Transcript Wide Area Networks

Wide Area Networking
1
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
Site B
Site C
Site A
WAN
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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
Customer
Premises
Carrier
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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
Mesh
Leased Line
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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
5
10
25
Lines
10
45
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 Leased
Line
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
Access
Device
PSDN
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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
POP
– Separate from PSDN charges
Leased
Line
PSDN
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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
PSDN
efg
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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
Multiplexed
Trunk 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
Virtual
Circuit
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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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•ISDN
WAN Products
•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
64
kbps
64
kbps
BRI
2B+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
2B+D
BRI
23B+D
PRI
ISDN
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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 circuitswitched 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 lowspeed 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 at
the low end of the range
– Priced aggressively
– Both reasons
are 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
Price
FR
ATM
Speed
• In general, a smooth price-speed
curve across the two services
• At some speed, may offer both
– If so, usually price them the same
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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
E-Mail, WWW, Database, etc.
TCP
UDP
IPsec
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VPNs
• IPsec Transport Mode
– End-to-end security for hosts
Local
Network
Internet
Local
Network
Secure Communication
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VPNs
• IPsec Tunnel Mode
– IPsec server at each site
– Secure communication between sites
Local
Network
Secure Communication
Internet
Local
Network
IPsec
Server
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VPNs
• IPsec Modes Can be Combined
– End-to-end transport mode connection
– Within site-to-site tunnel connection
Local
Network
Tunnel Mode
Internet
Local
Network
Transport Mode
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VPNs
• Another Security System for VPNs is the Pointto-Point Tunneling Protocol (PPTP)
– For dial-up connections, based on PPP
– Connects user with securely to a remote access
server at a site
Dial-Up
Connection
PPTP Connection
Internet
Local
Network
Remote Access Server
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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 2
Peering
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