Transcript Intro to MANs and WANs - Pennsylvania State University

Data Communications and
Computer Networks: A
Business User’s Approach
Chapter 10
Introduction to Metropolitan Area
Networks and Wide Area Networks
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This time/last time
• Focus has been on LANs
• Now MANs and WANs
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Want to connect WANs
Internet
Telephony
Cell
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Data Communications and Computer Networks
Chapter 10
Introduction
As we have seen, a local area network covers a room, a
building or a campus.
A metropolitan area network (MAN) covers a city or a region
of a city.
A wide area network (WAN) covers multiple cities, states,
countries, and even the solar system.
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Metropolitan Area Network Basics
MANs borrow technologies from LANs and WANs.
MANs support
high-speed disaster recovery systems
real-time transaction backup systems
interconnections between corporate data centers and
internet service providers, and government, business,
medicine, and education high-speed interconnections.
Almost exclusively fiber optic systems
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Metropolitan Area Network Basics
MANs have very high transfer speeds
MANs can recover from network faults very quickly (failover
time)
MANs are very often a ring topology (not a star-wired ring)
Some MANs can be provisioned dynamically
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Failover
Failover is a backup operational mode in which the functions
of a system component (such as a processor, server,
network, or database, for example) are assumed by
secondary system components when the primary
component becomes unavailable through either failure or
scheduled down time.
Used to make systems more fault-tolerant, integral part of
mission-critical systems that must be constantly available.
The procedure involves automatically offloading tasks to a
standby system component so that the procedure is as
seamless as possible to the end user.
Failover can apply to any aspect of a system: network
component or system of components, such as a connection
path, storage device, or Web server.
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Data Communications and Computer Networks
Chapter 10
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Remember SONET
Synchronous Time Division
Multiplexing
Three types popular today:
•T-1 multiplexing (the classic)
•ISDN multiplexing
•SONET (Synchronous Optical NETwork)
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SONET versus Ethernet MANs
Most MANs are SONET networks built of multiple rings (for
failover purposes)
•SONET is well-proven but complex, fairly expensive, and
cannot be provisioned dynamically.
•SONET is based upon T-1 rates and does not fit nicely into 1
Mbps, 10 Mbps, 100 Mbps, 1000 Mbps chunks, like Ethernet
systems do.
Ethernet MANs
•Well understood, scale well and best technology to carry IP
traffic (internet)
•Have high failover times (slow recovery to failure)
•Growing in popularity
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SONET systems are comprised of multiple rings
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The Ethernet MAN Topology
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Wide Area Network Basics
WANs used to be characterized with slow, noisy lines. Today
WANs are very high speed with very low error rates.
WANs often follow a mesh topology.
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Characteristics of WANs
• Similarities to LANs
– Interconnect computers.
– Use some form of media for the interconnection.
– Support network applications.
• Differences to LANs
– Include both data networks, such as the Internet, and voice
networks, such as telephone systems.
– Interconnect more workstations, so that any one
workstation can transfer data to any other workstation.
– Cover large geographic distances, including the earth.
– Subnet
• Originally routers and lines, now also means a section of network
addressing
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Why WANs?
Federal Express package routing system.
American Airlines reservation system.
Amazon.com.
Visa International payment process system.
Any application system that is based on the
Internet.
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Data Communications and Computer Networks
Chapter 10
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Wide Area Network Basics
A station is a device that interfaces a user to a network.
A node is a device that allows one or more stations to access
the physical network and is a transfer point for passing
information through a network.
A node is often a computer, a router, or a telephone switch.
The subnet (old terminology) or physical network is the
underlying connection of nodes and telecommunication links.
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Data Communications and Computer Networks
Chapter 10
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Types of Network Subnets
Categorize network by the way it
transfers information from one node to
another – physical entity
•Circuit switched
•Packet switched
•Broadcast
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Circuit Switched Network
Circuit switched network - a network in which a dedicated
circuit is established between sender and receiver and all data
passes over this circuit.
The telephone system is a common example.
The connection is dedicated until one party or another
terminates the connection.
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Data Communications and Computer Networks
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Packet Switched Network
Packet switched network - a network in which all data
messages are transmitted using fixed-sized packages, called
packets.
More efficient use of a telecommunications line since packets
from multiple sources can share the medium.
One form of packet switched network is the datagram. With
a datagram, each packet is on its own and may follow its own
path.
Virtual circuit packet switched network create a logical path
through the subnet and all packets from one connection
follow this path.
Virtual path is not physically real, but acts like a circuit. It exists
only in software of the routers. Not used in today’s internet.
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Packets vs frames
Packet is a more general term.
Messages transmitted over the network
Packets for messages at the OSI network layer (routing).
Frames for messages at the OSI data link layer.
Datagrams are packets built such that each packet is on its
own and may follow its own path.
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Switching Comparison:
Packet switching: transmission of messages by dividing the
message up into packets and transmitting it over many lines.
The message is reconstructed at the end. Ex. internet
Circuit switching: transmission of messages over a dedicated
line; all packets go other this line.
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Broadcast Network
Broadcast network - a network typically found in local area
networks but occasionally found in wide area networks.
A workstation transmits its data and all other workstations
“connected” to the network hear the data. Only the
workstation(s) with the proper address will accept the data.
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Connection-oriented versus Connectionless
The network structure is the underlying physical component
of a network. What about the software or application that
uses the network?
What is the logical entity of the connection?
A network application can be either connection-oriented
or connectionless.
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Connection-oriented versus Connectionless
A connection-oriented application requires both sender and
receiver to create a connection before any data is transferred.
Applications such as large file transfers and sensitive
transactions such as banking and business are typically
connection-oriented.
A connectionless application does not create a connection
first but simply sends the data.
Electronic mail is a common example.
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Connection-oriented telephone call
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Connectionless postal system
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Logical on the physical
A connection-oriented application can operate over
both a circuit switched network or a packet switched
network.
A connectionless application can also operate over
both a circuit switched network or a packet switched
network but a packet switched network may be more
efficient.
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Routing Choices
Each node in a WAN is a router that accepts an input packet,
examines the destination address, and forwards the packet on
to a particular telecommunications line.
How does a router decide which line to transmit on?
A router must select the one transmission line that will best
provide a path to the destination and in an optimal manner.
Often many possible routes exist between sender and receiver.
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Data Communications and Computer Networks
Chapter 10
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How Routing Works
The subnet with its nodes and telecommunication
links is essentially a weighted network graph.
The edges, or telecommunication links, between
nodes, have a cost associated with them.
The cost could be a delay cost, a queue size cost, a
limiting speed, or simply a dollar amount for using
that link.
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A Simple Example of a Network Graph
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Network Graphs
What do we notice about network graphs?
•Connectivity
•Values on the links – weighted network
graph
•Transitivity
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Routing Method
The routing method, or algorithm, chosen to move packets
through a network should be:
• Optimal, so the least cost can be found
• Fair, so all packets are treated equally
• Robust, in case link or node failures occur and the network
has to reroute traffic.
• Not too robust so that the chosen paths do not oscillate too
quickly between troubled spots.
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Find the minimal cost
route from one node to
another.
Need an algorithm to
do that; “eyeballing”
does not always work
This is what a routing
algorithm does
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Forward Search Least Cost Routing
Algorithm
Dijkstra’s least cost algorithm finds all possible paths
between two locations.
By identifying all possible paths, it also identifies the least
cost path.
Cost is reflected in value on the links. Examples?
The algorithm can be applied to determine the least cost path
between any pair of nodes.
Only calculated periodically since one has to get through the
entire net to do it.
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Flooding Routing
When a packet arrives at a node, the node sends a copy of the
packet out every link except the link the packet arrived on.
Traffic grows very quickly when every node floods the
packet.
To limit uncontrolled growth, each packet has a hop count.
Every time a packet hops, its hop count is incremented.
When a packet’s hop count equals a global hop limit, the
packet is discarded.
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Network
with
Flooding
Routing
Starting
from
Node A
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Flooding
has
continued
and is
now being
done at
Nodes B,
C&D
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Centralized Routing
One routing table is kept at a “central” node.
Whenever a node needs a routing decision, the central node is
consulted.
To survive central node failure, the routing table should be
kept at a backup location.
The central node should be designed to support a high amount
of traffic consisting of routing requests.
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Data Communications and Computer Networks
Chapter 10
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Data Communications and Computer Networks
Chapter 10
Centralized Routing
Problems:
Centralized routers can easily fail.
How does the routing table scale?
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Distributed Routing
Each node maintains its own routing table.
No central site holds a global table.
Somehow each node has to share information with other
nodes so that the individual routing tables can be created.
Possible problem with individual routing tables holding
inaccurate information.
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Data Communications and Computer Networks
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Isolated Routing
Each node uses only local information to create its own
routing table.
Advantage - routing information does not have to be passed
around the network.
Disadvantage - a node’s individual routing information could
be inaccurate, or out of date.
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Adaptive Routing versus Static Routing
With adaptive routing, routing tables can change to reflect
changes in the network
Static routing does not allow the routing tables to change.
Static routing is simpler but does not adapt to network
congestion or failures.
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Network Congestion
When a network or a part of a network becomes so saturated
with data packets that packet transfer is noticeably impeded,
network congestion occurs.
Preventive measure include providing backup nodes and links
and preallocation of resources.
To handle network congestion, you can perform buffer
preallocation, choke packets, or permit systems.
Forward and backward explicit congestion control also used
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Quality of Service (QoS)
Before making a connection, user requests how much
bandwidth is needed, or if connection needs to be real-time
Network checks to see if it can satisfy user request
If user request can be satisfied, connection is established
If a user does not need a high bandwidth or real-time, a
simpler, cheaper connection is created
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Data Communications and Computer Networks
Chapter 10
WANs In Action: Making Internet
Connections
Home to Internet connection - modem and dial-up telephone
provide circuit switched subnet, while connection through the
Internet is a packet switched subnet.
The application can be either a connection-oriented
application or a connectionless application.
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Data Communications and Computer Networks
Chapter 10
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Data Communications and Computer Networks
Chapter 10
WANs In Action: Making Internet
Connections
A work to Internet connection would most likely require a
broadcast subnet (LAN) with a connection to the Internet
(packet switched subnet).
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Data Communications and Computer Networks
Chapter 10
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What we covered
• WANs and MANs
• Physical network connections
– Circuit, Packet, Broadcast switching
• Logical network connections
– Connection oriented vs connectionless
• Network graphs
– Nodes and edges (links)
• Routing algorithms
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What we covered
• Routing algorithms find the path
– Global vs decentralized
– Flooding
– Dynamic vs static
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