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Wide Area Networking

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Outline Topics

• Wide Area Networks– Link sites together

– Carriers and regulation

– Leased Line Networks

– Public Switched Data Networks (PSDNs)

– Virtual Private Networks

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Wide Area Networks• WANs Link Sites (Locations)

– Usually sites of the same organization– Sometimes, sites of different

organizations

WANSite A Site C

Site B

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Carriers• You can only install wires on your own property

– Called your customer premises

• To send signals between sites or to customers, you must use a carrier

• Carriers transport data and voice traffic between customer premises, charging a price for their services

• Receive rights of way from the government to lay wires and radio links

CarrierCustomerPremises

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Carrier Regulation• Traditionally, Carriers Have been Regulated

– Given rights of way– Given monopoly protection from competition– In return, services normally must be tariffed

• Tariff specifies exact terms of the service to be provided, and

• Tariff specifies price to be charged• Prevents special deals, which would be inappropriate for a

regulated monopoly• Regulators must approve price for reasonableness

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Carrier Regulation• There is a Strong Trend Toward

Deregulation– Gradual removal of monopoly protections

– Allows competition, so lower prices and more service options

– Fewer services need to be tariffed, allowing price negotiation

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Carrier Regulation• Service Level Agreements (SLAs)

– Even under competition, carriers may guarantee specific levels of service for certain service parameters in an SLA

• Throughput• Latency• Availability• Error Rates, etc.

– Penalties are paid to customers if carrier fails to meet agreed-upon service levels

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High Cost of Long-Distance• LAN Communication is Inexpensive per Bit

Transmitted– So most LANs operate at 10 Mbps to a few gigabits

per second

• Long-Distance Communication is Very Expensive per Bit Transmitted– So Most WANs use low speeds– Most WAN demand is 56 kbps to a few Mbps

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Leased Lines• Leased Lines are Circuits

– Often goes through multiple switches and trunk lines

– Looks to user like a simple direct link

– Limited to point-to-point communication• Limits who you can talk to

– Carriers offer leased lines at an attractive price per bit sent to keep high-volume customers

Switch Trunk Line

Leased Line

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Leased Line Meshes• If you have several sites, you need a mesh

of leased lines among sites

Leased Line

Mesh

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Leased Line Speeds• Largest Demand is 56 kbps to a few Mbps

• 56 kbps (sometimes 64 kbps) digital leased lines– DS0 signaling

• T1 (1.544 Mbps) digital leased lines– 24 times effective capacity of 56 kbps– Only about 3-5 times cost of 56 kbps– DS1 signaling

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Leased Line Speeds• Fractional T1

– Fraction of T1’s speed and price– Often 128, 256, 384 kbps

• T3: is the next step– 44.7 Mbps in U.S.

• Europe has E Series– E1: 2.048 Mbps– E3: 34 Mbps

• SONET/SDH lines offer very high speeds– 156 Mbps, 622 Mbps, 2.5 Gbps, 10 Gbps

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SONET/SDH

• Created as Trunk Lines for Internal Carrier Traffic– As were other leased lines

• The Trunk Line Breakage Problem– Problem: unrelated construction products often break

carrier trunk lines, producing service disruptions

– The most common cause of disruptions

X

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SONET/SDH Uses a Dual Ring• Normally, Traffic Travels in One Direction on One Ring

• If Trunk Line Breakage, Ring is Wrapped; Still a Ring, So Service Continues

Switch

Normal Operation Wrapped

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Digital Subscriber Lines (DSLs)• Can Use Instead of Traditional Leased

Lines– Less expensive

• HDSL (High-Speed DSL)– Symmetrical: Same speed in each direction

– HDSL: 768 kbps (Half a T1) on a single twisted pair

– HDSL2: 1.544 Mbps (T1) on a single twisted pair

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Digital Subscriber Line• Normal Leased Lines Used Data Grade Wires

– High-quality, high-cost– Two pairs (one in each direction)

• DSLs Normally Use Voice Grade Copper– Not designed for high-speed data– So sometimes works poorly– Usually one pair (ADSL, HDSL)– Sometimes two pairs (HDSL2)

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Problems of Leased Lines• With many sites, meshes are expensive and difficult to

manage

• With N sites, N*(N-1)/2 leased lines for a mesh– May not need all links, but usually use many

• User firm must handle switching and ongoing management

– Expensive because this requires planning and the hiring, training, and retention of a WAN staff

Sites Lines5 1010 4525 300

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T1 Leased Lines• Voice Requirements

– Analog voice signal is encoded as a 64 kbps data stream

– 8 bits per sample

– 8,000 samples per second

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T1 Leased Lines• T1 lines are designed to multiplex 24 voice

channels of 64 kbps each

• T1 lines use time division multiplexing (TDM)– Time is divided into 8,000 frames per second

• One frame for each sampling period

– Each frame is divided into 24 8-bit slots• One for each channel’s sample in that time period• (24 x 8) 192 bits• Plus one framing bit for 193 bits per frame

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T1 Leased Lines• Speed Calculation

– 193 bits per frame– 8,000 frames per second– 1.544 Mbps

• Framing Bit– One per frame– 8,000 per second– Used to carry supervisory information (in groups of 12

or 24 framing bits)

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PSDNs• Public Switched Data Networks

– Designed for data rather than voice

– Site-to-site switching is handled for you

– You merely connect each site to the PSDN “cloud” (No need to know internal details)

PSDN

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PSDNs• Connect each site to the PSDN using one

leased line– Only one leased line per site– With N sites, you only need N leased lines,

not N* (N-1)/2 as with a full mesh

1 LeasedLine

PSDN

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PSDNs• Access Device Needed at Each Site

– Connects each site to access line– Often a router– Sometimes a device specific to a particular

PSDN Technology

PSDNAccessDevice

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PSDNs• Point of Presence (POP)

– Place where you connect to the cloud– May be several in a city– May not have any POP close– Need leased line to POP– Separate from PSDN charges

LeasedLine

PSDN

POP

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PSDNs in Perspective• Simpler than Networks of Leased Lines

– Less staffing– Fewer leased lines to support

• Less Expensive than Networks of Leased Lines– Less staffing– PSDN prices are very low– PSDN is less expensive overall– PSDNs are replacing many leased line mesh

networks

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Circuit-Switched PSDNs• End-to-End Capacity is Guaranteed

– If you need it, it is always there– When you don’t need it, you still pay for it– Expensive for data traffic, which usually has

short bursts and long silences

A bcd efg

PSDN

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Packet-Switched PSDNs• Messages are divided into small units called

packets

– Short packets load switches more effectively than fewer long messages

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Packet-Switched PSDNs • Packet-Switched PSDNs Usually Operate

at Layer 2 (Data Link Layer for Single Subnets)

– Should be called frame-switched networks

– Still called packet-switched networks

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Packet Switched PSDNs• Packets are multiplexed on trunk lines

– Cost of trunk lines is shared– Packet switching lowers transmission costs– Dominates PSDN service today

MultiplexedTrunk Line

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Packet Switched PSDNs: Virtual Circuits

• All commercial packet switched PSDNs use virtual circuits– Eliminates forwarding decisions for individual packets– Reduces switching load, so reduces switching costs

VirtualCircuit

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Unreliable PSDNs• Most commercial PSDNs are Unreliable

– (Only obsolete X.25 PSDN technology was reliable)

– No error correction at each hop between switches

– Reduces costs of switching

– Note that both virtual circuits and unreliable service reduce switching costs

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PSDN Cost Savings• Packet Switching

– Reduces costs of transmission lines through multiplexing

• Virtual Circuits– Reduces costs of switches because they do not have

to make decisions for each frame

• Unreliability– Reduces costs of switches because they do not have

to do error correction

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WAN Products•ISDN•X.25•Frame Relay•ATM•Virtual Private Networks (VPNs)

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ISDN• Integrated Services Digital Network

• 2B+D Basic Rate Interface (BRI) to the desktop– Two 64-kbps B channels– Can be bonded for 128 kbps service– One 16-kbps D channel, usually for supervisory

signals

64kbps

64kbps

BRI2B+D

ISDN Modem

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ISDN• Primary Rate Interface (PRI)

– Connection between firm and ISDN carrier– 23B+D (on a T1 line)– 30B+D (on an E1 line)– One 64 kbps D channel for supervision

ISDNPRIBRI

2B+D 23B+D

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ISDN

• Circuit-Switched– Dedicated capacity– Expensive for data

• Dial-Up Connection– Must connect each time you wish to communicate– Other PSDNs are dedicated (always on)

• Unreliable

• Only Popular PSDN that is either circuit-switched or dial-up

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ISDN• Never achieved strong market use

• Being overtaken by PSDNs that are both faster and less expensive

• Often, ISDN is spelled out as “It still does nothing”

• However, there is enough ISDN in use that you must know it

• Also, if connectivity is only needed a short time each day, ISDN is still a good choice for low-speed transmission

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X.25• First Packet-Switched PSDN Standard

– Developed in the 1970s– Now obsolete– But still used, especially in third-world countries and

Europe

• Slow: Usually 64 kbps or slower– Some faster X.25 services are available

• Reliable, so costs of switches are high– So cost of service is high– But works even if transmission lines are poor

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Frame Relay• Most Popular PSDN Today

– Offers speeds of 64 kbps to about 40 Mbps; This covers the range of greatest corporate demand

– Most demand is atthe low end of the range

– Priced aggressively

– Both reasonsare critical

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Frame Relay• Low-Cost Service

– Packet-Switched– Uses virtual circuits to cut costs– Unreliable– Relatively low speeds

• Dedicated Connections– Always ready to send

and receive

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ATM• Like Frame Relay:

– Packet switched– Virtual circuits– Dedicated (Always On) Connections

• Unlike Frame Relay– Much faster top speed

• 1 Mbps, 25 Mbps, 45 Mbps, 156 kbps, 622 kbps, several Gbps

– May offer quality of service (QoS) guarantees• Maximum latency for time-critical applications• Exact cell-by-cell timing

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ATM• Very Expensive

– Complexity because of basic transmission mechanisms

– Complexity because of quality of service mechanisms

– High-speed transmission

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Frame Relay and ATM

• Most Vendors Offer Both

• To cover speeds from 56 kbps to a few gigabits per second

• In general, a smooth price-speed curve across the two services

• At some speed, may offer both– If so, usually price them the same

Speed

Price ATMFR

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Frame Relay and ATM

• Both are widely used

• Frame Relay is more popular today because it serves the range of greatest corporate need (56 kbps to a few megabits per second) at an attractive price

• As demand for higher-speed links grows, ATM should become more widely used– Unless other alternatives to ATM appear, such as 10

Gbps Ethernet for WANs

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VPNs

• Virtual Private Networks– Use the Internet for transmission instead of a PSDN– Sometimes called VPNs if use Frame Relay or ATM with

added security

• Why use the Internet?– Inexpensive– Business partners are already connected to the same

network (the Internet)• May use different PSDNs, but everybody is connected to the

Internet

Internet

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VPNs• Problems with the Internet

– Congestion: slows transmissions

– Reliability: cannot always connect, sometimes fails during transmissions

– Lack of security

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VPNs• IETF developing IPsec security standards

– IP security– At the internet layer– Protects all messages at the transport and application

layers

IPsec

TCP UDP

E-Mail, WWW, Database, etc.

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VPNs• IPsec Transport Mode

– End-to-end security for hosts

LocalNetwork

Internet LocalNetwork

Secure Communication

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VPNs• IPsec Tunnel Mode

– IPsec server at each site– Secure communication between sites

LocalNetwork

Internet LocalNetwork

Secure CommunicationIPsecServer

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VPNs• IPsec Modes Can be Combined

– End-to-end transport mode connection– Within site-to-site tunnel connection

LocalNetwork

Internet LocalNetwork

Tunnel Mode Transport Mode

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VPNs• Another Security System for VPNs is the Point-

to-Point Tunneling Protocol (PPTP)– For dial-up connections, based on PPP– Connects user with securely to a remote access

server at a site

Internet LocalNetwork

Remote Access Server

Dial-UpConnection

PPTP Connection

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Virtual Private Networks

• Other Problems Remain

• Internet Congestion is Still a Problem– Internet throughput tends to be low

• Internet Reliability is Low– Cannot get connections– Backbone fails occasionally

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Virtual Private Networks• Alternative

– Avoid the congested and unreliable backbone!

– Use one ISP that serves all sites

– Should offer QoS service level agreement (SLAs) for latency and reliability

Site 1 ISP Site 2

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Virtual Private Networks• Alternative

– Avoid the congested backbone

– Use ISPs that “peer” with one another: connect with one another not through the Internet backbone

– May offer end-to-end SLAs

Site 1 ISP A ISP B Site 2Peering