Frame
Relay Terms
Access Line
A communications line (e.g.
circuit) interconnecting a frame-relay-compatible device(DTE) to a frame-relay
switch (DCE). See also Trunk Line.
Access Rate (AR)
The data rate of the user
access channel. The speed of the access channel determines how rapidly (maximum
rate) the end user can inject data into a frame relay network.
American National
Standards Institute (ANSI)
Devises and proposes
recommendations for international communications standards. See also Comite
Consultatif International Telegraphique et Telephonique (CCITT).
Backward Explicit
Congestion Notification (BECN)
A bit 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, 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 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 local area network (LAN) segments to other LAN
segments or to a wide area network (WAN). They route traffic on the Level 2 LAN
protocol (e.g., the Media Access Control address), which occupies the lower sub
layer of the LAN OSI data link layer. See also Router.
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, and in between data transmissions the channel experiences
idle time waiting for the DTEs to respond to the transmitted data user's input
of waiting for the user to send more data.
Channel
Generically 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 of how the
line is configured.
Unchannelized
The entire T1/E1 line is
considered a channel, where:
·
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 20 E1 time slots.
Channelized
The channel is any one of N
time slots within a given line, where:
·
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 56/64 Kbps to 1.536 Mbps, with aggregate speed not
exceeding 1.536 Mbps.
·
The E1 line consists of one or more
channels. Each channel is any one of 31 time slots. The E1 line operates at
speeds in multiples of 64 Kbps to 1.984 Mbps, with aggregate speed not exceeding
1.984 Mbps.
Fractional
The T1/E1 channel is one of
the following groupings of consecutively or nonconsecutively assigned time
slots:
·
N T/1 time slots (NX56/64Kbps where N =
1 to 23 T1 time slots per FT1 channel).
·
N E1 time slots (NX64Kbps, where N = 1
to 30 time slots per E1 channel).
Channel Service Unit (CSU)
An ancillary device needed to
adapt the V.35 interface on a F.R. 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).
Comite Consultatif
International Telegraphique et Telephonique (CCITT)
International Consultative
Committee for Telegraphy and Telephony, a standards organization that devises
and proposes recommendations for international communications. See also American
National Standards Institute (ANSI).
Committed Information
Rate (CIR)
The committed rate (in bits
per second) at which the ingress access interface trunk interfaces, and egress
access interface of a frame relay network transfer information to the
destination frame relay end system under normal conditions. The rate is averaged
over a minimum time interval Tc.
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)
as 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).
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 recomputed value of the function at
the destination device. See also Frame Check Sequence (FCS).
Data Communications
Equipment (DCE)
Term defined by both frame
relay and X.25 committees, that applies to switching equipment and is
distinguished from the devices that attach to the network (DTE). Also see DTE.
Data Link Connection
Identifier (DLCI)
A unique number assigned to a
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. Frames
with the DE bit set are considered Be excess data. See also Excess burst Size
(Be).
Egress
Frame relay frames leaving a
frame relay network in the direction toward the destination device. Contrast
with Ingress.
End Device
The ultimate source or
destination of data flowing through a frame relay network sometime referred to
as a 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 (i.e., the frame relay frame is
stripped off, leaving only the user's data) from the interface device. Also see
DCE
NOTE: An end device can be an
application program or some operator-controlled device (e.g., workstation). In a
LAN environment, the end device could be a file server or host.
Encapsulation
A process by which an
interface device places an end device's protocol-specific frames inside a frame
relay frame. The network accepts only frames formatted specifically for frame
relay; hence, interface devices acting as interfaces to an frame relay network
must perform encapsulation. See also Interface device or Frame-Relay-Capable
Interface Device.
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. This data (Be) generally is
delivered with a lower probability than Bc. The network treats Be data as
discard eligible. See also Committed burst Size (Bc).
E1
Transmission rate of 2.048
Mbps on E1 communications lines. An E1 facility carriers a 2.048 Mbps digital
signal. See also T1 and channel.
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 recover and flow
control functions as well as end-to-end acknowledgment of data during data
transfer, thereby significantly reducing overhead within the frame relay
network.
Forward Explicit
Congestion Notification (FECN)
A bit 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 BECN.
Frame Check Sequence (FCS)
The standard 16-bit cyclic
redundancy check used for HDLC and frame relay frames. The FCS detects bit
errors occurring in the bits of the frame between the opening flag and the FCS,
and is only effective in detecting errors in frames no larger than 4096 octets.
See also Cyclic Redundancy Check (CRC).
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 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. See also Q.922A.
Frame Relay Network
A telecommunications network
based on frame relay technology. Data is multiplexed. Contrast with
Packet-Switching Network.
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).
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 programs to perform such functions as text
editing, program execution, access to data bases, etc.
Ingress
Frame relay frames from an
access device toward the frame relay network. Contrast with Egress.
Interface Device
Provides the interface
between the end device(s) 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.
Link Access Procedure
Balanced (LAPB)
The balanced-mode, enhanced,
version of HDLC. Used in X.25 packet-switching networks. Contrast with LAPD.
Link Access Procedure
on the D-channel (LAPD)
A protocol that operates at
the data link layer (layer 2) of the OSI architecture. LAPD is used to convey
information between layer 3 entities across the frame relay network. The
D-channel carries signaling information for circuit switching. Contrast with
LAPB.
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.
Packet
A group of fixed-length
binary digits, including the data and call control signals, that are transmitted
through an X.25 packet-switching network as a composite whole. The data, call
control signals, and possible error control information are arranged in a
predetermined format. Packets do not always travel the same pathway but are
arranged in proper sequence at the destination side before forwarding the
complete message to an addressee. Contrast with Frame Relay Frame.
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.
Parameter
A numerical code that
controls an aspect of terminal and/or network operation. Parameters control 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 (often referred to as 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. Data terminating equipment with a
need for continuous communication use PVCs. See also Data Link Connection
Identifier (DLCI).
Q.922 Annex A (Q.922A)
The international draft
standard that defines the structure of frame relay frames. Based on the Q.922A
frame format developed by the CCITT. All frame relay frames entering a frame
relay network automatically conform to this structure. Contrast with Link Access
Procedure Balanced (LAPB).
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 (i.e., it contains no flow control information
). Contrast with Packet. See also Frame Relay Frame.
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 Level 3 LAN protocol (e.g., the Internet Protocol
address). See also Bridge.
Statistical
Multiplexing
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.
Synchronous Data Link
Control (SDLC)
A link-level communications
protocol used in an International Business Machines (IBM) Systems Network
Architecture (SNA) network that manages synchronous, code-transparent, serial
information transfer over a link connection. SDLC is a subset of the more
generic High-Level Data Link Control (HDLC) protocol developed by the
International Organization for Standardization (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.