Download Frame Relay: Functionality, Implementation, and LMI and more Study Guides, Projects, Research Communication in PDF only on Docsity!
C H A P T E R
Internetworking Technologies Handbook 1-58705-001-
Chapter Goals
- Describe the history of Frame Relay. - Describe how Frame Relay works. - Describe the primary functionality traits of Frame Relay. - Describe Frame Relay network implementation. - Describe the format of Frame Relay frames.
Frame Relay
Introduction
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. The following two techniques are used in packet-switching technology:
- Variable-length packets - Statistical multiplexing Variable-length packets are used for more efficient and flexible data transfers. These packets are switched between the various segments in the network 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 last 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.
Internetworking Technologies Handbook 1-58705-001-
Frame Relay Devices
Frame Relay Standardization
Initial proposals for the standardization of Frame Relay were presented to the Consultative Committee on International Telephone and Telegraph (CCITT) in 1984. Because of 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, Digital Equipment Corporation (DEC), 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 it 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 Telecommunication Union—Telecommunications Standards Section (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 the following two general categories:
- Data terminal equipment (DTE) - 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.
Internetworking Technologies Handbook 1-58705-001-
Frame Relay Virtual Circuits
- Call setup —The virtual circuit between two Frame Relay DTE devices is established. - Data transfer —Data is transmitted between the DTE devices over the virtual circuit. - Idle —The connection between DTE devices is still active, but no data is transferred. If an SVC remains in an idle state for a defined period of time, the call can be terminated. - Call termination —The virtual circuit between DTE devices is terminated. After the virtual circuit is terminated, the DTE devices must establish a new SVC if there is additional data to be exchanged. It is expected that SVCs will be established, maintained, and terminated using the same signaling protocols used in ISDN. Few manufacturers of Frame Relay DCE equipment support switched virtual circuit connections. Therefore, their actual deployment is minimal in today’s Frame Relay networks. Previously not widely supported by Frame Relay equipment, SVCs are now the norm. Companies have found that SVCs save money in the end because the circuit is not open all the time.
Permanent Virtual Circuits
Permanent virtual circuits (PVCs) are permanently established connections that are used for frequent and consistent data transfers between DTE devices across the Frame Relay network. Communication across a PVC does not require the call setup and termination states that are used with SVCs. PVCs always operate in one of the following two operational states:
- Data transfer —Data is transmitted between the DTE devices over the virtual circuit. - Idle —The connection between DTE devices is active, but no data is transferred. Unlike SVCs, PVCs will not be terminated under any circumstances when in an idle state. DTE devices can begin transferring data whenever they are ready because the circuit is permanently established.
Data-Link Connection Identifier
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 their values are unique in the LAN, but not necessarily in the Frame Relay WAN. Figure 10-2 illustrates how two different DTE devices can be assigned the same DLCI value within one Frame Relay WAN.
Internetworking Technologies Handbook 1-58705-001-
Congestion-Control Mechanisms
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 is 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 Discard Eligibility
The Discard Eligibility (DE) bit is used to indicate that a frame has lower importance than other frames. The DE bit is part of the Address field in the Frame Relay frame header. DTE devices can set the value of the DE bit of a frame to 1 to indicate that the frame has lower importance than other frames. When the network becomes congested, DCE devices will discard frames with the DE bit set before discarding those that do not. This reduces the likelihood of critical data being dropped by Frame Relay DCE devices during periods of congestion.
Virtual circuits
Frame Relay DTE network^ DTE
DLCI DLCI 12
62
89
22
36
62
Internetworking Technologies Handbook 1-58705-001-
Frame Relay Network Implementation
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 that 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.
Public Carrier-Provided Networks
In public carrier-provided Frame Relay networks, the Frame Relay switching equipment is located in the central offices of a telecommunications carrier. Subscribers are charged based on their network use but are relieved from administering and maintaining the Frame Relay network equipment and service. Generally, the DCE equipment also is owned by the telecommunications provider. DTE equipment either will be customer-owned or perhaps will be owned by the telecommunications provider as a service to the customer. The majority of today’s Frame Relay networks are public carrier-provided networks.
Router
T1 MUX
T1 MUX
PBX
Token Ring
Video/teleconference
Ethernet
Frame Relay interface
Non-Frame Relay interface
Frame Relay interface
Non-Frame Relay interface
Ethernet
Token Ring
WAN
Internetworking Technologies Handbook 1-58705-001-
Frame Relay Frame Formats
Private Enterprise Networks
More frequently, organizations worldwide are deploying private Frame Relay networks. In private Frame Relay networks, the administration and maintenance of the network are the responsibilities of the enterprise (a private company). All the equipment, including the switching equipment, is owned by the customer.
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- 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 as 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.
8
Field length, in bits
16 Variable 16 8
Flags Address^ Data FCS Flags
Internetworking Technologies Handbook 1-58705-001-
Summary
- 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 Typ e—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 keepalives 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 1 or more bytes containing encapsulated upper-layer data. - Frame Check Sequence (FCS) —Ensures the integrity of transmitted data.
Summary
Frame Relay is a networking protocol that works at the bottom two levels of the OSI reference model: the physical and data link layers. It is an example of packet-switching technology, which enables end stations to dynamically share network resources. Frame Relay devices fall into the following two general categories:
- Data terminal equipment (DTEs), which include terminals, personal computers, routers, and bridges - Data circuit-terminating equipment (DCEs), which transmit the data through the network and are often carrier-owned devices (although, increasingly, enterprises are buying their own DCEs and implementing them in their networks) Frame Relay networks transfer data using one of the following two connection types: - Switched virtual circuits (SVCs), which are temporary connections that are created for each data transfer and then are terminated when the data transfer is complete (not a widely used connection) - Permanent virtual circuits (PVCs), which are permanent connections The DLCI is a value assigned to each virtual circuit and DTE device connection point in the Frame Relay WAN. Two different connections can be assigned the same value within the same Frame Relay WAN—one on each side of the virtual connection. In 1990, Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment Corporation developed a set of Frame Relay enhancements called the Local Management Interface (LMI). The LMI enhancements offer a number of features (referred to as extensions) for managing complex internetworks, including the following: - Global addressing - Virtual circuit status messages - Multicasting
Internetworking Technologies Handbook 1-58705-001-
Review Questions
Review Questions
Q— What kind of technology is Frame Relay? A— Packet-switched technology. Q— Name the two kinds of packet-switching techniques discussed in this chapter, and briefly describe each. A— 1. In variable-length switching, variable-length packets are switched between various network segments to best use network resources until the final destination is reached. 2. Statistical multiplexing techniques essentially use network resources in a more efficient way, Q— Describe the difference between SVCs and PVCs. A— A SVC, switched virtual circuit, is created for each data transfer and is terminated when the data transfer is complete. PVC, permanent virtual circuit, is a permanent network connection that does not terminate when the transfer of data is complete. Previously not widely supported by Frame Relay equipment, SVCs are now used in many of today’s networks. Q— What is the data-link connection identifier (DLCI)? A— The DLCI is a value assigned to each virtual circuit and DTE device connection point in the Frame Relay WAN. Two different connections can be assigned the same value within the same Frame Relay WAN—one on each side of the virtual connection. Q— Describe how LMI Frame Relay differs from basic Frame Relay. A— LMI Frame Relay adds a set of enhancements, referred to as extensions, to basic Frame Relay. Key LMI extensions provide the following functionality: global addressing, virtual circuit status messages, and multicasting.
