Wide Area Networks WANs

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• Wide Area Networks (WANs)

2G1316 Data Communications and Computer
Networks 2E1623 Data Links and Local Area
Networks
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Illustrations in this material are collected from
Behrouz A Forouzan, Data Communications and
Networking, 3rd edition, McGraw-Hill.
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Todays Lecture

• Networking in Wide Area
• Switched networks for WANs
• Circuit and Packet Switching
• datagram and virtual circuit networks
• Routing
• Congestion control
• ATM

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Wide Area Networking

• Wide area communication includes
• Public telephony
• Public Internet communication
• Corporate networking, data exchange for banks,
  etc.
• Public and private networks
• Public networks unlimited access, management and
  maintenance not the users responsibility
• Private networks secure, controlled access,
  large investment, technical expertise
• Virtual Private Networks (VPNs)

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Switched Networks
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• Switching nodes
• not concerned with contents of data
• purpose provide switching facility
• in general not fully connected
• End nodes
• provides data to transfer
• connected via switching nodes
• Links
• physical connections between nodes

A
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Switched Networks

• How to switch information from one link to the
  other?
• Store-and-forward
• Synchronous switching
• Time slots
• How to select the transmission path (route)
  through the network?
• routing
• How to ensure that links do not get overloaded?
• congestion control

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Switching
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Circuit Switching

• Establishment
• Channels over internal links
• TDMA, FDMA, WDMA
• At each node
• Make routing decision
• Reserve resources

• Data transfer
• Fixed capacity of connection
• Disconnect
• Free resources

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TDM Circuit Switching With Time-Slot Interchange
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Properties of Circuit Switching

• Fixed data rate with low delay and small delay
  variations
• Suitable for voice and video
• Delay before connection can be used
• Resources are permanently allocated to connection
• Much idle time ? low utilisation
• Variable bit rate traffic ? channel has to be
  allocated for peak rate (maximum rate)

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Packet Switching

• Data transmitted in short packets
• Large messages broken up into a number of small
  packets
• Store and forward switching
• Receive-store-transmit

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Properties of Packet Switching

• Link capacity dynamically shared
• Traffic with changing bit rate
• Connections with different bandwidth needs
• Packets stored and delayed at switching nodes
• Larger and variable (non-deterministic) traffic
  delay
• Voice and video transmission more difficult

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Packet Switch (Router)

• Functions
• receives packet from end nodes and other switches
• decides where to forward the packet
• stores packet until the transmission link becomes
  free
• transmits packet

Input/output controllers
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Packet Switch Buffer Designs
switch fabric
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Packet Switching Service Models

• How to transmit a sequence of packets?
• Datagram
• each packet is treated independently
  (connectionless)
• each packet contains full address information
• Virtual Circuit Switching
• a connection is set up prior to data transfer
  (connection oriented)
• each packet only contains a virtual connection
  (VC) identifier

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Datagram Versus Virtual Circuit
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Performance Comparison
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Routing

• Find the best path through the network
• Best by some criteria
• Requirements
• Universal
• For each destination
• Correct
• Should lead to destination
• Simple
• Robust
• Should work in face of overload and congestion
• Stable
• Should not oscillate between extreme points
• Fair
• Should not lock out any packets
• Optimal and efficient
• should use the resources as efficiently as
  possible

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Routing Table

• Routing information stored at each switch next
  hop for each destination
• In this example Next hop specified by link
  endpoints (from, to)

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Routing Strategies

• Static routing
• Routing table entries are defined when the switch
  starts
• Simple, low network overhead
• Inflexible, does not adopt to failures and
  traffic conditions
• Dynamic routing
• Routing table entries change as conditions in the
  network change
• Flexible, can improve performance
• Complex, adds overhead

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Routing Strategies (contd)

• Optimality criteria
• path with the fewest number of links
• The shortest output queue
• The lowest end to end delay
• Administrative policies
• A router computes least cost path to all other
  routers
• Dijkstras algorithm
• Bellman-Ford algorithm
• Graph representation
• switches, links ? nodes, edges
• Weight on edgesvarious definitions of least cost
  path
• Shortest path, inverse capacity,
• Routing tables constructed during the computation

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Link State Routing

• A node advertizes its link state
• All directly connected nodes (neighbours), and
  the cost to reach them
• Information is broadcast to all other nodes
• flooding
• Dijkstras algorithm for computing shortest path
  (least cost path)

a
b
3
L1
L2
L3
1
1
c
2
2
4
d
e
1
Link state advertisement from a
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Dijkstras Algorithm (Shortest Path First)
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3
a
b
a
b
Find shortest paths from a to all other nodes!
1
1
1
1
2
2
c
c
2
2
4
4
d
e
d
e
1
1
M Db (path) Dc (path) Dd (path) De (path)
3 (a-b) 1 (a-c) 8 (--) 2 (a-e)
a
2 (a-c-b) 1 (a-c) 8 (--) 2 (a-e)
a, c
2 (a-c-b) 1 (a-c) 4 (a-c-b-d) 2 (a-e)
a, c, b
2 (a-c-b) 1 (a-c) 3 (a-e-d) 2 (a-e)
a, c, b, e
2 (a-c-b) 1 (a-c) 3 (a-e-d) 2 (a-e)
a, c, b, e, d
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Dijkstras Algorithm

• Initialize
• M s
• For all n ?N Dn dsn
• Find w, the untreated node with lowest cost
• Find w ? M such thatDw Dj for all j ? M
• If no such w exists, we are done
• Otherwise add w to M
• Update cost to (untreated) nodes is there a
  lower cost path via w?
• For all n ? MDn min(Dn, Dwdwn)
• Go back to 2

• Input
• N set of network nodes
• s source node
• dij link cost from i to j
• 8 if i and j are not directly connected
• Output
• Dn minimum cost of path from s to n
• Temporarily use
• M set of nodes treated

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Distance Vector Routing
a
b

• A node advertizes its distance-vector
• A list (vector) of all nodes that the node knows
  about
• The distance to each of them
• Advertizements are sent to neighbours only
• Each neighbour updates its routing table and
  sends the new distance-vectors to its neighbours
• Bellman-Ford algorithm

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1
1
c
2
2
4
d
e
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Distance-vector from e
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Bellman-Ford Algorithm

• Input
• N set of network nodes
• s source node
• dij link cost from i to j
• Output
• Dhn min. cost of path from s to n in no more
  than h hops

• Initialize
• For all n ? N, n ? s D0n 8
• For all h 0 Dhs 0
• Iterate through all path lengths for each
  length, find least cost paths for all nodes.
• For each successive h 0
• For all n ? N Dh1n minj Dhj djn
• If Dh1n Dhj for all n, we are done

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Congestion in Packet Switching Networks

• End nodes transmit data independently from each
  other
• What happens if more packets arrive than what is
  possible to forward?

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Congestion Scenario

• Output buffer becomes full
• discard packets
• source retransmits
• more messages in the network
• more buffers saturated
• delay increases
• source times out
• more retransmissions
• capacity drops towards zero

• Need congestion control

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Congestion Control

• Long term congestions network dimensioning
  (Erlang)
• Short term congestion congestion control
• Transmitting nodes have to be informed and have
  to slow down
• Congested node sends congestion information to
  sources
• Congested node sends information to the
  destination, destination reports to the node
• Source node relies on routing information with
  end-to-end delay
• Source node relies on packet loss information
  (e.g., TCP)
• Admission control applied for new connections
  (VCS networks)

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WAN Examples

• Datagram networks
• Internet
• From ARPANET project
• late 60s
• US Department of Defense
• Based on TCP/IP
• Circuit switching networks
• Telephone network
• TDM hierarchy
• DS-0, DS-1, etc
• E1/T1
• SONET/SDH
• OC12/STM 4

• Connection-oriented networks
• ATM
• Virtual circuit switching
• Different traffic types
• Video, telephony, data
• AAL

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Virtual Circuit Switching

• Connection-oriented packet switching
• Virtual circuit identifier (VCI)
• Switch scopeonly valid between two switches
• Altered when frames are switched

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Virtual Circuit Switching from A to B
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ATMAsynchronous Transfer Mode

• Multiplexing of frames of different sizes
  regarded as a big problem
• In particular for real-time applications, such as
  audio and video
• Smaller (audio, video) frames get delayed by
  large frames

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ATM multiplexing

• Data stream divided into fixed sized frames
  called cells
• Asynchronous TDM multiplexing
• Each time slot can take one cell
• Slots are filled with cells from inputs where
  there are cells

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Connections in ATM

• Two levels
• Virtual path (VP) is a collection of Virtual
  Circuits (VCs)
• Hierarchical routing
• Many switches need only route using VP

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ATM Cells

• Fixed size 53 bytes
• 48 bytes of payload, 5 bytes of header
• Addressing by Virtual Path Identifier and Virtual
  Circuit Identifier (VPI VCI)

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ATM Networks

• Based on SONET/SDH
• 155 Mb/s, 622 Mb/s
• Both telephone traffic and data
• Application Adaption Layer (AAL) protocols
• AAL1 (constant bit rate), AAL2 (low-bit-rate,
  short-frame traffic, e.g. Mobile telephony)
• AAL3/4, AAL5 (data)

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Summary

• WAN requirements
• Switched networks
• Circuit and Packet Switching
• Packet Switching
• the packet switch
• datagram and virtual circuit switching
• routing
• congestion control

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Reading Instructions

• Behrouz A. Forouzan, Data Communications and
  Networking, third edition
• 18 Virtual Circuit Switching Frame Relay and ATM
• 18.1 Virtual Circuit Switching
• 18.3 ATM
• 19 Host-to-Host Delivery ...
• 19.1 Internetworks