1 wide area networking. 2 outline topics wide area networks –link sites together –carriers and...
Post on 22-Dec-2015
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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 10
10 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
PSDN
AccessDevice
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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 Communication
IPsecServer
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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