This document defines common Frame Relay terms.
There are no specific requirements for this document.
This document is not restricted to specific software or hardware
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access line—A communications line (for example, a
circuit) interconnecting a Frame-Relay-compatible device (DTE) to a Frame Relay
switch (DCE). See also “trunk line” below.
access rate (AR)—The data rate of the user access
channel. The speed of the access channel determines how rapidly (the maximum
rate) that the end user can inject data into a Frame Relay network.
American National Standards Institute (ANSI)—A
private, non-profit organization that administers and coordinates the U.S.
voluntary standardization and conformity assessment system by devising and
proposing recommendations for international communications standards. See also
“International Telecommunication Union Telecommunication Standardization
Sector” (ITU-T, formerly Consultative Committee for International Telegraph and
Telephone [CCITT]) below.
backward explicit congestion notification (BECN)—A bit
sent in reverse direction to the data flow. It is set by a Frame Relay network
to notify an interface device (DTE) that congestion avoidance procedures should
be initiated by the sending device.
bandwidth—The range of frequencies, expressed in
kilobits per second (kbps), that can pass over a given data transmission
channel within a Frame Relay network. The bandwidth determines the rate at
which information can be sent through a channel: the greater the bandwidth, the
more information that can be sent in a given amount of time.
bridge—A device that supports LAN-to-LAN
communications. Bridges may be equipped to provide Frame Relay support to the
LAN devices that they serve. A Frame-Relay-capable bridge encapsulates LAN
frames in Frame Relay frames and feeds those Frame Relay frames to a Frame
Relay switch for transmission across the network. A Frame-Relay-capable bridge
also receives Frame Relay frames from the network, strips the Frame Relay frame
off each LAN frame, and passes the LAN frame on to the end device. Bridges are
generally used to connect LAN segments to other LAN segments or to a WAN. They
route traffic on the Layer 2 (L2) LAN protocol (for example, the MAC address),
which occupies the lower sublayer of the LAN Open System Interconnection (OSI)
data-link layer. See also “router” below.
burstiness—In the context of a Frame Relay network,
data that uses bandwidth only sporadically; that is, information that does not
use the total bandwidth of a circuit 100 percent of the time. During pauses,
channels are idle and no traffic flows across them in either direction.
Interactive and LAN-to-LAN data is bursty in nature because it is sent
intermittently. Between data transmissions, the channel experiences idle time
waiting for the DTEs to respond to the transmitted data user’s input and
waiting for the user to send more data.
channel—Generally, channel refers to the user access
channel across which Frame Relay data travels. Within a given T1 or E1 physical
line, a channel can be one of the following, depending upon how the line is
unchannelized—The entire T1 or E1 line is considered
a channel, where the following is true:
The T1 line operates at speeds of 1.536 Mbps and is a single
channel consisting of 24 T1 time slots.
The E1 line operates at speeds of 1.984 Mbps and is a single
channel consisting of 30 or 31 E1 time slots, depending upon the
channelized—The channel is any one of
n time slots within a given line, where the following is
The T1 line consists of any one, or more, channels. Each channel is
any one of 24 time slots. The T1 line operates at speeds in multiples of 56 or
64 Kbps to 1.536 Mbps, with aggregate speed not exceeding
The E1 line consists of one or more channels. Each channel is any
one of 30 or 31 time slots. The E1 operates at speeds in multiples of 64 Kbps
to 1.984 Mbps, with aggregate speed not exceeding 1.984
fractional—The T1 or E1 channel is one of the
following groupings of consecutively or non-consecutively assigned time
n T1 time slots (n � 56
or 64Kbps, where n is equal to 1 to 23 T1 time slots per
n E1 time slots (n �
64Kbps, where n is equal to 1 to 30 time slots per E1
channel service unit (CSU)—An ancillary device needed
to adapt the V.35 interface on a Frame Relay DTE to the T1 (or E1) interface on
a Frame Relay switch. The T1 (or E1) signal format on the Frame Relay switch is
not compatible with the V.35 interface on the DTE; therefore, a CSU or similar
device, placed between the DTE and the Frame Relay switch, is needed to perform
the required conversion.
committed burst size (Bc)—The maximum amount of data
(in bits) that the network agrees to transfer, under normal conditions, during
a time interval Tc. See also “excess burst size (Be)” below.
Consultative Committee for International Telegraph and
Telephone (CCITT)—See “International Telecommunication Union
Telecommunication Standardization Sector (ITU-T)” below.
committed information rate (CIR)—The rate at which a
Frame Relay network agrees to transfer information under normal conditions,
averaged over time interval Tc. CIR, measured in bits per second (bps), is one
of the key negotiated tariff metrics.
committed rate measurement interval (Tc)—The time
interval during which the user can send only Bc-committed amount of data and
Be-excess amount of data. In general, the duration of Tc is proportional to the
burstiness of the traffic. Tc is computed (from the subscription parameters of
CIR and Bc) with the formula Tc = Bc � CIR. Tc is not a periodic time interval.
Instead, it is used only to measure incoming data, during which it acts like a
sliding window. Incoming data triggers the Tc interval, which continues until
it completes its commuted duration. See also “committed information rate (CIR)”
and “committed burst size (Bc)” above.
cyclic redundancy check (CRC)—A computational means to
ensure the accuracy of frames transmitted between devices in a Frame Relay
network. The mathematical function is computed, before the frame is
transmitted, at the originating device. Its numerical value is computed based
on the content of the frame. This value is compared with a re-computed value of
the function at the destination device. There is no limit to the size of the
frame to which the CRC can be applied; when the frame length increases,
however, so does the probability that an undetected error may occur. Frame
Relay uses CRC-16, a 16-bit Frame Check Sequence (FCS) that will detect all
types of bit errors for frames less than 4096 bytes in length. As the frames
get larger, rare erroroneous bit patterns can occur that the CRC-16 will not
detect. See also “frame check sequence (FCS)” below.
data communications equipment (DCE)—Defined by both
the Frame Relay and the X.25 committees, DCE applies to switching equipment and
is distinguished from devices that attach to the network (DTE). See also “end
data-link connection identifier (DLCI)—A unique number
assigned to a permanent virtual circuit (PVC) end point in a Frame Relay
network. Identifies a particular PVC endpoint within a user’s access channel in
a Frame Relay network and has local significance only to that channel.
discard eligibility (DE)—A user-set bit indicating
that a frame may be discarded in preference to other frames if congestion
occurs, to maintain the committed quality of service within the network. The
network side can also set the DE bit and, on congestion, will first drop frames
that have this DE bit set. Frames with the DE bit set are considered
“Be-excess” data. See also “excess burst size (Be)” below.
E1—Transmission rate of 2.048 Mbps on E1
communications lines. An E1 facility carriers a 2.048 Mbps digital signal. See
also T1 below and Channel above.
egress—Frame Relay frames which leave a Frame Relay
network heading toward the destination device. Contrast with “ingress”
end device—The ultimate source or destination of data
flowing through a Frame Relay network, sometimes referred to as Data Terminal
Equipment (DTE). As a source device, it sends data to an interface device for
encapsulation in a Frame Relay frame. As a destination device, it receives
de-encapsulated data from the interface device (in other words, the Frame Relay
frame is stripped off, leaving only the user’s data). An end device can be an
application program or some operator-controlled device (for example, a
workstation). In a LAN environment, the end device can be a file server or a
host. See also “data communications equipment (DCE)” above.
encapsulation—A process by which an interface device
places the protocol-specific frames of an end device inside a Frame Relay
frame. The network accepts only those frames formatted specifically for Frame
Relay; hence, devices acting as interfaces to a Frame Relay network must
perform encapsulation. See also “interface device” or “Frame-Relay-capable
interface device” below.
excess burst size (Be)—The maximum amount of
uncommitted data (in bits) in excess of Bc that a Frame Relay network can
attempt to deliver during a time interval Tc. Generally, Be data is delivered
with a lower probability than Bc, and the network treats it as discard
eligible. See also “committed burst size (Bc)” above.
file server—In the context of Frame Relay network
supporting LAN-to-LAN communications, a device connecting a series of
workstations within a given LAN. The device performs error recovery and flow
control functions, as well as end-to-end acknowledgment of data during data
transfer, thereby significantly reducing overhead within the Frame Relay
forward explicit congestion notification (FECN)—A bit
sent in the same direction as the data flow. It is set by a Frame Relay network
to notify an interface device (DTE) that congestion avoidance procedures should
be initiated by the receiving device. See also “backward explicit congestion
notification (BECN)” above.
frame check sequence (FCS)—A 16-bit field for the CRC
used in High-Level Data Link Control (HDLC) and Frame Relay frames. The FCS is
used to detect bit errors that may occur during transmission of the frame. The
bits between the opening flag and the FCS are checked. See also “cyclic
redundancy check (CRC)” above.
Frame-Relay-capable interface device—A communications
device that performs encapsulation. Frame-Relay-capable routers and bridges are
examples of interface devices used to interface the customer’s equipment to a
Frame Relay network. See also “interface device” below and “encapsulation”
Frame Relay frame—A variable-length unit of data, in
Frame Relay format, that is transmitted through a Frame Relay network as pure
data. Contrast with “packet” below. See also “Q.922 Annex A (Q.992A)”
Frame Relay network—A telecommunications network based
on Frame Relay technology. Data is multiplexed. Contrast with “packet-switching
high-level data link control (HDLC)—A generic
link-level communications protocol developed by the International Organization
for Standardization (ISO). HDLC manages synchronous, code-transparent, serial
information transfer over a link connection. See also “Synchronous Data Link
Control (SDLC)” below.
hop—A single trunk line between two switches in a
Frame Relay network. An established PVC consists of a certain number of hops,
spanning the distance from the ingress access interface to the egress access
interface within the network.
host computer—A communications device that enables
users to run applications to perform such functions as text editing, program
execution, access to databases, and so on.
ingress—Frame Relay frames heading from an access
device toward the Frame Relay network. Contrast with “egress” above.
interface device—A device that provides the interface
between the end device (or devices) and a Frame Relay network by encapsulating
the user’s native protocol in Frame Relay frames and sending the frames across
the Frame Relay backbone. See also “encapsulation” and “Frame-Relay-capable
interface device” above.
International Telecommunication Union Telecommunication
Standardization Sector (ITU-T)—A standards organization that devises
and proposes recommendations for international communications. Formerly known
as Comite Consultatif International Telegraphique et Telephonique (CCITT). See
also “American National Standards Institute (ANSI)” above.
Link Access Procedure, Balanced (LAPB)—The
balanced-mode, enhanced version of HDLC used in X.25 packet-switching networks.
Contrast with “Link Access Procedure on the D-channel (LAPD)” below.
Link Access Procedure on the D-channel (LAPD)—A
protocol that operates at the data-link layer (L2) of the OSI architecture.
LAPD is used to convey information between Layer 3 (L3) entities across the
Frame Relay network. The D-channel carries signaling information for circuit
switching. Contrast with “Link Access Procedure, Balanced (LAPB)” above.
local area network (LAN)—A privately owned network
that offers high-speed communications channels to connect information
processing equipment in a limited geographic area.
LAN protocols—A range of LAN protocols supported by a
Frame Relay network, including Transmission Control Protocol/Internet Protocol
(TCP/IP), Apple Talk, Xerox Network System (XNS), Internetwork Packet Exchange
(IPX), and Common Operating System used by DOS-based PCs.
LAN segment—In the context of a Frame Relay network
supporting LAN-to-LAN communications, a LAN linked to another LAN by a bridge.
Bridges enable two LANs to function like a single, large LAN by passing data
from one LAN segment to another. To communicate with each other, the bridged
LAN segments must use the same native protocol. See also “bridge” above.
Local Management Interface (LMI)—A set of enhancements
to the basic Frame Relay specification. LMI includes support for a keepalive
mechanism, which verifies that data is flowing, and for a status mechanism,
which provides an on-going status report on the DLCIs known to the switch.
There are three types of LMI: The Frame Relay Forum’s LMI, ANSI T1.617 (Annex
D), and CCITT Q922 (Annex A).
packet—A group of fixed-length binary digits—including
the data and call control signals—that are transmitted as a composite whole
through an X.25 packet-switching network. The data, call control signals, and
possible error control information are arranged in a predetermined format.
Packets do not always travel the same pathway; rather, they are arranged in
proper sequence at the destination side before forwarding the complete message
to an addressee. Contrast with “Frame Relay frame” above.
packet-switching network—A telecommunications network
based on packet-switching technology, wherein a transmission channel is
occupied only for the duration of the transmission of the packet. Contrast with
“Frame Relay network” above.
parameter—A numerical code that controls an aspect of
terminal or network operation, such aspects as page size, data transmission
speed, and timing options.
permanent virtual circuit (PVC)—A Frame Relay logical
link whose endpoints and class of service are defined by network management.
Analogous to an X.25 permanent virtual circuit, a PVC consists of the
originating Frame Relay network element address, originating data-link control
identifier, terminating Frame Relay network element address, and termination
data-link control identifier. “Originating” refers to the access interface from
which the PVC is initiated. “Terminating” refers to the access interface at
which the PVC stops. Many data network customers require a PVC between two
points. DTE that needs continuous communication uses PVCs. See also “data-link
connection identifier (DLCI)” above.
Q.922 Annex A (Q.992A)—The international draft
standard, based on the Q.922A frame format developed by the ITU-T, that defines
the structure of Frame Relay frames. All Frame Relay frames entering a Frame
Relay network automatically conform to this structure. Contrast with “Link
Access Procedure, Balanced (LAPB)” above.
Q.922A frame—A variable-length unit of data, formatted
in Frame Relay (Q.922A) format, that is transmitted through a Frame Relay
network as pure data (that is, it contains no flow control information).
Contrast with “packet” above. See also “Frame Relay frame” above.
router—A device that supports LAN-to-LAN
communications. Routers may be equipped to provide Frame Relay support to the
LAN devices they serve. A Frame-Relay-capable router encapsulates LAN frames in
Frame Relay frames and feeds those Frame Relay frames to a Frame Relay switch
for transmission across the network. A Frame-Relay-capable router also receives
Frame Relay frames from the network, strips the Frame Relay frame off each
frame to product the original LAN frame, and passes the LAN frame on to the end
device. Routers connect multiple LAN segments to each other or to a WAN.
Routers route traffic on the L3 LAN protocol (for example, the IP address). See
also “bridge” above.
statistical multiplexing—A method of interleaving the
data input of two or more devices on a single channel or access line for
transmission through a Frame Relay network. Interleaving of data is
accomplished using the DLCI.
switched virtual circuit (SVC)—A virtual circuit that
is dynamically established on demand and is torn down when transmission is
complete. SVCs are used in situations where data transmission is sporadic.
Called a switched virtual connection in ATM terminology.
Synchronous Data Link Control (SDLC)—A link-level
communications protocol used in an International Business Machines (IBM)
Systems Network Architecture (SNA) network which manages synchronous,
code-transparent, serial information transfer over a link connection. SDLC is a
subset of the more generic HDLC protocol developed by the ISO.
T1—Transmission rate of 1.544 Mbps on T1
communications lines. A T1 facility carriers a 1.544 Mbps digital signal. Also
referred to as digital signal level 1 (DS-1). See also “E1” and “channel”
trunk line—A communications line connecting two Frame
Relay switches to each other.