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Frame
Relay is a packet-switched technology that uses bridges, routers, or FRADs (Frame-Relay
access devices). These devices aggregate and convert data into Frame-Relay packets
at - 56kbps, FT1, T1 speeds.
Frame
Relay Technology
Today's
LANs and computing equipment have the potential to run at much higher speeds and
transfer very large quantities of data. With the diversity and complexity of today's
networks, management can be a mammoth task if you don't have the proper tools.
Each environment is a unique combination of equipment from different vendors.
Frame Relay, which is a relatively new wide area networking method, is gaining
in popularity. It uses a packet-switching technology, similar to X.25, but is
more efficient. As a result, it can make your networking quicker, simpler, and
less costly.
Frame
Relay was developed to solve communication problems that other protocols could
not: the increased need for higher speeds, an increased need for large bandwidth
efficiency, particularly for clumping ("bursty" traffic), an increase
in intelligent network devices that lower protocol processing, and the need to
connect LANs and WANs.
Like
X.25, Frame Relay is a packet-switched protocol. But the Frame-Relay process is
streamlined. There are significant differences that make Frame Relay a faster,
more efficient form of networking. A Frame-Relay network doesn't perform error
detection, which results in a considerably smaller amount of overhead and faster
processing than X.25. Frame Relay is also protocol independent-it accepts data
from many different protocols. This data is encapsulated by the Frame-Relay equipment,
not the network.
The
intelligent network devices connected to a Frame-Relay network are responsible
for the error correction and frame formatting. Processing time is minimized, so
the transmission of data is much faster and more efficient.
In
addition, Frame Relay is entirely digital, which reduces the chance of error and
offers excellent transmission rates. Frame Relay typically operates at 56 kbps
to 1.544 mbps.
What
does Frame Relay do?
Frame
Relay sends information in packets called frames through a shared Frame-Relay
network. Each frame contains all the information necessary to route it to the
correct destination. So in effect, each endpoint can communicate with many destinations
over one access link to the network. And instead of being allocated a fixed amount
of bandwidth, Frame-Relay services offer a CIR (committed information rate) at
which data is transmitted. But if traffic and your service agreement allow, data
can burst above your committed rate.
Why
choose Frame Relay?
Since
Frame Relay has a low overhead, it's a perfect fit for today's complex networks.
You get several clear benefits: First, multiple logical connections can be sent
over a single physical connection, reducing your internetworking costs. By reducing
the amount of processing required, you get improved performance and response time.
And because Frame Relay uses a simple link layer protocol, your equipment usually
requires only software changes or simple hardware modifications, so you don't
have to invest a lot of money to upgrade your system.
Since
Frame Relay is protocol independent, it can process traffic from different networking
protocols like IP, IPX, and SNA.
Frame
Relay is an ideal choice for connecting Wide Area Networks (WANs) that have unpredictable,
high-volume, and bursty traffic. Typically, these applications include data transfer,
CAD/CAM, and client-server applications.
Frame
Relay also offers advantages for interconnecting WANs. In the past, setting up
WANs required the use of private lines or circuit switching over a leased line.
Single, dedicated lines are not needed to make each WAN-to-WAN connection with
Frame Relay, reducing costs.
Permanent
Virtual Circuits.
Essentially,
a permanent virtual circuit (PVC) is your dedicated connection through the shared
Frame-Relay network replacing a dedicated end to-end line. A PVC is needed for
each site in the network, just as a private line is. But in a Frame Relay network,
the bandwidth is shared among multiple users. So any single site can communicate
with any other single site without the need for multiple dedicated lines.
PVCs
function via a Local Management Interface (LMI), which provides control procedures.
The control procedures function in three ways: link integrity verification initiated
by the user device, network status report giving details of all PVCs, and network
notification of whether a PVC's status changes from active to inactive or vice
versa. Data-Link Connections (DLCs) are PVCs pre-configured by both sides of the
connection. The DLC identifier (DLCI) is used as the logical address for frame-layer
multiplexing.
What
do I need to get started?
First,
you need a Frame-Relay Bearer Service (FRBS), which is offered by the local telephone
company. You'll sign up for a committed information rate (CIR), which might be
64 kbps. That means you're guaranteed the data will go through your PVC at this
rate. But, depending on network traffic and what type of line you have, such as
a fractional T1 line capable of 128 kbps, you may actually get higher transmission
rates thanks to 2-second bursts of speed across the network. At peak times when
there is a lot of congestion, you may only transmit at 64 kbps.
Next,
you need Frame-Relay equipment. Since Frame Relay doesn't provide protocol conversion
and error detection/correction, the end-user devices need to be intelligent. Typically
you can access the Frame-Relay service through Frame-Relay devices, such as Frame-Relay
Assembler/Disassemblers (FRADs), frame routers, bridges, or switches.
Frame
Routers.
Frame
routers translate existing data communications protocols for transmission over
a Frame-Relay network, then route the data across the network to another frame
router or other Frame-Relay compatible device. Frame routers can handle many types
of protocols, including LAN protocols. They're used in environments that require
T1 or slower network access speeds. Each router supports one of many physical
data interfaces and can provide several user ports.
Bridges,
Routers, and FRAIDs.
You
can also use bridges, routers, or FRADs (Frame-Relay access devices). These devices
aggregate and convert data into Frame-Relay packets.
Bridges
are easy to configure and maintain, and they usually connect a branch off ice
to a hub location.
Routers
can handle traffic from other WAN protocols, re-route a connection if a line fails,
or provide support for flow control and congestion control.
FRADs
format outgoing data into the format required by a Frame-Relay network, and some
even function as routers. They work well in applications where a site already
has bridges and routers or when sending mainframe traffic over a Frame-Relay network.
What's
next for Frame Relay?
While
Frame Relay offers many benefits, a host of problems have to be overcome before
it can be used effectively as a carrier for voice, fax, or video traffic. Until
recently, the advancements were vendor-specific solutions that offered no interoperability.
Recently ratified industry standards have addressed such issues as compression,
packetization, and prioritization. This move towards standardization has been
led by the Frame-Relay Forum (FRF) and the International Telegraphic Union (ITU).
Compression-
in
February 1998, the ITU ratified an umbrella standard for simultaneous transmission
of voice, data, and video traffic over I P. Known as H.323, this standard incorporates
other newly adopted criteria, such as G.729 and G.723. These standards specify
algorithms for compressing voice traffic (which usually travels over a full 64-kbps
telco circuit) down to 8 kbps for (Voice Over Frame Relay) VOFR.
Packetization-
The
FRF recently agreed to ratify two new procedures for VOFR. FRFA 1 specifies a
process for connecting PBXs over Frame Relay to carry voice, data, and fax traffic
over one PVC. FRF. 12 addresses packetization and (consequently) prioritization.
It standardizes a procedure for Frame Relay to break down larger frames into a
series of smaller ones.
This
technique helps alleviate network congestion problems that occur during peak usage
periods when larger data blocks queue up ahead of time-sensitive voice traffic.
In lieu of a formal (Quality of Service) QoS protocol, such as that implemented
by asynchronous transfer mode (ATM), FRFA 2 relies on smaller-sized packets to
ensure predictable delay patterns and therefore maintain the quality and integrity
of voice transmissions. Instead of traffic-snarling data packets clogging up the
circuitry, smaller, fragmented data frames are interleaved with delay-sensitive
traffic, reducing jitter and delay and clearing the path for voice calls.
Prioritization-
Currently,
(resource reservation protocol) RSVP is the only industry standard specifically
designed to support traffic prioritization. While RSVP is rather limited compared
to ATM's QoS capabilities, it is a dynamic mechanism that helps keep traffic flowing
by activating automatically whenever voice packets are present on the line.
The
future.
As
new standards continue to emerge, we predict you'll see more VOFR in data centers
that rely heavily on international communications, where the potential for savings
looms largest
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.
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.
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.
Frame
Relay Virtual Circuits
Frame
Relay provides connection-oriented data link layer communication. This means that
a defined communication exists between each pair of devices and that these connections
are associated with a connection identifier. This service is implemented by using
a Frame Relay virtual circuit, which is a logical connection created between two
data terminal equipment (DTE) devices across a Frame Relay packet-switched network
(PSN).
Virtual
circuits provide a bidirectional communication path from one DTE device to another
and are uniquely identified by a data-link connection identifier (DLCI). A number
of virtual circuits can be multiplexed into a single physical circuit for transmission
across the network. This capability often can reduce the equipment and network
complexity required to connect multiple DTE devices.
A
virtual circuit can pass through any number of intermediate DCE devices (switches)
located within the Frame Relay PSN.
Frame
Relay virtual circuits fall into two categories: switched virtual circuits (SVCs)
and permanent virtual circuits (PVCs).
Switched Virtual Circuits
Switched
virtual circuits (SVCs) are temporary connections used in situations requiring
only sporadic data transfer between DTE devices across the Frame Relay network.
A communication session across an SVC consists of the following four operational
states:
•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.
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.
Frame Relay Error Checking
Frame
Relay uses a common error-checking mechanism known as the cyclic redundancy check
(CRC). The CRC compares two calculated values to determine whether errors occurred
during the transmission from source to destination. Frame Relay reduces network
overhead by implementing error checking rather than error correction. Frame Relay
typically is implemented on reliable network media, so data integrity is not sacrificed
because error correction can be left to higher-layer protocols running on top
of Frame Relay.
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
