Frame Relay Local Management Interface
The Local Management Interface (LMI) is a set of enhancements to the basic Frame Relay specification. The LMI was developed in 1990 by Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment Corporation. It offers a number of features (called extensions) for managing complex internetworks. Key Frame Relay LMI extensions include global addressing, virtual circuit status messages, and multicasting.
The LMI global addressing extension gives Frame Relay data-link connection identifier (DLCI) values global rather than local significance. DLCI values become DTE addresses that are unique in the Frame Relay WAN. The global addressing extension adds functionality and manageability to Frame Relay internetworks. Individual network interfaces and the end nodes attached to them, for example, can be identified by using standard address-resolution and discovery techniques. In addition, the entire Frame Relay network appears to be a typical LAN to routers on its periphery.
LMI virtual circuit status messages provide communication and synchronization between Frame Relay DTE and DCE devices. These messages are used to periodically report on the status of PVCs, which prevents data from being sent into black holes (that is, over PVCs that no longer exist).
The LMI multicasting extension allows multicast groups to be assigned. Multicasting saves bandwidth by allowing routing updates and address-resolution messages to be sent only to specific groups of routers. The extension also transmits reports on the status of multicast groups in update messages.
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-Kb 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 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.
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 10-5 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 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.
–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 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 3 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.
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 capability 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 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 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
The LMI global addressing extension gives Frame Relay data-link connection identifier (DLCI) values global rather than local significance. DLCI values become DTE addresses that are unique in the Frame Relay WAN. The global addressing extension adds functionality and manageability to Frame Relay internetworks. Individual network interfaces and the end nodes attached to them, for example, can be identified by using standard address-resolution and discovery techniques. In addition, the entire Frame Relay network appears to be a typical LAN to routers on its periphery.
LMI virtual circuit status messages provide communication and synchronization between Frame Relay DTE and DCE devices. These messages are used to periodically report on the status of PVCs, which prevents data from being sent into black holes (that is, over PVCs that no longer exist).
The LMI multicasting extension allows multicast groups to be assigned. Multicasting saves bandwidth by allowing routing updates and address-resolution messages to be sent only to specific groups of routers. The extension also transmits reports on the status of multicast groups in update messages.
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-Kb 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 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.
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 10-5 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 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.
–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 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 3 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.
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 capability 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 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 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
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