wide area networking
DESCRIPTION
Wide Area Networking. Wide Area Networks Link sites together Carriers and regulation Leased Line Networks Public Switched Data Networks (PSDNs) Virtual Private Networks. Outline Topics. Wide Area Networks. WANs Link Sites (Locations) Usually sites of the same organization - PowerPoint PPT PresentationTRANSCRIPT
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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