegraph (CCITT) in 1984. Due to lack of interoperability and lack of complete standardization, however, Frame Relay did not experience significant deployment during the late 1980s.
A major development in Frame Relay's history occurred in 1990 when Cisco Systems, Digital Equipment, Northern Telecom, and StrataCom formed a consortium to focus on Frame Relay technology development. This consortium developed a specification that conformed to the basic Frame Relay protocol that was being discussed in CCITT but extended the protocol with features that provide additional capabilities for complex internetworking environments. These Frame Relay extensions are referred to collectively as the Local Management Interface (LMI).
Since the consortium's specification was developed and published, many vendors have announced their support of this extended Frame Relay definition. ANSI and CCITT have subsequently standardized their own variations of the original LMI specification, and these standardized specifications now are more commonly used than the original version.
Internationally, Frame Relay was standardized by the International Telecommunications Union - Telecommunications Sector (ITU-T). In the United States, Frame Relay is an American National Standards Institute (ANSI) standard.
Frame Relay Devices
Devices attached to a Frame Relay WAN fall into two general categories: data terminal equipment (DTE) and data circuit-terminating equipment (DCE). DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer. In fact, they may be owned by the customer. Examples of DTE devices are terminals, personal computers, routers, and bridges.
DCEs are carrier-owned internetworking devices. The purpose of DCE equipment is to provide clocking and switching services in a network, which are the devices that actually transmit data through the WAN. In most cases, these are packet switches. Figure 10-1 shows the relationship between the two categories of devices.
Figure 10-1: DCEs generally reside within carrier-operated WANs.
The connection between a DTE device and a DCE device consists of both a physical-layer component and a link-layer component. The physical component defines the mechanical, electrical, functional, and procedural specifications for the connection between the devices. One of the most commonly used physical-layer interface specifications is the recommended standard (RS)-232 specification. The link-layer component defines the protocol that establishes the connection between the DTE device, such as a router, and the DCE device, such as a switch. This chapter examines a commonly utilized protocol specification used in WAN networking---the Frame Relay protocol.
Data-Link Connection Identifier (DLCI)
Frame Relay virtual circuits are identified by data-link connection identifiers (DLCIs). DLCI values typically are assigned by the Frame Relay service provider (for example, the telephone company). Frame Relay DLCIs have local significance, which means that the values themselves are not unique in the Frame Relay WAN. Two DTE devices connected by a virtual circuit, for example, may use a different DLCI value to refer to the same connection. Figure 10-2 illustrates how a single virtual circuit may be assigned a different DLCI value on each end of the connection.
Figure 10-2: A single Frame Relay virtual circuit can be assigned different DLCIs on each end of a VC.
Congestion-Control Mechanisms
Frame Relay reduces network overhead by implementing simple congestion-notification mechanisms rather than explicit, per-virtual-circuit flow control. Frame Relay typically is implemented on reliable network media, so data integrity is not sacrificed because flow control can be left to higher-layer protocols. Frame Relay implements two congestion-notification mechanisms:
Forward-explicit congestion notification (FECN)
Backward-explicit congestion notification (BECN)
FECN and BECN each are controlled by a single bit contained in the Frame Relay frame header. The Frame Relay frame header also contains a Discard Eligibility (DE) bit, which is used to identify less important traffic that can be dropped during periods of congestion.
The FECN bit is part of the Address field in the Frame Relay frame header. The FECN mechanism is initiated when a DTE device sends Frame Relay frames into the network. If the network is congested, DCE devices (switches) set the value of the frames' FECN bit to 1. When the frames reach the destination DTE device, the Address field (with the FECN bit set) indicates that the frame experienced congestion in the path from source to destination. The DTE device can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow-control may be initiated, or the indication may be ignored.
The BECN bit is part of the Address field in the Frame Relay frame header. DCE devices set the value of the BECN bit to 1 in frames traveling in the opposite direction of frames with their FECN bit set. This informs the receiving DTE device that a particular path through the network is congested. The DTE device then can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow-control may be initiated, or the indication may be ignored.
Frame Relay Network Implementation
A common private Frame Relay network implementation is to equip a T1 multiplexer with both Frame Relay and non-Frame Relay interfaces. Frame Relay traffic is forwarded out the Frame Relay interface and onto the data network. Non-Frame Relay traffic is forwarded to the appropriate application or service, such as a private branch exchange (PBX) for telephone service or to a video-teleconferencing application.
A typical Frame Relay network consists of a number of DTE devices, such as routers, connected to remote ports on multiplexer equipment via traditional point-to-point services such as T1, fractional T1, or 56 K circuits. An example of a simple Frame Relay network is shown in Figure 10-3.
Figure 10-3: A simple Frame Relay network connects various devices to different services over a WAN.
The majority of Frame Relay networks deployed today are provisioned by service providers who intend to offer transmission services to customers. This is often referred to as a public Frame Relay service. Frame Relay is implemented in both public carrier-provided networks and in private enterprise networks. The following section examines the two methodologies for deploying Frame Relay.
Frame Relay Frame Formats
To understand much of the functionality of Frame Relay, it is helpful to understand the structure of the Frame Relay frame. Figure 10-4 depicts the basic format of the Frame Relay frame, and Figure 10-5 illustrates the LMI version of the Frame Relay frame.
Flags indicate the beginning and end of the frame. Three primary components make up the Frame Relay frame: the header and address area, the user-data portion, and the frame-check sequence (FCS). The address area, which is 2 bytes in length, is comprised of 10 bits representing the actual circuit identifier and 6 bits of fields related to congestion management. This identifier commonly is referred to as the data-link connection identifier (DLCI). Each of these is discussed in the descriptions that follow.
Standard Frame Relay Frame
Standard Frame Relay frames consist of the fields illustrated in Figure 10-4.
Figure 10-4: Five fields comprise the Frame Relay frame.
The following descriptions summarize the basic Frame Relay frame fields illustrated in Figure 10-4.
Flags---Delimits the beginning and end of the frame. The value of this field is always the same and is represented either as the hexadecimal number 7E or the binary number 01111110.
Address---Contains the following information:
DLCI: The 10-bit DLCI is the essence of the Frame Relay header. This value represents the virtual connection between the DTE device and the switch. Each virtual connection that is multiplexed onto the physical channel will be represented by a unique DLCI. The DLCI values have local significance only, which means that they are unique only to the physical channel on which they reside. Therefore, devices at opposite ends of a connection can use different DLCI values to refer to the same virtual connection.
Extended Address (EA): The EA is used to indicate whether the byte in which the EA value is 1 is the last addressing field. If the value is 1, then the current byte is determined to be the last DLCI octet. Although current Frame Relay implementations all use a two-octet DLCI, this capability does allow for longer DLCIs to be used in the future. The eighth bit of each byte of the Address field is used to indicate the EA.
C/R: The C/R is the bit that follows the most significant DLCI byte in the Address field. The C/R bit is not currently defined.
Congestion Control: This consists of the three bits that control the Frame Relay congestion-notification mechanisms. These are the FECN, BECN, and DE bits, which are the last three bits in the Address field.
Forward-explicit congestion notification (FECN) is a single bit field that can be set to a value of 1 by a switch to indicate to an end DTE device, such as a router, that congestion was experienced in the direction of the frame transmission from source to destination. The primary benefit of the use of the FECN and BECN fields is the ability of higher-layer protocols to react intelligently to these congestion indicators. Today, DECnet and OSI are the only higher-layer protocols that implement these capabilities.
Backward-explicit congestion notification (BECN) is a single bit field that, when set to a value of 1 by a switch, indicates that congestion was experienced in the network in the direction opposite of the frame transmission from source to destination.
Discard eligibility (DE) is set by the DTE device, such as a router, to indicate that the marked frame is of lesser importance relative to other frames being transmitted. Frames that are marked as "discard eligible" should be discarded before other frames in a congested network. This allows for a fairly basic prioritization mechanism in Frame Relay networks.
Data---Contains encapsulated upper-layer data. Each frame in this variable-length field includes a user data or payload field that will vary in length up to 16,000 octets. This field serves to transport the higher-layer protocol packet (PDU) through a Frame Relay network.
Frame Check Sequence---Ensures the integrity of transmitted data. This value is computed by the source device and verified by the receiver to ensure integrity of transmission.
LMI Frame Format
Frame Relay frames that conform to the LMI specifications consist of the fields illustrated in Figure 10-5.
Figure 10-5: Nine fields comprise the Frame Relay that conforms to the LMI format.
The following descriptions summarize the fields illustrated in Figure 10-5.
Flag---Delimits the beginning and end of the frame.
LMI DLCI---Identifies the frame as an LMI frame instead of a basic Frame Relay frame. The LMI-specific DLCI value defined in the LMI consortium specification is DLCI = 1023.
Unnumbered Information Indicator---Sets the poll/final bit to zero.
Protocol Discriminator---Always contains a value indicating that the frame is an LMI frame.
Call Reference---Always contains zeros. This field currently is not used for any purpose.
Message Type---Labels the frame as one of the following message types:
Status-inquiry message: Allows a user device to inquire about the status of the network.
Status message: Responds to status-inquiry messages. Status messages include keep-alives and PVC status messages.
Information Elements---Contains a variable number of individual information elements (IEs). IEs consist of the following fields
IE Identifier: Uniquely identifies the IE.
IE Length: Indicates the length of the IE.
Data: Consists of one or more bytes containing encapsulated upper-layer data.
Frame Check Sequence (FCS)---Ensures the integrity of transmitted data.
