Longdistance and local loop digital technologies

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Long-distance and local loop digital technologies
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Digital Telephony

• (0,2,4,4,7,1,1,1,,4,4,4)
• Sampling rate 8000 times/sec
• 125 mu sec
• Pulse Code Modulation (PCM)
• 8bit800064 kbps

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Digital Circuits, NIUs, and DSU/CSU

• Digital circuits leased form the fundamental
  building blocks for long-distance computer
  networks.
• Network interface Unit (NIU)
• Data service unit (DSU)
• Channel service unit (CSU)
• The DSU/CSU translates between the digital
  representation used by phone companies and the
  digital representation used by the computer
  industry

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Telephone Standards

• T1 24 voice channels
• T3 28 T1s

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• Digital circuits are classified according to a
  set of telephone standards. Two of the most
  popular circuit types in North America are T1 and
  T3.
• A company that does not need T1 capacity can save
  money by leasing a fractional T1 digital circuit.
  The phone company uses the term Time Division
  Multiplexing for the technology used to subdivide
  a T1 circuit. One of the most popular fractional
  T1 capacities is 64 Kbps.

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Intermediate capacity digital circuits.
High capacity circuits
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Synchronous optical network (SONET)

• Sonet specifies details such as how data is
  framed, how lower-capacity circuits are
  multiplexed into a high-capacity circuits, and
  how syn clock information is sent along with
  data.
• Above figure shows a sonet frame on an STS-1
  circuit contains 810 octets.
• STS-1 51.840 Mbps, ? 6480 bits/ 125 us? 810
  Octets/125 us

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• The local subscriber loop refers to the
  connection between central office (CO) and
  individual subcribers residence or place of
  business.
• ISDN 2BD
• B?64kbps, D?16kbps
• 2BD channels are known as the ISDN basic rate
  interface (BRI).

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Asymmetric Digital Subscribe Line Technology

• ADSL is a local loop technology that is optimized
  for typical users who receive much more
  information than they send. To accommodate such
  use, ADSL provides a higher bit rate downstream
  than upstream.

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• ADSL uses a scheme known as Discrete multi tone
  modulation (DMT), which combines FDM and inverse
  multiplexing techniques.
• FDM in DMT 286 subchannels
• 255? downstream
• 31? upsteam
• 2?control information
• 4.1325 KHz bandwidth

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Cable Modem Technology

• Engineers have devised ways to use the existing
  cable TV infrastructure as a local loop
  technology that delivers digital data to
  subscribers.
• Addresschannel
• Upstream Communication
• Dual paths

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Wide Area Networks (WANs),Routing, and
ShortestPaths
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Motivation

• Connect multiple computers
• Span large geographic distance
• Cross public right-of-way
• Streets
• Buildings
• Railroads

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Building Blocks

• Point-to-point long-distance connections
• Packet switches

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Packet Switch

• Hardware device
• Connects to
• Other packet switches
• Computers
• Forwards packets
• Uses addresses

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Illustration Of A Packet Switch

• Special-purpose computer system
• CPU
• Memory
• I/O interfaces
• Firmware

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Building A WAN

• Place one or more packet switches at each site??
• Interconnect switches
• LAN technology for local connections
• Leased digital circuits for long-distance
  connections

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Illustration Of A WAN

• Interconnections depend on
• Estimated traffic
• Reliability needed

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Store And Forward

• Basic paradigm used in packet switched network
• Packet
• Sent from source computer
• Travels switch-to-switch
• Delivered to destination
• Switch
• Stores packet in memory
• Examines packets destination address
• Forwards packet toward destination

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Addressing In A WAN

• Need
• Unique address for each computer
• Efficient forwarding
• Two-part address
• Packet switch number
• Computer on that switch

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Illustration Of WAN Addressing

• Two-part address encoded as integer
• High-order bits for switch number
• Low-order bits for computer number

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Next-Hop Forwarding

• Performed by packet switch
• Uses table of routes
• Table gives next hop

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Forwarding Table Abbreviations

• Many entries point to same next hop
• Can be condensed (default)
• Improves lookup efficiency

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Routing in a WAN
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Default routes
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Source Of Routing Table Information

• Manual
• Table created by hand
• Useful in small networks
• Useful if routes never change
• Automatic routing
• Software creates/updates table
• Needed in large networks
• Changes routes when failures occur

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Relationship Of Routing To Graph Theory

• Graph
• Node models switch
• Edge models connection

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Shortest Path Computation

• Algorithms from graph theory
• No central authority (distributed computation)
• A switch
• Must learn route to each destination
• Only communicates with directly attached
  neighbors

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Illustration Of Minimum Weight Path

• Label on edge represents distance
• Possible distance metric
• Geographic distance
• Economic cost
• Inverse of capacity
• Darkened path is minimum 4 to 5

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Algorithms For Computing Shortest Paths

• Distance Vector (DV)
• Switches exchange information in their routing
  tables
• Link-state
• Switches exchange link status information
• Both used in practice

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Distance Vector

• Periodic, two-way exchange between neighbors
• During exchange, switch sends
• List of pairs
• Each pair gives (destination, distance)
• Receiver
• Compares each item in list to local routes
• Changes routes if better path exists

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Distance Vector Algorithm
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Distance Vector Intuition

• Let
• N be neighbor that sent the routing message
• V be destination in a pair
• D be distance in a pair
• C be D plus the cost to reach the sender
• If no local route to V or local route has cost
  greater than C, install a route with next hop N
  and cost C
• Else ignore pair

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Example Of Distance Vector Routing

• Consider transmission of one DV message
• Node 2 sends to nodes 3, 5, and 6
• Node 6 installs cost 8 route to node 2
• Later, node 3 sends update
• Node 6 changes route to make node 3 the next hop
  for destination 2

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Example
B

• Destination Cost NextHop
• A 1 A
• C 1 C
• D 2 C
• E 2 A
• F 2 A
• G 3 A

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Round 1
Distance to Reach Node
Distance to Reach Node
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A B C D E F G
0 1 1 8 1 1 8
1 0 1 8 8 8 8
1 1 0 1 8 8 8
8 8 1 0 8 8 1
1 8 8 8 0 8 8
1 8 8 8 8 0 1
8 8 8 1 8 1 0
A B C D E F G
1 0 1 8 2 2 8
1 1 0 1 2 2 8
8 8 1 0 8 8 1
1 2 2 8 0 2 8
1 2 2 8 2 0 1
8 8 8 1 8 1 0
0 1 1 8 1 1 8
A sends a message to B C E F B C E
F update
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Distance to Reach Node
Distance to Reach Node
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A B C D E F G
1 0 1 8 2 2 8
1 1 0 1 2 2 8
8 8 1 0 8 8 1
1 2 2 8 0 2 8
1 2 2 8 2 0 1
8 8 8 1 8 1 0
0 1 1 8 1 1 8
A B C D E F G
1 0 1 8 2 2 8
1 1 0 1 2 2 8
8 8 1 0 8 8 1
1 2 2 8 0 2 8
1 2 2 8 2 0 1
8 8 8 1 8 1 0
0 1 1 8 1 1 8
B sends a message to A C A C
update
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Distance to Reach Node
Distance to Reach Node
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A B C D E F G
1 0 1 2 2 2 8
1 1 0 1 2 2 8
2 2 1 0 8 8 1
1 2 2 3 0 2 8
1 2 2 3 2 0 1
8 8 8 1 8 1 0
0 1 1 2 1 1 8
A B C D E F G
1 0 1 8 2 2 8
1 1 0 1 2 2 8
8 8 1 0 8 8 1
1 2 2 8 0 2 8
1 2 2 8 2 0 1
8 8 8 1 8 1 0
0 1 1 8 1 1 8
C sends a message to A BD A BD
update
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Distance to Reach Node
Distance to Reach Node
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 8 8 1
1 2 2 3 0 2 4
1 2 2 3 2 0 1
8 8 2 1 8 1 0
0 1 1 2 1 1 3
A B C D E F G
1 0 1 2 2 2 8
1 1 0 1 2 2 8
2 2 1 0 8 8 1
1 2 2 3 0 2 8
1 2 2 3 2 0 1
8 8 8 1 8 1 0
0 1 1 2 1 1 8
D sends a message to C G C G
update
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Distance to Reach Node
Distance to Reach Node
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 8 8 1
1 2 2 3 0 2 4
1 2 2 3 2 0 1
8 8 2 1 8 1 0
0 1 1 2 1 1 3
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 8 8 1
1 2 2 3 0 2 4
1 2 2 3 2 0 1
8 8 2 1 8 1 0
0 1 1 2 1 1 3
E sends a message to A A
update
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Distance to Reach Node
Distance to Reach Node
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 8 8 1
1 2 2 3 0 2 3
1 2 2 3 2 0 1
2 8 2 1 8 1 0
0 1 1 2 1 1 2
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 8 8 1
1 2 2 3 0 2 4
1 2 2 3 2 0 1
8 8 2 1 8 1 0
0 1 1 2 1 1 3
F sends a message to A G A G
update
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Distance to Reach Node
Distance to Reach Node
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 8 2 1
1 2 2 3 0 2 3
1 2 2 2 2 0 1
2 8 2 1 8 1 0
0 1 1 2 1 1 2
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 8 8 1
1 2 2 3 0 2 3
1 2 2 3 2 0 1
2 8 2 1 8 1 0
0 1 1 2 1 1 2
G sends a message to F D F D
update
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Round 2
Distance to Reach Node
Distance to Reach Node
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 3 2 1
1 2 2 3 0 2 3
1 2 2 2 2 0 1
2 3 2 1 3 1 0
0 1 1 2 1 1 2
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 8 2 1
1 2 2 3 0 2 3
1 2 2 2 2 0 1
2 8 2 1 8 1 0
0 1 1 2 1 1 2
A sends a message to B C E F
B C E F update
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Distance to Reach Node
Distance to Reach Node
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 3 2 1
1 2 2 3 0 2 3
1 2 2 2 2 0 1
2 3 2 1 3 1 0
0 1 1 2 1 1 2
A B C D E F G
1 0 1 2 2 2 3
1 1 0 1 2 2 2
2 2 1 0 3 2 1
1 2 2 3 0 2 3
1 2 2 2 2 0 1
2 3 2 1 3 1 0
0 1 1 2 1 1 2
B sends a message to A C A C
update
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Link-State Routing

• Overcomes instabilities in DV
• Pair of switches periodically
• Test link between them
• Broadcast link status message
• Switch
• Receives status messages
• Computes new routes
• Uses Dijkstras algorithm

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Example Of Link-State Information

• Assume nodes 2 and 3
• Test link between them
• Broadcast information
• Each node
• Receives information
• Recomputes routes as needed

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Dijkstras Shortest Path Algorithm

• Input
• Graph with weighted edges
• Node n
• Output
• Set of shortest paths from n to each node
• Cost of each path
• Called Shortest Path First (SPF) algorithm

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Dijkstras Algorithm
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Algorithm Intuition

• Start with self as source node
• Move outward
• At each step
• Find node u such that it
• Has not been considered
• Is closest to source
• Compute
• Distance from u to each neighbor v
• If distance shorter, make path from u go
  through v

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Result Of Dijkstras Algorithm

• Example routes from node 6
• To 3, next hop 3, cost 2
• To 2, next hop 3, cost 5
• To 5, next hop 3, cost 11
• To 4, next hop 7, cost 8

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Early WAN Technologies

• ARPANET
• Historically important in packet switching
• Fast when invented slow by current standards
• X.25
• Early commercial service
• Still used
• More popular in Europe

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Recent WAN Technologies

• SMDS
• Offered by phone companies
• Not as popular as Frame Relay
• Frame Relay
• Widely used commercial service
• Offered by phone companies
• ATM