Descartes Fr

7/28/2019 Descartes Fr

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Relais Del Marco

El relais del captulo es un estndar de protocolo para la interred del LAN que proporciona unmtodo rpido y eficiente de transmitir la informacin de un dispositivo del usuario a lospuentes y a las rebajadoras del LAN.El protocolo del relais del captulo utiliza un similar estructurado marco a el de LAPD, exceptoque el jefe del marco es substituido por un campo del jefe del relais del captulo 2-byte. El jefe

del relais del captulo contiene el campo user-specified de DLCI, que es la direccin dedestinacin del bastidor. Tambin contiene las seales de la congestin y del estado que la redenva al usuario.Circuitos VirtualesEl marco del relais del captulo se transmite a su destinacin por los circuitos virtuales(trayectorias lgicas de un punto que origina en la red) a un punto de la destinacin. Loscircuitos virtuales pueden ser permanentes (PVCs) o cambiaron (SVCs). PVCs es instaladoadministrativo por el encargado de red para un punto dedicado para sealar la conexin; SVCsse instala en llamar-por-llama base.Ventajas del relais del marcoEl relais del captulo ofrece un alternativa atractivo las redes dedicadas a lneas y a X.25 paraconectar LANs con los puentes y las rebajadoras. El xito del protocolo del relais del captulose basa en los dos factores subyacentes siguientes:

Porque los circuitos virtuales consumen anchura de banda solamente cuandotransportan datos, muchos circuitos virtuales pueden existir simultneamente a travsde una lnea dada de la transmisin. Adems, cada dispositivo puede utilizar ms de laanchura de banda como necesarios, y funciona as a velocidades ms altas.

La confiabilidad mejorada de las lneas de comunicacin y creciente error-dirigiendo la

sofisticacin en las estaciones del extremo permite que el protocolo del relais delcaptulo deseche marcos errneos y elimine as el proceso de error-direccio'ndesperdiciador de tiempo.

Estos dos factores hacen que el captulo retransmite una opcin deseable para la transmisinde datos; sin embargo, tambin hacen necesario la prueba para determinarse que el sistematrabaja correctamente y que los datos no estn perdidos.

Estructura Del Relais Del MarcoLos estndares para el protocolo del relais del captulo han sido desarrollados por ANSI yCCITT simultneamente. La especificacin separada de LMI se ha incorporado bsicamente enla especificacin del ANSI. La discusin siguiente de la estructura del protocolo incluye lospuntos principales de estas especificaciones.La estructura del marco del relais del captulo se basa en el protocolo de LAPD. En laestructura del relais del captulo, el jefe del marco se altera levemente para contener elidentificador de la conexin de trasmisin de datos (DLCI) y los pedacitos de la congestin, enlugar de los campos normales de la direccin y del control. Este jefe nuevo del relais delcaptulo es 2 octetos en longitud y tiene el formato siguiente:

DLCIel campo de 10-bit DLCI representa la direccin del bastidor y corresponde a un PVC.

C/R

Seala si el marco es un comando o una respuesta.


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EaEl campo de direccin extendido significa hasta dos octetos adicionales en el jefe delrelais del captulo, as grandemente ampliando el nmero de direcciones posibles.

FECNNotificacin explcita delantera de la congestin (vase ECN abajo).

BECNNotificacin explcita posterior de la congestin (vase ECN abajo).

DeDeseche la elegibilidad (vase el DE abajo).

InformacinEl campo de informacin puede incluir otros protocolos dentro de l, tal como unpaquete del X.25, del IP o del SDLC (SNA).

Pedacitos Explcitos De la Notificacin De la Congestin (ECN)

Cuando la red se congestiona al punto que no puede procesar nuevas transmisiones dedatos, comienza a desechar marcos. Se retransmiten estos marcos desechados, ascausando ms congestin. En un esfuerzo de prevenir esta situacin, variosmecanismos se han desarrollado para notificar los dispositivos del usuario en el iniciode la congestin, para poder reducir la carga ofrecida.

Dos pedacitos en el jefe del relais del captulo se utilizan para sealar el dispositivo delusuario que la congestin est ocurriendo en la lnea: Son la notificacin explcitadelantera de la congestin (FECN) mordida y el pedacito explcito posterior de lanotificacin de la congestin (BECN). El FECN se cambia a 1 mientras que un marco seenva ro abajo hacia la localizacin de la destinacin cuando la congestin ocurredurante la transmisin de datos. De esta manera, todos los nodos enes sentidodescendiente y el dispositivo unido del usuario aprenden sobre la congestin en la lnea.El BECN se cambia a 1 en un marco que viaja detrs hacia la fuente de la transmisin dedatos en una trayectoria donde est ocurriendo la congestin. As el nodo de la fuente senotifica para retrasar la transmisin hasta que se desploma la congestin.

Los octetos con los pedacitos de la congestin fijaron segn valor de DLCI


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Gerencia De Capa Consolidada De Acoplamiento (CLLM)

Puede ocurrir que no hay marcos que viajan de nuevo al nodo de la fuente que estcausando la congestin. En este caso, la red desear enviar su propio mensaje al nodo

problemtico de la fuente. El estndar, sin embargo, no permite que la red enve suspropios marcos con el DLCI del circuito virtual deseado.

Para tratar este problema, el ANSI defini la gerencia de capa consolidada deacoplamiento (CLLM). Con CLLM, un DLCI separado (nmero 1023) es reservado paraenviar mensajes del control de la capa del acoplamiento de la red al dispositivo delusuario. El estndar del ANSI (T1.618) define el formato del mensaje de CLLM. Contieneun cdigo para la causa de la congestin y de un listado de todo el DLCIs que debaactuar para reducir su transmisin de datos a una congestin ms baja.

Estado de las conexiones (LMI)

Cada DLCI corresponde a un PVC (circuito virtual permanente). Es a veces necesariotransmitir la informacin sobre esta conexin (e.g., si el interfaz sigue siendo activo) elDLCIs vlido para el interfaz y el estado de cada PVC. Se transmite esta informacinusando el DLCI reservado 1023 o DLCI 0, dependiendo del estndar usado.

Un estado del multicast se puede tambin enviar con el LMI. Multicasting es donde unarebajadora enva un marco en un DLCI reservado conocido como grupo del multicast. Lared despus repliega el marco y lo entrega a una lista predefinida de DLCIs, asdifundiendo un solo marco a una coleccin de destinaciones.

Deseche La Elegibilidad (De)

Cuando hay congestin en la lnea, la red debe decidir a qu marcos a desechar paraliberar la lnea. La elegibilidad del descarte provee de la red una seal de determinarsequ marcos a desechar. La red desechar marcos con un DE value de 1 antes dedesechar otros marcos.

El DE bit puede ser fijado por el usuario en algunos de sus bastidores de la bajo-prioridad. Alternativomente, la red puede fijar al DE bit para indicar a otros nodos que unmarco se debe seleccionar preferencial para el descarte, en caso de necesidad.

Interesado en ms detalles sobre la prueba de este protocolo?

Estndares Del Relais Del Marco

ANSI T1.618

T1.618 describe el protocolo que apoya la fase de transferencia de datos del servicio deportador del relais del captulo, como definido en ANSI T1.606. T1.618 se basa en unsubconjunto de ANSI T1.602 (LAPD) llamado los "aspectos de la base" y es utilizado porllamadas virtuales cambiadas y permanentes.

T1.618 tambin incluye el mecanismo consolidado de la capa del acoplamiento (CLLM).La generacin y el transporte de CLLM es opcionales. Con CLLM, DLCI 1023 esreservado para enviar mensajes del control de la capa del acoplamiento.

T1.618 publica la notificacin implcita de la congestin de la red al dispositivo delusuario. La notificacin de la congestin contiene un cdigo que indica la causa de la


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congestin y enumera todos los DLCIs que tengan que reducir su trfico para bajar lacongestin.

ANSI T1.617

Para establecer una conexin cambiada del circuito virtual (SVC), los usuarios del relais

del captulo deben establecer un dilogo con la red usando la especificacin que sealaen T1.617. El procedimiento da lugar a la asignacin de un DLCI. Una vez que seestablezca el dilogo, los procedimientos T1.618 se aplican.

Para establecer un circuito virtual permanente (PVC), un protocolo de la disposicin seutiliza que es idntico al protocolo del D-canal del ISDN y se define en T1.617.

Con el ISDN, los usuarios pueden utilizar el D-canal para la disposicin. Para losllamadores no-Non-ISDN, no hay D-canal, as que el dilogo entre el usuario y la red sedebe separar de los procedimientos ordinarios de la transferencia de datos. En T1.617,DLCI 0 es reservado.

T1.617 tambin contiene especificaciones en cmo se negocian los parmetros delservicio del relais del captulo.

ANSI LMI

El ANSI LMI es un sistema de gerencia del circuito virtual permanente (PVC) definido enel anexo D de T1.617. El ANSI LMI es virtualmente idntico al LMI de los fabricantes, sinlas extensiones opcionales. El ANSI LMI utiliza DLCI 0.

LMI De los Fabricantes

LMI de los fabricantes es una especificacin del relais del captulo con el extensio'n-

documento nmero 001-208966, de septiembre el 18 de 1990.

LMI de los fabricantes define un servicio genrico del relais del captulo basado en PVCspara interconectar los dispositivos del DTE con el equipo de la red del relais del captulo.Adems del estndar del ANSI, LMI de los fabricantes incluye extensiones y lasfunciones y los procedimientos de LMI. LMI de los fabricantes utiliza DLCI 1023.

Pvc Del Relais NNI Del Marco (FRF.2)

Su analizador tambin apoya descifrar de los bastidores del PVC de la Red-a-Red (NNI)segn el acuerdo FRF.2 de la puesta en prctica del foro del relais del captulo. Elinterfaz de NNI se refiere a la transferencia de la informacin del C-plano y del U-plano

entre dos nodos de red que pertenecen a dos diversas redes del relais del captulo. Talesmarcos son reconocidos automticamente por el analizador y exhibidos correctamente.


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El PVC de NNI descifra

FRF.3

FRF.3 proporciona la encapsulacin multiprotocol sobre el relais del captulo dentro deun marco del ANSI T1.618. La estructura de tal bastidor del relais del captulo es comosigue:

8

7 6 5 4 3 2 1

Octeto

Bandera (tuerca hexagonal 7E) 1

Direccin T1.618(10-bit incluyendo DLCI)

23

Control Q.922 (UI o yo enmarcamos) 4

Cojn Opcional (0x00) 5

NLPID 6

Datos..

7
http://216.239.39.104/translate_c?hl=es&sl=en&u=http://www.protocols.com/pbook/frame.htm&prev=/images/image33.gif


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Secuencia Del Cheque Del Marco n-2n-1

Bandera (tuerca hexagonal 7E) n

Estructura del marco FRF.3

El campo de NLPID (identificacin del protocolo del nivel de red) seala quencapsulacin o qu protocolo sigue. El diagrama siguiente detalla los valores posiblespara NLPID y los protocolos que sean sealados por cada valor. Por ejemplo, un valor de0xCC indica un marco encapsulado del IP.

Relais excesivo del captulo de la encapsulacin multiprotocol

UNI SVC (FRF.4)

FRF.4 es acuerdo cambiado relais del interfaz de la usuario-a-red de la conexin virtualdel captulo. Se aplica con el equipo unido a una red del relais del captulo no-Non-ISDN

o a una red del ISDN usando solamente el caso A.
http://216.239.39.104/translate_c?hl=es&sl=en&u=http://www.protocols.com/pbook/frame.htm&prev=/images/image31.gif


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UNI SVC descifran

Lo que sigue es una lista de los tipos de mensaje vlidos del SVC:

Procedimiento de la llamada. Conecte. Conecte Reconocen.

Desconecte. Progreso. Lanzamiento. Lance completo. Disposicin. Estado. Investigacin del estado.

FRF.5

FRF.5 define el relais del captulo de la interred de la red que conecta sobre la atmsferase refiere ( relais del captulo sobre la atmsfera ) para ms detalles.

FRF.8

FRF.8 define el relais del captulo de la interred del servicio que conecta sobre laatmsfera se refiere (relais del captulo sobre la atmsfera) para ms detalles.

DCP (FRF.9)

FRF. 9RFC 1661

sta es la encapsulacin del relais del captulo del excedente del protocolo de lacompresin de datos (DCP). Se aplica a la informacin innumerable (UI) enmarca
http://216.239.39.104/translate_c?hl=es&u=http://www.protocols.com/pbook/atmenca.htm%23Frame&prev=/search%3Fq%3D%2B%2522DISCARD%2BELIGIBILITY%2522%26hl%3Des%26lr%3D%26sa%3DG%26as_qdr%3Dallhttp://216.239.39.104/translate_c?hl=es&u=http://www.protocols.com/pbook/atmenca.htm%23Frame&prev=/search%3Fq%3D%2B%2522DISCARD%2BELIGIBILITY%2522%26hl%3Des%26lr%3D%26sa%3DG%26as_qdr%3Dallftp://ftp.isi.edu/in-notes/rfc1661.txthttp://216.239.39.104/translate_c?hl=es&sl=en&u=http://www.protocols.com/pbook/frame.htm&prev=/images/image32.gifhttp://216.239.39.104/translate_c?hl=es&u=http://www.protocols.com/pbook/atmenca.htm%23Frame&prev=/search%3Fq%3D%2B%2522DISCARD%2BELIGIBILITY%2522%26hl%3Des%26lr%3D%26sa%3DG%26as_qdr%3Dallftp://ftp.isi.edu/in-notes/rfc1661.txt


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encapsulado usando Q.933 el anexo E [ 9 ] y FRF.3.1 [ 3 ] puede ser utilizada en lasconexiones del relais del captulo que se intertrabajan con la atmsfera usando FRF.5.

DCP se descompone lgicamente en dos subcapas: la subcapa del control de DCP, y lasubcapa de la funcin de DCP.

The Data Compression Protocol (DCP) is encapsulated with Multiprotocol Encapsulationover a Frame Relay network using Q.933 Annex E. A DCP PDU is the combination of aDCP Header and DCP Payload or DCPCP PDU.

DCP PROTOCOL DATA UNIT FORMATThe DCP PDU is used by a DCP entity to communicate data or control information to theremote peer DCP entity. The most significant (earliest sent) octet(s) of the DCP PDU mustbe the DCP Header. The C/D bit of the DCP Header signals whether the DCP PDU is forcontrol or for data.

DCP Data PDU FormatA DCP data PDU encapsulates compressed or uncompressed user data for transport to

the peer DCP entity.

The DCP Header is one or three octets in length and is described by the first two lines ofthe diagram below.

/ 1 2 3 4 5 6 7 8bits

Header C/D Reserved R-R R-A C/U E

\ DCCI(0 or 2 octets)

DCFD Data (payload)(0 to n octets)

DCP header structure

E0 extension1 no extensionThe E bit is always 1 for DCP data PDUs.

C/U0 uncompressed mode

1 for compressed mode

R-A0 no reset acknowledge1 reset acknowledge

R-R0 no reset request1 reset request

C/DShows whether the frame is for control or data.

0 DCP data PDUs


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DCCIZero or two-octet DC Context Identifier.

ReservedReserved for future use. Set to 0.

DCPCP

The DCP Control Protocol (DCPCP) is used to enable, disable, and optionally configure DCP.DCPCP has two modes of operation: Mode-1 and Mode-2. Mode-2 provides full negotiationcapabilities to enable, disable, and configure DCP using the Point-to-Point Protocol (PPP) LinkControl Protocol (LCP) negotiation procedures. Mode-1 uses a subset of the Mode-2 negotiationprimitives with simplified procedures to enable and disable DCP with the default DCFD anddefault parameter values. Mode-1 operation is required; Mode-2 operation is optional.

The length of a DCP Header for DCP control PDUs is one octet.

DCPCP PDUs use the same formats as the PPP LCP as defined in RFC 1661. DCPCP Mode-1

uses a subset of the DCPCP PDU formats (Configure-Request and Configure-Ack with theMode-1 Configuration Option only).

The format of the DCPCP packet consists of the header followed by the PDU which uses thesame format as the PPP LCP as shown below.

Code Identifier Length Data

1 byte 1 byte 2 bytes Variable

DCPCP PDU structure

CodeDecimal value which indicates the type of packet:1 Configure-Request.2 Configure-Ack.3 Configure-Nak.4 Configure-Reject.5 Terminate-Request.6 Terminate-Ack.7 Code-Reject.8 Protocol-Reject.9 Echo-Request.10 Echo-Reply.

11 Discard-Request.12 Link-Quality Report.

IdentifierDecimal value which aids in matching requests and replies.

LengthLength of the packet, including the Code, Identifier, Length and Data fields.

DataVariable length field which may contain one or more configuration options. The format of theconfiguration options is as follows:


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Type Length Revision

DCPCP configuration options

TypeOne-byte indication of the type of the configuration option is set to 254 for mode 1 messages.

LengthLength of the configuration option including the Type, Length and Data fields and is set to 3.

RevisionThe Revision field shall be one octet in length and contains the revision number. The currentrevision is 1.

Mode 1 Request and response messagesThe Mode-1 Request message is a DCPCP Configure-Request packet with the Code field set to1, the Mode-1 Response message is a DCPCP Configure-Ack packet with the Code field set to2.

Mode-2 FormatsMode-2 formats are the same as the LCP packet formats as shown above, with a unique set ofConfigurations options. The LCP packets with codes 1 through 7 are required. The other LCPpackets specified in RFC 1661 and listed above are optional.

NNI SVC (FRF.10)

Your analyzer also supports the decoding of Network-to-Network (NNI) SVC frames accordingto the Frame Relay Forum implementation agreement FRF.10. This implementation agreement

applies to SVCs over Frame Relay NNIs and to SPVCs. It is applicable at NNIs whether bothnetworks are private, both are public or one is private and the other public. Such frames areautomatically recognized by the analyzer and correctly displayed.

FRF.11

Frame Relay is now a major component of many network designs. The protocol provides aminimal set of switching functions to forward variable sized data payloads through a network.The basic frame relay protocol, described in the Frame Relay Forum User to Network (UNI) andNetwork to Network (NNI) Implementation Agreements, has been augmented by additionalagreements which detail techniques for structuring application data over the basic Frame Relayinformation field. These techniques enabled successful support for data applications such as

LAN bridging, IP routing, and SNA.

FRF.11 extends Frame Relay application support to include the transport of digital voicepayloads. Specifically FRF.11 addresses the following requirements:

Transport of compressed voice within the payload of a Frame Relay frame.

Support a diverse set of voice compression algorithms.

Effective utilization of low-bit rate Frame Relay connections.

Multiplexing of up to 255 sub-channels on a single frame relay DLCI.

Support of multiple voice payloads on the same or different sub-channel within a single

frame.

Support of data sub-channels on a multiplexed Frame Relay DLCI.

FRF.12


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FRF.12 is the Frame Relay Fragmentation Implementation Agreement. Fragmentation queuingreduces both delay and delay variation in Frame Relay networks by dividing large data packetsinto smaller packets and then reassembling the data into the original frames at the destination.This ability is particularly relevant to users who wish to combine voice and other time-sensitiveapplications, such as SNA mission-critical applications, with non-time-sensitive applications orother data on a single Permanent Virtual Circuit (PVC). The main benefit of fragmentation is the

ability to utilize common User to Network Interface (UNI) access lines or Network to NetworkInterface (NNI) lines and/or PVCs for communications combining large data frames and real-time protocols.

Fragmenting frames enhances the utility and uniformity of Frame Relay networks, reducingdelay and delay variation while upgrading application responsiveness, quality and reliability. Asa result, multiple types of traffic, such as voice, fax and data, can be transparently combined ona single UNI, NNI and/or PVC.

The Fragmentation Implementation Agreement provides for transmission of Frame Relay DataTerminal Equipment (DTE) and Data Communications Equipment (DCE) with the ability tofragment long data frames into a sequence of shorter frames that are then reassembled into theoriginal frame by the receiving peer DTE or DCE. Frame fragmentation is necessary to control

delay and delay variation when real-time traffic, such as voice, is carried across the sameinterfaces as data traffic. Fragmentation enables the interleaving of delay-sensitive traffic on onePVC with fragments of long data frames on another PVC using the same interface.

FRF.12 supports three fragmentation applications:

1. Locally across a Frame Relay UNI interface between the DTE/DCE peers.2. Locally across a Frame Relay NNI interface between DCE peers.3. End-to-End between two Frame Relay DTEs interconnected by one or more Frame

Relay networks.

When used end-to-end, the fragmentation procedure is transparent to Frame Relay network(s)

between the transmitting and receiving DTEs.

FREther

FREther is a variant of Frame Relay which is comprised of the Frame Relay header followed bythe EtherType field. This is an additional form of encapsulation over Frame Relay which is usedby some customers.

Interested in more details about testing this protocol?

Timeplex (BRE2)

BRE (Bridge Relay Encapsulation) is a proprietary Ascom Timeplex protocol that extendsbridging across WAN links by means of encapsulation. BRE2 is an improved form of thestandard, providing better performance due to the fact that it sits directly on the link layerprotocol, requires less configuration and provides its own routing protocol. BRE2 was deployedin 4.0 router software and is available in all 4.x and 5.x versions of the software.

The format of the BRE2 frame is as follows:

1 byte ----------------- 1 byte------------------

Frame Type F Priority


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Source BRE Bridge No.

Source Bridge Domain ID =1

VLAN ID SRB #

Remainder of BRE2 Header

7 bytes in unfragmented format

12 bytes in fragmented format

Data

BRE2 frame format

If F is 0 than the frame is unfragmented and the BRE2 header is 17 bytes long. If F is 1, theframe is fragmented and the BRE2 header is 22 bytes long. SRB # is the Source Route BridgeNumber (4 bits).


Cascade

In order to provide a Frame Relay service, Regional Bell Operating Companies (RBOCs) deployCascade switches in multiple LATAs and interconnect them to provide service to customersacross the LATAs, as well as manage the switches in multiple LATAs from a single networkmanagement station.

The trunk header format for the Cascade STDX family of switches conform to ANSI T1.618-1991 ISDN Core Aspects of Frame Protocol for use with Frame Relay Bearer Service.

The Cascade trunk header format is as shown below:

1 2 3 4 8

R C/R Version Reserved

ODE DE BECN FECN VC priority

Channel ID


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User Management data depending

Cascade trunk header format

RReserved.

C/RCommand/Response field bit.

VersionHeader version. Defines the version of the trunk header format for the Cascade STDX family ofswitches. This field is currently set to 0.

ODESet to 1 if the ingress rate is greater than the excess burst size.

DEDiscard eligibility indicator which is implemented based on the definitions specified in ANSIT1.618.

BECNBackward explicit congestion notification according to ANSI T1.618.

FECNForward explicit congestion notification according to ANSI T1.618.

VC priority

Virtual circuit priority. Used to differentiate the sensitive traffic from traffic not sensitive to delays,such as file transfers or batch traffic. The priority may be 1, 2 or 3, 1 being the highest.

For management data, a 5th byte contains information concerning the type of PDU informationto follow. Values are as follows:

0 Call request PDU1 Confirmation PDU2 Rejected PDU3 Clear PDU4 Disrupt PDU5 Hello PDU

6 Hello Acknowledgment PDU7 Defined Path Hello PDU8 Defined Path Hello Acknowledgment PDU


LAPF

The purpose of LAPF is to convey data link service data units between DL-service users in theU-plane for frame mode bearer services across the ISDN user-network interface on B-, D- or H-channels. Frame mode bearer connections are established using either procedures specified in


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Recommendation Q.933 [3] or (for permanent virtual circuits) by subscription. LAPF uses aphysical layer service, and allows for statistical multiplexing of one or more frame mode bearerconnections over a single ISDN B-, D- or H-channel by use of LAPF and compatible HDLCprocedures.

The format of the header is shown in the following illustration:

Defaultaddress field

format

(2 octets)

8 7 6 5 4 3 2 1

Upper DLCI C/REA0

Lower DLCIFECN(Note)

BECN(Note)

DE(Note)

EA1

LAPF address format

Default

address fieldformat

(2 octets)

8 7 6 5 4 3 2 1

Upper DLCI C/R EA0

Lower DLCIFECN(Note)

BECN(Note)

DE(Note)

EA1

LAPF address format

EAAddress field extension bit.

C/R

Command Response bit.

FECNForward Explicit Congestion Notification.

BECNBackward Explicit Congestion Notification.

DLCIData Link Connection Identifier.

DE

Discard Eligibility indicator.

D/CDLCI or DL-CORE control indicator.

N(S)Transmitter send sequence number.

N(R)Transmitter receive sequence number.

P/F

Poll bit when used as a command, final bit when used as a response.


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XReserved and set to 0.

SuSupervisory function bit.

MModifier function bit.


Multiprotocol over Frame Relay

RFC 1490http://www.cis.ohio-state.edu/htbin/rfc/rfc1490.htmlRFC 2427http://www.cis.ohio-state.edu/htbin/rfc/rfc2427.html

Multiprotocol over Frame Relay is a method of encapsulating various LAN protocols over FrameRelay. In this case, all protocols encapsulate their packets within a Q.922 Annex A frame.Additionally, frames must contain information necessary to identify the protocol carried withinthe protocol data unit (PDU), thus allowing the receiver to properly process the incoming packet.The format of such frames is as shown in the following illustration:

Flag (7E Hex)

Q.922 address

Control

Optional Pad (0x00)

NLPID

data

FCS

Flag (7E Hex)

Muliprotocol over Frame Relayframe

Q.922 address2-octet address field containing the 10-bit DLCI field. On some networks the Q.922 address mayoptionally contain 3 or 4 octets.
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ControlQ.922 control field. The UI value is of 0x03 is used unless negotiated otherwise. The use of XID(0xAF or 0xBF) is permitted.

PadUsed to align the remainder of the frame to a two octet boundary. There may be 0 or 1 pad octet

within the pad field. The value is always 0.

NLPIDNetwork Level Protocol ID, adminstered by ISO and CCITT. Identifies the encapsulatedprotocol.

FCS2-byte frame check sequence.

There are two basic types of data packets that travel within the Frame Relay network: routedpackets and bridged packets. These packets have distinct formats and therefore, must containan indicator that the destination may use to correctly interpret the contents of the frame. This

indicator is embedded within the NLPID and SNAP header information.

For those protocols that do not have a NLPID already assigned, it is necessary to provide amechanism to allow easy protocol identification. There is a NLPID value defined indicating thepresence of a SNAP header. The format of the SNAP header is as follows:

Organizationally Unique Identifier(OUI)

Protocol Identifier (PID)

3 bytes 2 bytes

SNAP header

All stations must be able to accept and properly interpret both the NLPID encapsulation and theSNAP header encapsulation for a routed packet.

The three-octet Organizationally Unique Identifier (OUI) identifies an organization whichadministers the meaning of the Protocol Identifier (PID) which follows. Together they identify adistinct protocol. Note that OUI 0x00-00-00 specifies that the following PID is an Ethertype.

Some protocols have an assigned NLPID, but because the NLPID numbering space is solimited, not all protocols have specific NLPID values assigned to them. When packets of suchprotocols are routed over Frame Relay networks, they are sent using the NLPID 0x80 followedby SNAP. If the protocol has an Ethertype assigned, the OUI is 0x00-00-00 (which indicates anEthertype follows), and PID is the Ethertype of the protocol in use. There is one pad octet toalign the protocol data on a two octet boundary.

The second type of Frame Relay traffic is bridged packets. These are encapsulated using theNLPID value of 0x80 indicating SNAP. As with other SNAP encapsulated protocols, there is onepad octet to align the data portion of the encapsulated frame. The SNAP header which followsthe NLPID identifies the format of the bridged packet. The OUI value used for this encapsulationis the 802.1 organization code 0x00-80-C2. The PID portion of the SNAP header (the two bytesimmediately following the OUI) specifies the form of the MAC header, which immediately followsthe SNAP header. Additionally, the PID indicates whether the original FCS is preserved withinthe bridged frame.

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