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WAN Basics

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Agenda

WAN Basics Point-to-Point Protocol (PPP) Integrated Services Digital

Network (ISDN) Frame Relay

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WAN Basics

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A network that serves users across a broad geographic area

Often uses transmission devices provided by public carriers (Pacific Bell, AT&T, etc.) This service is commonly referred to as “plain

old telephone service” (POTS) WANs function at the lower three layers of the

OSI reference model Physical layer, data link layer, and network layer

What Is a WAN?

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WAN Devices

WAN Switch Switches traffic such as Frame Relay, X.25, and SMDS, and operates at the data link layer

Modem Interprets digital and analog signals, enabling data transmission over telephone lines

Access Server A concentration point for dial-in and dial-out connections

CSU/DSU Adapts a terminal physical interface to a switch interface in a switched-carrier network

ISDN Terminal Connects ISDN Basic Rate Interface (BRI) to other interfaces, such as EIA/TIA-232

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WAN Terms

Customer premises equipment (CPE) Demarcation point Local loop Central office (CO) Toll network

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WAN Terminating Equipment

Modem

Data Terminal EquipmentDTE

Data Circuit-Terminating Equipment

The Service Providers The Service Providers EquipmentEquipment

DCE

EIA/TIA-232V.35X.21HSSI

To Corporate Network

The Customer’s The Customer’s EquipmentEquipment

WAN Provider(Carrier) Network

Physical Cable Types

Usually on the Customer’sPremises

Router

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Wan Connection Types

Leased line Circuit switching Packet switching

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Leased Line

One connection per physical interface Bandwidth: 56 kbps–1.544 Mbps

T1/E1 and fractional T1/E1 Cost effective at 4–6 hours daily usage Dedicated connections with predictable

throughput Permanent Cost varies by distance

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Dedicated physical circuit established, maintained, and terminated through a carrier network for each communication session

Datagram and data stream transmissions Operates like a normal telephone call Example: ISDN

WANModem Modem

Circuit Switching

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Network devices share a point-to-point link to transport packets from a source to a destination across a carrier network

Statistical multiplexing is used to enable devices to share these circuits

Examples: ATM, Frame Relay, SMDS, X.25

WANModem Modem

MultiplexingDemultiplexing

Packet Switching

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A logical circuit ensuring reliable communication between two devices

Switched virtual circuits (SVCs) Dynamically established on demand Torn down when transmission is complete Used when data transmission is sporadic

Permanent virtual circuits (PVCs) Permanently established Save bandwidth for cases where certain

virtual circuits must exist all the time Used in Frame Relay, X.25, and ATM

WAN Virtual Circuits

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OSI Reference Model

PhysicalLayer

DataLink

Layer

Network Layer

LLCSublayer

MACSublayer

SM

DS

X.2

1bis

EIA/TIA-232EIA/TIA-449V.24 V.35

HSSI G.703EIA-530

HD

LC

PP

PWAN Protocols

LA

PB

X.2

5 P

LP

Fra

me

Rel

ay

SD

LC

WAN Protocols

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SDLC IBM’s SNA data link layer communications protocol

HDLC Bit-oriented synchronous data link layer protocol

LAPB Data link layer protocol in the X.25 protocol stack

PPP Provides router-to-router and host-to-network connections over sync and async circuits

X.25 Defines connections for remote terminal access and computer communications in PDNs

ISDN Permits telephone networks to carry data, voice, and other source traffic

Frame Relay Switched data link layer protocol that handles multiple virtual circuits using HDLC; replacing X.25 due to higher efficiency

WAN Protocols

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Point-to-Point Protocol (PPP)

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HDLC

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Point-to-Point Protocol (PPP)

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LCP Configuration Option

Authentication PAP (Password Authentication Protocol) CHAP (Challenge Handshake Authentication

Protocol) Compression Error detection Multilink PPP callback

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PPP Session Establishment

Link-establishment phase Authentication phase Network layer protocol phase

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Integrated Services Digital Network (ISDN)

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Integrated Services Digital Network (ISDN)

High bandwidth Up to 128 Kbps per basic rate interface Dial on demand Multiple channels Fast connection time Monthly rate plus cost-effective,

usage-based billing Strictly digital

LAN Server

Company Network

Telecommuter/After-Hours, Work-at-Home

BRI2B+D

BRI/PRI23B+D

30B+D (Europe)

ISDN

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ISDN Terminals and Reference Points

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ISDN Protocols

Defined by ITU-T There are several series:

E ISDN on the existing telephone network. I concepts, aspects, and services. Q switching and signaling

• Q.921 layer 2• Q.931 layer 3

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ISDN Switch Types

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ISDN

One physical connection to the ISDN network

23 logical connections (U.S./Canada)

30 logical connections (Europe)

Used at central site

Primary Rate Interface (PRI)

1.536 Mbps

23B

30B

D

64 Kbps

64 Kbps}

• One physical connection to the ISDN network

• Two logical connections

• Used at remote telecommuter site

64 Kbps64 Kbps

16 Kbps

144 Kbps

2B

D }{

Basic Rate Interface (BRI)

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Bandwidth-on-Demand

Adds bandwidth when needed Configurable thresholds Controls usage costs

Start FileStart FileTransferTransfer

Remote SiteMain Office PSTN

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Dial Backup

If a primary link goes down or is too busy

Load balancing Completely customizable

Secondary

Primary

Autodial

DSU/CSUXX

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Dial-on-Demand Routing

Dials connection only when needed Ideal for low-volume, periodic traffic Controls usage costs

Interesting Interesting TrafficTraffic

Remote SiteMain OfficePSTN

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Configuring DDR

1) Define static routes2) Specify the interesting traffic3) Configure the dialer information

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Configuration Example (1)

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Configuration Example (2)

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Configuration Example (3)

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Frame Relay

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X.25

Very robust protocol for low-quality lines Packet-switched Bandwidth: 9.6 kbps–64 kbps Well-established technology;

large installed base Worldwide availability

X.25DCE

DTE DTE

DCE

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Frame Relay Permanent, not dialup Multiple connections per

physical interface (permanent virtual circuits)

Efficient handling of bursty (peak performance period) data

Guaranteed bandwidth (typical speeds are 56/64 Kbps, 256 Kbps, and 1.544 Mbps)—committed information rate (CIR)

Cost varies greatly by region

Permanent Virtual Circuit (PVC)

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Committed Information Rate (CIR)

Access rate the maximum speed at which the Frame Relay interface can transmit.

CIR the maximum bandwidth of data guaranteed to be delivered.

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Free If Available

Traffic

Time

Peak

CIRWhat You Pay

For{

{Frame Relay

Connecting Offices with Frame Relay

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Data Link Connection Identifier (DLCI)

Used to identify virtual circuits. Locally significant.

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Local Management Interface (LMI)

Signaling standard between router and the first Frame Relay switch.

It communicates information about: Keepalives Multicasting Global addressing Status of virtual circuits

Virtual circuit status: Active state Inactive state Deleted state

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Frame Relay Congestion Control

Discard Eligibility (DE) Forward Explicit Congestion

Notification (FECN) Backward Explicit Congestion

Notification (BECN)

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Operate beyond the local LAN’s control Customers pay telephone service providers

for WAN connections such as ISDN, xDSL, Frame Relay, leased line, X.25, etc.

Switching methods include point-to-point, circuit switching, packet switching, dialup, and WAN virtual circuits

Key devices include WAN switches, access servers, modems, and CSU/DSUs

Bandwidth optimization features are essential for controlling WAN costs

WAN Summary