© 1999, Cisco Systems, Inc. Module 6: WAN Basics.

© 1999, Cisco Systems, Inc. www.cisco.com

Module 6: WAN Basics

6-2CSE: Networking Fundamentals—WAN Basics © 1999, Cisco Systems, Inc. www.cisco.com

Agenda

• WAN Basics

• Transmission Options

• WAN Requirements & Solutions


WAN BasicsWAN Basics



• 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?What Is a WAN?


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


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


• 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 SwitchingCircuit Switching


• 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 SwitchingPacket Switching


• 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 CircuitsWAN Virtual Circuits


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 ProtocolsWAN Protocols


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 ProtocolsWAN Protocols


Transmission Options

Transmission Options



Transmission Options or WAN Services

Type of ServiceType of Service Analog or DigitalAnalog or Digital PermanentPermanentor Temporaryor Temporary

POTS POTS

ISDNISDN

Leased line —T1/E1Leased line —T1/E1

Frame RelayFrame Relay

X.25X.25

AnalogAnalog

DigitalDigital

DigitalDigital

DigitalDigital

DigitalDigital

Temporary Temporary

PermanentPermanent

Temporary Temporary

PermanentPermanent

Permanent Permanent

DSLDSL DigitalDigital Temporary Temporary


POTS Using Modem Dialup

• Widely available

• Easy to set up

• Dial on demand

• Asynchronous transmission

• Low cost, usage-based

• Lower bandwidth access requirements

Telecommuters

Mobile Users

Modem

Corporate Network

ServerModem

Access Router

Basic Telephone

Service


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


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)


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


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)


Free If Available

Traffic

Time

Peak

CIRWhat You Pay

For{

{Frame Relay

Connecting Offices with Frame Relay

Connecting Offices with Frame Relay


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


DSL“Modem”

End User

DSL“Modem”

DSL

• DSL is a pair of “modems” on each end of a copper wire pair

• DSL converts ordinary phone lines into high-speed data conduits

• Like dial, cable, wireless, and T1, DSL by itself is a transmission technology, not a complete end-to-end solution

• End-users don’t “buy” DSL, they “buy” services, such as high-speed Internet access, intranet, leased line, voice, VPN, and video on demand

• Service is limited to certain geographical areas

Copper Loop

Ethernet ATM

Digital Subscriber Line (xDSL)Digital Subscriber Line (xDSL)

Server

Corporate

Network


DSL TechnologyMax. Data Rate

Down/Uplink (bps)Line Coding Technology

Baseband Voice?

VDSL—Very-high-data-rate DSL

51–55M / 1.6–2.3M 13M / 1.6–2.3M TBD Yes

ADSL—Asymmetric DSL

8M / 1M1.5M / 640K

CAP, DMT,G.lite

Yes

IDSL—ISDN DSL

144K / 144K 2B1Q No

SDSL—Symmetric DSL

768K / 768K 2B1Q/CAP No

HDSL2—High-data-rate DSL

1.5M–2M / 1.5M–2M(T1–E1 Symmetric) OPTIS No

• Trade-off is reach versus bandwidth

• Reach numbers are best-case assuming “clean copper”

• Different Layer 1 transmission technologies, need a common upper protocol layer to tie them together

Max. ReachFeet (km)

1,000 (0.3)4,500 (1.5)

18,000 (5.5)

18,000 (5.5)+ (w/repeaters)

22,000 (6.9)

15,000 (4.6)

Key Attributes

Very fast—Short reachNo standard yet

Coexists with POTSTechnology of choice

for residential

Uses existing ISDN CPERelatively slow

SymmetricNo standard

Standard still under development

DSL Modem TechnologyDSL Modem Technology


Asynchronous Transfer Mode (ATM)

• Technology capable of transferring voice, video, and data through private and public networks

• Uses VLSI technology to segment data, at high speeds, into units called cells

– 5 bytes of header information

– 48 bytes of payload

– 53 bytes total

• Cells contain identifiers that specify the data stream to which they belong

• Capable of T3 (44 Mbps), E3 (34 Mbps), and SONET transmission speeds (OC-1 at 51.84 Mbps to OC-12+)

• Primarily used in enterprise backbones or WAN links

DataDataHeaderHeader

55 4848


????

Which Service?

• Before deciding, determine the answers to some questions:– Will employees use the Internet

frequently?

– Will the Internet be used for conducting business?

– Is a large volume of traffic between branch offices of the business anticipated?

– Is videoconferencing or video training needed between locations?


How Services Stack Up

• Frame Relay 56,000 bps to 45,000,000 bps

• ISDN BRI 56,000 to 128,000 bps

• ISDN PRI 1,544,000 bps

• T1/E1/DS1 1,544,000 bps/2,090,000 bps

• Analog modems 33,600 bps

• 56K modems 56,000 bps

• Cable modems 30,000,000 bps

• ADSL modems 9,000,000 bps


Wide-Area Network Requirements






• Minimize bandwidth costs• Maximize efficiency• Maximize performance• Support new/emerging applications• Maximize availability• Minimize management and maintenance

• Multiservice consolidation• Bandwidth efficiency• Performance and QoS guarantees• Emerging IP services• Carrier-class reliability• Ease of operation and management


Manage Bandwidth to Control Cost

• Dial-on-demand routing

• Bandwidth on demand

• Snapshot routing

• IPX protocol spoofing

• Compression

Software Costs 2.7%

Hardware Costs 8.0%

Transmission CostsTransmission Costs87.8%87.8%

Maintenance1.5%

Source: Data Communications


Dial-on-Demand Routing

• Dials connection only when needed

• Ideal for low-volume, periodic traffic

• Controls usage costs

Interesting Interesting TrafficTraffic

Remote SiteMain OfficePSTN


• Adds bandwidth when needed

• Configurable thresholds

• Controls usage costs

Bandwidth-on-Demand

Start FileStart FileTransferTransfer

Remote SiteMain Office PSTN


ISDNRoutingRouting

TableTableRoutingRouting

TableTable

Link Up

Update Request

Routing Updates

Routing Table

Updated

• Controls exchange of routing updates

• Client initiates request

• Server responds

Snapshot RoutingSnapshot Routing


IPX Protocol Spoofing

• Without spoofing

• With spoofing

• LAN traffic can be very chatty• WAN links are expensive• Solution: Limit unnecessary traffic across WAN

High overhead traffic across WAN

Spoofing SpoofingMuch reduced overhead across WAN


Compression

• Three types– Header

– Link

– Payload

• Van Jacobson header compression– RFC 1144

– Reduces header from 40 to ~5 bytes

Compression

DataData

DataData

CRC

CRC

TCP/IP Header

hdr


Dial Backup

• If a primary link goes down or is too busy

• Load balancing

• Completely customizable

Secondary

Primary

Autodial

DSU/CSUXX


• 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 SummaryWAN Summary

© 1999, Cisco Systems, Inc. Module 6: WAN Basics.

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