Frame Relay and X.25

7/30/2019 Frame Relay and X.25

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WAN Services

A WAN is a Data CommunicationsNetworkoperating beyond a LAN's geographic scope.

You must subscribe to a WAN service provider,

such as a regional Bell operating company(RBOC) to use WAN carrier network services.

A WAN connects the locations of an organizationto: each other locations of other organizations

external services (such as databases)

remote user


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WAN Services

WANs carry many types of , such as:

voice

data

video

WAN technologies function at three layers of the OSI model:

physical

data link

Network


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WAN Services and are the most commonly used

WAN services.

Telephone and data services are connected from thecustomer building point of connection to the WAN

provider's central office (CO). The CO is the local telephone company office to which all

local loops (the cable coming from customer to Co) in thatarea connect.

The services offered by the WAN provider are of 3 maintypes: Call setup (also called signalling)

Time Division Multiplexing (TDM)

Frame Relay


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WAN Services Call setup (also called signalling):

Sets up and clears calls between telephone users. Mostcommonly used call setup is Signaling System 7 (SS7)

Time Division Multiplexing (TDM): Information from many sources has bandwidth allocation on

a single medium. Basic telephone service and ISDN useTDM circuits.

Frame Relay:

Data contained in frames shares bandwidth with other WANFrame Relay subscribers. Frame relay is atelecommunication service designed for cost-efficient datatransmission for intermittent traffic between local areanetworks (LANs) and between end-points in a wide areanetwork (WAN).


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A T1 Internet line is a fast, multi-channel line that offers higherbandwidth and capacity than typical business-class cable or DSLalternatives. A T1 line allows you to transmit voice and data on thesame line at the same time.

Time Division Multiplexing is the process of dividing up one

communication time slot into smaller time slots. We will use theexample of a T1 which is time-division multiplexed at the DS1 rate.A T1 consists of 24 channels.

DS0 64 kilobits per second T1 1.544 megabits per second (24 DS0 lines)

T3 43.232 megabits per second (28 T1s) OC3 155 megabits per second (100 T1s) OC12 622 megabits per second (4 OC3s) OC48 2.5 gigabits per seconds (4 OC12s) OC192 9.6 gigabits per second (4 OC48s)

WAN Services


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CPE, demarc, "last mile", CO switch, toll network The most commonly used terms associated with the main parts

of WAN services:

Customer premises equipment (CPE) -- Devices physically located onthe subscriber's premises.

Demarcation (ordemarc) -- The point at which the CPE ends and thelocal loop portion of the service begins.

Localloop (or "last-mile") -- Cabling (usually copper wiring) thatextends from the demarc into the WAN service provider's central office.

COswitch -- A switching facility that provides the nearest point of

presence for the provider's WAN service. Tollnetwork -- The collective switches and facilities (called trunks)

inside the WAN provider's cloud.


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CPE, demarc, "last mile", CO switch, toll network


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CPE, demarc, "last mile", CO switch, toll network

A key interface is between the data terminal equipment (DTE) and

the data circuit-terminating equipment (DCE).

Typically, the DTE is the router, and the DCE is the device used toconvert the user data from the DTE into a form acceptable to the WAN

service's facility (eg. MODEM, CSU/DSU, TA/NT1).

The WAN path between the DTEs is called the

link

circuit

channel

Line


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Fundamental WAN Devices

The router is the essential WAN device, but it is

also considered a LAN device.

It offers many services, including LAN and

WAN interface ports WAN switches connect to WAN bandwidth for

voice, data, and video communication.

Modems interface voice-grade services

(telephone lines). They include: CSUs/ DSUs devices that interface T1/E1 lines

TA/NT1 devices that interface ISDN services.

Communication servers concentrate dial-in and

dial-out user communication.


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Routers and WAN Switches Routers provide interfaces for a wide range of links and subnetworks at a

wide range of speeds.

A WAN switch is a multiport networking device that operates at the data

link layer of the OSI reference model. A WAN switch typically switches

traffic such as:

Frame Relay X.25

Switched Multimegabit Data Service (SMDS)


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CSU/DSUs on a WANA CSU/DSU is :

A digital-interface device that connects a T1/E1 Sometimes two separate digital devices

Sometimes its integrated into the router Short for ChannelService Unit/Data Service Unit. The CSU is a device

that connects a terminal to a digital line. Typically, the two devices arepackaged as a single unit. The dSU is a device that performs protectiveand diagnostic functions for a telecommunications line. You can think ofit as a veryhigh-powered and expensive modem. Such a device isrequired for both ends of aT-1 or T-3 connection, and the units at bothends must be set to the same communications standard.

Is a hardware device about the size of an external modem that converts aDigital data frame from the communications technology used on a localarea network (LAN) into a frame appropriate to a wide-area network(WAN) and vice versa.


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Frame Relay Operation Generally, the greater the distance covered by a

leased line, the most expensive service.


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WAN Virtual Circuits A virtual circuit is a pathway through a packet-switched network

(providers network) that appears to be a dedicated.

Virtual circuits are connection oriented

Two types of virtual circuits exist:

switched virtual circuits (SVCs)

permanent virtual circuits (PVCs).

In a PVC, the customer and the carrier have negotiated the endpointsand characteristics of the virtual circuit ahead of time, and they areconstantly available.

The end points and a stated bandwidth called a Committed

Information Rate ) constitute a PVC, which is defined to theframe relay network devices

In a SVC, the virtual circuit is available only " ".

circuit establishment, data transfer, and circuit termination.


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Frame Relay OperationA packet-switched service such as Frame Relay

requires that a customer maintain only one

circuit, typically a T1, to the provider's CentralOffice (CO).

Frame Relay provides tremendous cost-

effectiveness, since one site can connect to

many geographically distant sites using a singleT1 (and single CSU/DSU) to the local CO.


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Frame Relay Operation Frame Relay networks, support both permanent

virtual circuits (PVCs) and switched virtual

circuits (SVCs). The PVC is the most common type of Frame

Relay virtual circuit. PVCs are permanently

established connections that are used when

there is frequent and consistent data transferbetween DTE devices across a Frame Relaynetwork.


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Frame Relay Operation SVCs are temporary connections, used when

there is only periodic or infrequent data transfer

between DTE devices across the Frame Relaynetwork.

Because they are temporary, SVC connections

require call setup and termination for each

connection. Many Frame Relay providers only support PVCs.


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WAN Line Types

OC-768 now exists running at 40Gbps


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Name and describe 6 data-link encapsulations

The WAN data link layer defines how data is encapsulated for

transmission to remote sites Frame Relay: uses simplified encapsulation with no error correction over

high-qualitydigital facilities. A very fast protocol compared to the otherWAN protocols.

Point-to-Point Protocol (PPP): Point-to-Point protocol is a Data Linklayer protocol that can be used over a telephone lines media. The basic

purpose of PPP is to transport layer-3 packets over a Data Link layer point-to-point link. Like connecting ur computer to server. ISDN: a set of digital services that transmitsvoice and data over existing

phone lines. Link Access Procedure, Balanced (LAPB): For packet-switched networks

used to encapsulate packets at Layer 2 of the X.25 stack. Provides reliabilityand flow control on a point-to-point basis.

Cisco/IETF: Used to encapsulate Frame Relay traffic. The Cisco option isproprietary and can be used only between Cisco routers.

High-Level Data Link Control (HDLC): an ISO standard, HDLC notcompatible between different vendors because of the way each vendor haschosen to implement it. HDLC supports both point-to-point andmultipoint configurations.


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Serial line frame fields The two most common point-to-point WAN

encapsulations are HDLC and PPP

All the serial line encapsulations share a common

frame format, which has the following fields

The choice of encapsulation protocol depends onthe WAN technology and the communicatingequipment.


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PPP and HDLC PPP is a standard serial-line encapsulation method

This protocol can check for link quality during connectionestablishment.

Provides authentication through Password Authentication Protocol

(PAP) and Challenge Handshake Authentication Protocol (CHAP).

HDLC is Cisco's default encapsulation for seriallines No windowing or flow control

A proprietary type code is inserted in the frame which means thatHDLC framing is not interoperable with other vendors' equipment.

Used when both ends of a dedicated-line connection are routersrunning Cisco IOS


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Frame Relay Designed to be used over high-speed, high quality digital

facilities

Does not offer much error checking or reliability, butexpects upper-layer protocols to attend to these issues

Connect multiple network devices on a multipoint WAN

Frame Relay access is typically at 56 kbps, 64 kbps, or 1.544Mbps


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Frame Relay Frame Relay defines the interconnection process

between the customer's DTE (e.g. router) and the

service provider's DCE (Frame Relay switch).


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Frame Relay Frame Relay does not define the way the data is

transmitted within the service provider's network

once the traffic reaches the provider's switch. Thus,

a Frame Relay provider could use a variety of

technologies, such as ATM or PPP, to move datafrom one end of its network to another.


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Frame Relay Two common topologies can be used in a Frame

Relay solution:

Every Frame Relay networkdevice has a PVC to every other device on the

multipoint WAN.

also often called a star

topology or hub-and-spokes topology. In a partially

meshed topology, not every device on the Frame

Relay cloud has a PVC to every other device.


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Frame Relay Operation In a packet-switched network, such as Frame Relay, each end

of the virtual circuit is assigned a connection identifier-DLCI.

The service provider's switching equipment maintains a table

that maps these connection identifiers to outbound ports. When a frame is received, the Frame Relay switch analyzes

the connection identifier and delivers the frame to the

appropriate outbound port.

Each site can be connected to every other by a virtual circuit.

A data-link connection identifier (DLCI) identifies a PVC

The DLCI identifies the logical circuit between the source and

destination devices.


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DLCI In Frame Relay networks, a data-link connection

identifier (DLCI) identifies the virtual circuit betweenthe DCE and the Frame Relay switch.

The Frame Relay switch maps the DLCIs betweeneach pair of routers to create a PVC.

DLCIs have local significance.


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DLCIA locally significant DLCI does not reference the

other end of the PVC. In other words, two DTE

devices connected by a virtual circuit may use a

different DLCI value to refer to the same

connection.

In order for the router to know which PVC to use,

Layer 3 addresses must be mapped to DLCInumbers.


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Router R1 uses global DLCI 101. What does that mean? It means that all theother routers use 101 as their local DLCI on their PVC connected to R1. In thiscase, R2, R3, and R4 would all use local DLCI 101 on their PVCs to R1,


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DLCI Frame Relay does not operate at Layer 3.

Your Frame Relay service provider will assignthe DLCI numbers for your WAN. Usually, DLCIs0 to 15 and 1008 to 1023 are reserved forspecial purposes.

Therefore, service providers typically assignDLCIs in the range of 16 to 1007.


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DLCI In order to build a map of DLCIs to Layer 3

addresses, the router must first know what VCs are

available. Typically, the process of learning about

available VCs and their DLCI values is handled by asignaling standard called Local ManagementInterface (LMI).


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LMI Local Management Interface(LMI) is a signaling

standard between the DTE and the Frame Relay

switch. LMI is responsible formanagingthe

connection and maintainingthe status betweendevices.

It includes support for the following:


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LMI A keepalive mechanism - This verifies that data

is flowing.

A status mechanism - These messages providecommunication and synchronization between thenetwork and the user device. VC status

messages prevent the sending of data into black

holes, that is, over PVCs that no longer exist.


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LMI Global addressing - This gives connection

identifiers global rather than local significance, which

allows them to be used to identify a specific interface

to the Frame Relay network. Global addressingmakes the Frame Relay network resemble a local-

area network (LAN) in terms of addressing.


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LMI There are three types of LMI, none of which are

compatible with the other. Cisco, StrataCom,

Northern Telecom, and Digital Equipment

Corporation, collectively known as the "gang of four,"released one type of LMI, while the ANSI and the

ITU-T each released their own version. Cisco,ANSI, and Q933a.


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LMI The Frame Relay switch uses LMI to report the

status of each configured PVC. The threepossible PVC states are:

Active state - indicates that the connection isactive and that routers can exchange data.


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LMI Inactive state - indicates that the local

connection to the Frame Relay switch is working,

but the remote router connection to the Frame

Relay switch is not working.

Deleted state - indicates that no LMI is beingreceived from the Frame Relay switch, or that

there is no service between the CPE router andFrame Relay switch.


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Inverse ARP You can map DLCIs to Layer 3 addresses

manually on a router using the appropriateconfiguration commands.

Building static maps can require a great deal ofadministrative overhead in complex networks,and static maps cannot adapt to changes in theFrame Relay topology.

Through the exchange of LMI, a Frame Relayswitch may announce a new virtual circuit with itscorresponding DLCI.


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Inverse ARP Unfortunately, Layer 3 protocol addressing is not

included in the announcement.

Without a new configuration or a mechanism fordiscovering the protocol address of the other

side, this new virtual circuit is unusable.

Inverse ARP was developed to provide a

mechanism for dynamic DLCI to Layer 3 addressmaps.


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Inverse ARP Inverse ARP works much the same way Address

Resolution Protocol (ARP- map IP networkaddresses to the hardware addresses) works on a LAN.

Once the router learns from the switch aboutavailable PVCs and their corresponding DLCIs, the

router can send an Inverse ARP request to the other

end of the PVC.


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Inverse ARP In effect, the Inverse ARP request asks the remote

station for its Layer 3 address, while at the same

time providing the remote system with the local

system's Layer 3 address.

The return information from the Inverse ARP is thenused to build the Frame Relay map.


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Inverse ARP On a Cisco router, Inverse ARP is on by default

when you configure an interface to use Frame Relay

encapsulation. If you configure a static mapping for a

specific DLCI, Inverse ARP is automatically disabledfor the specified protocol on the specified DLCI.


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ENOUGH OF THEORY .

YOUR ASSIGNEMENT IN PACKET TRACER CISCO

Configuring Frame-Relay


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X.25


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X.25 History and Overview Designed to provide a low cost alternative for data

communication over public networks Pay only for bandwidth actually used

Ideal for bursty communication over low quality

circuits Standard provides error detection and correction

for reliable data transferX.25 was a basic of frame-relay, eventually was

replaced by it. Can support speeds of 9.6 Kbps to 2 Mbps Can provide multiplexing of up to 4095 virtual

circuits over on DTE-DCE link


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X.25 Devices

X.25 also uses the same terminology for its devices asFrame relay.

Data Terminal Equipment (DTE) Terminals, personal computers, and network hosts Located on premises of subscriber

Data Circuit-terminating Equipment (DCE) Modems and packet switches

Usually located at carrier facility Packet Switching Exchange (PSE)

Switches that make up the carrier network


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Sample X.25 Network

X.25WAN

Personal ComputerDTE

TerminalDTE

ServerDTE

Modem

DCE

ModemDCE

ModemDCE

PSE

PSE

PSE

PSE


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Packet Assembler/Disassembler (PAD)

Packet assembler/disassemblers (PADs) are a form ofdata communications equipment (DCE) forconnecting asynchronous data terminal equipment

(DTE) such as computers and dumb terminals to theX.25 packet-switching service.

Acts as intermediary device between DTE and DCE

Performs three functions Buffering to store data until a device is ready to process

it

Packet Assembly

Packet Disassembly


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PAD in Action

Terminal

DTEModem

DCE

PAD

Buffer

Data

Assembly/

Disassembly

Data

X.25 Packet

PSE

PSE


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X.25 mapping to OSI ModelApplication

Presentation

Session

Transport

Network

Data Link

Physical

PLP

LAPBx.21 bis, EIA/TIA-232, EIA/TIA-449,

EIA-530, G.703

Other Services

X.25

ProtocolSuite


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X.25 Data Link Layer Link Access Procedure, Balanced (LAPB) is the protocol

used for this layer in x.25.

LAPB is a data link-layer protocol that manages

communication and packet framing between DTE andDCE devices and is best known for its presence in the X.25

Makes sure that frames are delivered in sequence anderror-free

Uses sliding window of 8 or 128 frames


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X.25 Network Layer

Packet Layer Protocol (PLP) is the X.25 network layerprotocol

PLP manages calls between a pair DTE devices using a

Permanent Virtual Circuit (PVC) or a Switched VirtualCircuit (SVC)

PLP handles segmentation, reassembly, bit paddingand error and flow control


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PLP Operates in Five Modes

Call Setup Used to setup virtual circuit for SVC

Data Transfer

Used for transferring data with both SVC and PVC Idle

Used when SVC call has been established but no data is currentlybeing transferred

Call Clearing Used to end communication between DTEs for a SVC

Restarting Used to synchronize DTE and DCE for all virtual circuits that exist

between them


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X.25 vs. Frame Relay A major difference between X.25 and Frame Relay packet

switching are that X.25 is a reliable protocol, based on node-to-node automatic repeat request, while Frame Relay is a non-reliable protocol.

The X.25 protocol is a network layer protocol, whereas Frame relay

is a Data link layer. Frame relay is a further development of X.25.

The simplicity of Frame Relay made it considerably faster andmore cost effective than X.25 packet switching.

Is more cost effective to use X.25 on slower networks.

X.25 connection establishment and release (call control) use in-band signaling within the same virtual channel used for user datatransmission causing additional overhead. Frame Relay callcontrol uses separate virtual channels identified by reserved DLCIusing the LMI (Local Management Interface) protocol.


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Config Frame Relay


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Configuring Frame-Relay To configure an interface for Frame Relay, you must

specify frame relay encapsulation to encapsulate

data traffic end to end. There are two possible Frame

Relay encapsulations: cisco and ietf. By default, an interface will use the Cisco Frame Relay

encapsulation method. This method is Cisco proprietary,

and should not be used if the router is connected to

another vendor's equipment across a Frame Relaynetwork.


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Configuring Frame-Relay The default LMI type is cisco

Router(config-if)#encapsulation Frame-Relay [cisco | ietf ]

Router(config-if)#frame-relay lmi-type [ansi | cisco |q933a]


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Configuring Frame-Relay Maps Dynamic address mapping is enabled by default

for all protocols enabled on a physical interface.If your Frame Relay environment supports LMI

autosensing and Inverse ARP, dynamic addressmapping will take place automatically, thereforeno static address mapping is required.

If your environment does not support LMI

autosensing and Inverse ARP, you may have tomanually configure a Frame Relay map.


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Configuring Frame-Relay Maps The frame-relay map command is used to

configure static address mapping. Once you

configure a static map for a given DLCI, Inverse

ARP is disabled on that DLCI. The frame-relaymap uses the following syntax.

Router(config-if)#frame-relay mapprotocol

protocol-addressdlci [broadcast] [ietf | cisco].


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Verifying Frame-RelayAfter you configure Frame Relay, you can verify that

the connections are active by using several differentshowcommands:

show interfaces serial show frame-relay pvc

show frame-relay map

show frame-relay lmi

Frame Relay and X.25

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