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.
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.
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 bi-directional
communications 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 (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 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, however, support Switched
Virtual Connections. Therefore, their actual deployment is minimal in
today's Frame Relay networks.
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:
DTE devices can begin transferring data
whenever they are ready because the circuit is permanently established.
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.
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:
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.
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 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.
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.
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.
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. DCE equipment either will be
customer-owned or perhaps owned by the telecommunications provider as a
service to the customer.
The majority of today's Frame Relay
networks are public carrier-provided 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.
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 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.
-
- 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.
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.
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