1. Chapter 10 Data Communications and Computer Networks: A
Business User’s Approach
Chapter 10
Introduction to Metropolitan Area Networks and Wide Area Networks

2. This time/last time Focus has been on LANs Now MANs and WANs
Want to connect WANs
Internet
Telephony
Cell

3. Introduction 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.

4. 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

5. 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

6. 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.

7. Data Communications and Computer Networks
Chapter 10

8. Synchronous Time Division Multiplexing
Remember SONET
Synchronous Time Division Multiplexing
Three types popular today:
T-1 multiplexing (the classic)
ISDN multiplexing
SONET (Synchronous Optical NETwork)

9. 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

10. SONET systems are comprised of multiple rings

11. The Ethernet MAN Topology

12. 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.

13. 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

14. 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.

15. Data Communications and Computer Networks
Chapter 10

16. 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.

17. Data Communications and Computer Networks
Chapter 10

18. Types of Network Subnets
Categorize network by the way it transfers information from one node to another – physical entity
Circuit switched
Packet switched
Broadcast

19. 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.

20. Data Communications and Computer Networks
Chapter 10

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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.

27. Connection-oriented telephone call

28. Connectionless postal system

29. 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.

30. 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.

31. Data Communications and Computer Networks
Chapter 10

32. 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.

33. A Simple Example of a Network Graph

34. What do we notice about network graphs? Connectivity
Values on the links – weighted network graph
Transitivity

35. 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.

36. 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

37. 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.

38. 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.

39. Network with Flooding Routing Starting from Node A

40. Flooding has continued and is now being done at Nodes B, C & D

41. 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.

42. Data Communications and Computer Networks
Chapter 10

43. Centralized Routing Data Communications and Computer Networks
Chapter 10
Centralized Routing
Problems:
Centralized routers can easily fail.
How does the routing table scale?

44. 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.

45. Data Communications and Computer Networks
Chapter 10

46. 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.

47. 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.

48. 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

49. 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

50. WANs In Action: Making Internet Connections
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.

51. Data Communications and Computer Networks
Chapter 10

52. WANs In Action: Making Internet Connections
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).

53. Data Communications and Computer Networks
Chapter 10

54. 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

55. What we covered Routing algorithms find the path
Global vs decentralized
Flooding
Dynamic vs static