chapter 10 wide area networks. contents the need for wide area networks (wans) point-to-point...
TRANSCRIPT
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Chapter 10
Wide Area Networks
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Contents
• The need for Wide area networks (WANs)• Point-to-point approaches• Statistical multiplexing, TDM, FDM approaches• Dial-up, T/ DS links• X.25, Frame relay, ATM• SONET• DWDM• WANs and TCP/ IP stack
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X.25/FR/ATMNeed SONET DWDM
WANs and
TCP/ IP
Definition
• WANs are physical or logical networks that provide data communications to a large number of independent users. These users are usually spread over a larger geographic area than a LAN
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The need for WANs
• LANs are very effective at connecting computers within offices
• Links are short, so dedicated link to each PC is not too expensive
• But many organizations have offices in many states and countries
• Web pages, email servers are located world-wide• As users spread over large distances, link costs
become very high
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The need for WANs (contd.)
• Broadcast lowers costs of LAN equipment
• But as number of users increases, CSMA slows down the network significantly
• As number of network users increases, need mechanisms to merge traffic from multiple users
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Roads and computer networks
• There are many similarities between the challenges and design solutions used in road networks and computer networks
• Neighborhood networks are like LANs
• Interstate networks are like WANs
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Local intersection as LAN node
Stop sign promotes carrier sensingWhite car will wait till black car passes
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Interstate exit as WAN node
Merging lane: Entry ramp for local traffic
Existing traffic does not stop for merging traffic
Shared lanes
Merging lane: Exit ramp for local traffic
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Categories of WANs
• Point-to-point– Dial-up– T/ DS
• Statistical multiplexing– X.25, Frame relay, ATM
• TDM– SONET
• FDM/ WDM– Fiber optics
• MPLS
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#1UCLA
#4Utah
#3UCSB
#2SRI
Sigma 7
PDP 10
940
360
Point-to-point WANs
• Earliest WANs used dial-up networking– Use phone line to
connect to a remote computer
– Leverage existing communication network
– End stations perform routing
Phone lines
Internet in 1969
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T/ DS carriers
• Phone companies realized business opportunity in providing data services
• Combined (multiplexed) data carrying capacity of multiple phone lines to provide high speeds
• Offered as T/ DS carriers– T =– DS =
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Point - point
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WAN built using T-carriers
Branch
Branch
Branch
Branch
HQ
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T/ DS carriers
• Formally, t-carriers are the physical line, DS is the signal carried by the line– Both terms used interchangeably in the industry
• Offer point-to-point connection like dial-up
No. of phone lines aggregated T-carrier name DS name Data rate
1 DS-0 64 kbps
24 T-1 DS-1 1.544 mbps
96 T-2 DS-2 6.312 mbps
672 T-3 DS-3 44.736 mbps
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Statistically multiplexed WANs
• Point-to-point is very inefficient when network grows– No switching within network
– Inefficient use of bandwidth• Statistical multiplexing
allows WANs to aggregate traffic– Reduces “burstiness”
A
A
B
B2
B1
T1
T1T1
A
A
B
B2
B1
T1
Carrieraccesspoint
Carrieraccesspoint
T1
T1T1T1
X.25/ FR/ ATM
A
B
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Reducing “burstiness”
8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:000
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ABA+B
Time of day
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Virtual circuits
A
A
B
B2
B1Carrieraccesspoint
Carrieraccesspoint
X.25/ FR/ ATM
VC = 1VC = 2 VC = 3
A-A
B-B1
B-B2
Connection Virtual-circuit IDA-A 1B-B1 2B-B2 3
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X.25/ Frame relay/ ATM
• Shared network services offered by telcos– Multiple end users can share the same
infrastructure
– Aggregation similar to interstate system• End users connect to shared network using
point-to-point links such as T1/ T3
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X.25/ Frame relay/ ATM
• When data packets enter shared network, carrier assigns label based upon destination
• Shared network uses labels to direct packets to correct destinations
• Each label is called a virtual channel– Data link layer technologies– Many virtual channels can be carried over a single
physical link, limited only by link capacity
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X.25/ Frame relay/ ATM
• X.25– Standardized by CCITT in 1976– Data rates: 56 kbps – 2 mbps
• Frame relay– Specified/ standardized in 1990 (Cisco)/ 1992 (CCITT)– Data rates: 56 kbps – 45 mbps
• ATM– Standardized: 1992 by CCITT– Data rates: 1.544 mbps – 622.080 mbps– Pricing: ~ $400/ mbps/port (domestic) – upto $4,000/
mbps/port (internationally)
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TDM WANs
• Available line data rate divided into time slots– Physical layer technology
• Each virtual channel given one or more slots• Commercially available as SONET services
– Synchronous Optical NETwork– Synchronous Digital Hierarchy (SDH) in Europe– Offered as optical carrier (OC) services
• Pricing generally dependent on distance: ~ $15/ mbps/ mile
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TDM WANs
• X.25/ Frame relay/ ATM often transported over SONET links
• SONET data rates first standardized in 1988 by CCITT
SONET service names Data rate Data + overhead
OC-1 50.112 mbps 51.84 mbps
OC-3 150.336 mbps 155.52 mbps
OC-12 601.344 mbps 622.08 mbps
OC-48 2.405,376 gbps 2.488,320 gbps
OC-192 9.621,504 gbps 9.953,280 gbps
OC-768 38.486,016 gbps 39.813,120 gbps
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FDM WANs
• Optical fiber has very high bandwidth– Capable of supporting extremely large data rates
– No single user needs such high bandwidths• Available line bandwidth split into multiple
lower bandwidth channels– Like lanes on interstate highways– Vehicles are not wide enough to use entire road
width
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FDM WANs
• DWDM channel frequencies standardized by ITU-T as ITU grid in 2001– 3 bands: L band, C band, S band– 50 channels/ band = 150 channels total– Data rates up to 10 gbps possible per DWDM channel
• DWDM commonly used in network core– Considered below physical layer
• Each FDM channel on a DWDM link may transport a SONET signal, which in turn may transport multiple ATM channels
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WANs and TCP/ IP stack
• Where are WAN technologies positioned on the TCP/ IP stack?
• Typically, multiple WANs traversed by packet from source to destination
• Routers interface between WANs– Hence WANs typically considered a data link layer
technology
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WANs and TCP/ IP stack
• Traceroute to Google
Network layer
WAN 1 technology confirms router is WAN destination
WAN 2 technology frames data for transmission
Incoming frame from WAN 1
RouterDestination IP address passed to network layer to determine
next WAN
Outgoing frame to WAN 2
Data routed to correct WAN for onward transmission
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Summary
• WANs are long distance links that aggregate traffic from multiple networks
• WANs generally have very high data rates• WAN types include point-to-point, statistically
multiplexed, TDM and FDM• Carriers define virtual circuits for each source-
destination pair of nodes• WANs operate at the data link layer
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Case study – UAVs
• Remote wars were fought with soldiers• Now, increasingly de[end upon satellite based
WAN networks• UAVs
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Hands-on exercise
• OPNET– Download academic version of software
• Approx. 50 MB
– Run scenario– Collect statistics
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Network design exercise
• Choosing appropriate WAN technologies• Adding routers to the network