1. NT1210 Introduction to Networking
Unit 7:
Chapter 7, Wide Area Networks

2. Objectives
Identify the major needs and stakeholders for computer networks and network applications.
Identify the classifications of networks and how they are applied to various types of enterprises.
Explain the functionality and use of typical network protocols.
Analyze network components and their primary functions in a typical data network from both logical and physical perspectives. 2

3. Objectives
Differentiate among major types of LAN and WAN technologies and specifications and determine how each is used in a data network.
Explain basic security requirements for networks.
Use network tools to monitor protocols and traffic characteristics.
Use preferred techniques and necessary tools to troubleshoot common network problems.
Differentiate among WAN technologies available from service providers 3

4. Objectives Evaluate how WAN devices function
Define and describe WAN protocols
Evaluate troubleshooting techniques for WAN connections 4

5. Introducing Wide Area Networks: Basic Telco Services
Telephone, Telcos, and companies that grew from original Bell System impact how today’s WANs work
Telcos built huge networks to support voice traffic, long before computers could create and send bits
Timeline Comparison of Inventions Compared to Telephone 5
Figure 7-1

6. Introducing Wide Area Networks: Basic Telco Services – Circuit Switching
Early Voice: Telco Creates One Analog Electrical Circuit Between Phones 6
Figure 7-2

7. Introducing Wide Area Networks: Basic Telco Services – Circuit Switching
Switched Analog Circuits for Data: To create first WAN connections, early computing devices had to act like telephones
One computer device would “make phone call” to other computer, encoding its bits using analog electrical signals 7

8. Introducing Wide Area Networks: Basic Telco Services – Circuit Switching
Following the left-to-right example in the figure:
The PC sends bits over a (short) cable to the modem.
The modem converts the bits into analog electrical signals, with an encoding scheme that represents bits over time as different analog electrical signals (usually a different frequency).
The analog signals arrives at the far side of the circuit, where:
The modem translates back to a digital signal (bits).
Connecting from a PC to an ISP, Using Modems and an Analog Telco Circuit 8
Figure 7-4

9. Introducing Wide Area Networks: Basic Telco Services – Circuit Switching
Beginning mid-20th century Telcos transformed
Invention and commercialization of computers: Started with few computers being rare and unusual to world where most companies owned computers
Migration from Telcos as government monopolies to free- market competition: Governments started removing monopoly status from different parts of Telcos’ business so allowed competition
Computerization of Telco’s own network: Revolutionized how Telco built its internal network to create better services at lower cost 9

10. Introducing Wide Area Networks: Basic Telco Services – Circuit Switching
Digital Circuits and Leased Lines: Telcos started offering service that used digital circuit between customer devices
Endpoints still had circuit between them but could encode signal as bits with different electrical signals that followed encoding rules
More Modern Routers Using a Digital Leased Line 10
Figure 7-5

11. Introducing Wide Area Networks: Basic Telco Services – Circuit Switching
Switched Circuits and Circuit Switching: When user calls phone number, various circuit switches connect circuit on both sides of switch (see arrowed lines)
Circuit switches create effect of end-to-end circuit by switching/connecting circuits on various links
Circuit Switching 11
Figure 7-6

12. Introducing Wide Area Networks: Basic Telco Services – Circuit Switching
Circuit: Communication path between two endpoints
Circuit Switching: Logic used by Telco network and devices called “circuit switches” that allows them to switch circuits in and out of different physical trunks to create end-to-end circuit through network
Switched Circuit: End-to-end circuit through Telco that changes over time because user calls number, hangs up, calls another number, and so on
Dedicated Circuit (leased line): Circuit between two specific devices Telco never takes down 12

13. Introducing Wide Area Networks: Basic Telco Services – Packet Switching
Packet Switching: Telcos next started offering WAN services using packet switching services
General Timeline: Circuit Switching, Digital Circuits, and Packet Switching 13
Figure 7-7

14. Introducing Wide Area Networks: Basic Telco Services – Packet Switching
All customer devices need direct connection to WAN via circuit to packet switching service
Customers: All devices can send data to every other device connected to packet switched service
Telco (service provider): Must look at meaning of bits in customer’s headers and make forwarding decision per packet 14

15. Introducing Wide Area Networks: Basic Telco Services – Packet Switching
Packet Switching Example
For this example, the following steps occur:
Router R1 (owned by the customer) sends a message, with a header that lists R2 as the destination address.
Packet Switch A (owned by the WAN Service Provider) makes a choice to forward the message to packet switch B.
Packet switch B (owned by the WAN Service Provider) makes a choice to forward the message to R2.
Example of Packet Switching Service 15
Figure 7-8

16. Introducing Wide Area Networks: Routers
Connect LANs to WANs
Follow the steps in the figure
PC1’s IP logic tells it to send the IP packet to the nearby router (R1)
R1’s IP logic tells it to make a routing decision, based on the destination IP address; that decision is to forward the IP packet over the WAN to router R2.
R2’s IP logic tells it to make a routing decision, based on the destination IP address; that decision is to forward the IP packet over the LAN to host PC2.
Layer 3 IP Forwarding Logic 16
Figure 7-9

17. Introducing Wide Area Networks: Routers
LAN might be simple Ethernet-only LAN
LAN might be simple WLAN
LAN might be more complex campus LAN with both wired and wireless LANs
Example Enterprise Network, With LAN and WAN Details Revealed 17
Figure 7-10

18. Introducing Wide Area Networks: Routers
Encapsulation and De-encapsulation
Follow the steps in the figure:
PC1 sends an IP packet, inside an frame, to R1.
R1 strips off the header and trailer, adds a WAN header and trailer, and forwards the frame over the WAN to R2.
R2 strips off the WAN header and trailer, adds a new (different) header and trailer, and sends the frame to PC2.
Encapsulation that Happens During the IP Packet Forwarding Process 18
Figure 7-11

19. Introducing Wide Area Networks: Topologies
Point-to-Point Topology: Basic WAN service
LAN with10BASE-T or 100BASE-T cable has 2-pair: 1 pair for sending data in each direction
Both LAN and WAN topologies allow full duplex operation and can share 1 link
Point-to-Point Topologies in WAN and LAN 19
Figure 7-12

20. Introducing Wide Area Networks: Topologies
Hub and Spoke Topologies
Reduces number of leased lines
Provides way for packets to reach all sites
Connects one router (hub router) to all other routers using leased lines
WAN Hub and Spoke Topology Vs. LAN Star Topology 20
Figure 7-13

21. Introducing Wide Area Networks: Topologies
Multipoint topologies: Hub-and-spoke topology has some disadvantages
Uses leased lines that might have to run hundreds or thousands of miles at large expense
Packets that go from one spoke site to another spoke site have to cross multiple WAN links
WAN Multipoint Topology 21
Figure 7-14

22. Understanding Leased Line WAN Links
Distance limitations: No single circuit extends entire distance between two routers
“Point to point” circuits really series of circuits
Leased Line: Shorter Electrical Circuits, Knitted Together 22
Figure 7-16

23. Understanding Leased Line WAN Links
Telco installs physical cable between equipment in CO to customer site
2-pair cable typically runs underground into customer buildings terminating near customer’s router
Cables in a Relatively Short Leased Line 23
Figure 7-18

24. Understanding Leased Line WAN Links
Customer needs to plan for cabling at end of Telco’s leased line cable
Example: Customer’s router connects to cable installed by Telco
Components and Responsibilities on One Side of a Leased Line 24
Figure 7-19

25. Understanding Leased Line WAN Links
Leased line has Channel Services Unit/Data Services Unit (CSU/DSU) function on each side of line at customer site
Each site uses either internal or external CSU/DSU
Internal CSU/DSU sits inside router as part of serial interface card
Customer Equipment and Cabling with External CSU/DSU 25
Figure 7-20

26. Understanding Leased Line WAN Links
Example: Cisco router with two slots for removable router interface cards (WICs) where serial cards are install
Serial card on left has built-in CSU/DSU and uses RJ-48 connector
Serial card on right does not have CSU/DSU so relies on external CSU/DSU
1921 router…
WIC-1CSU:
Photos of Router and Removable WAN Cards 26
Figure 7-22

27. Understanding Leased Line WAN Links
Key steps for installing leased lines
Order leased line from Telco; include specs on line speed, cable connectors required, and exact location where cable should be installed (address, floor, identifying information for exact room)
Install router and serial interface cards in router as needed by leased line
If interface card does not have internal CSU/DSU, choose CSU/DSU and matching cable
Physically connect all cables
Configure devices (beyond scope of this chapter) 27

28. Break
Take 10 28

29. Understanding Leased Line WAN Links: Multiplexing
Possible solution: Telco could install three T1 trunk lines between CO switches
Telco Switching Connecting Incoming Customer T1s to T1 Trunks 29
Figure 7-24

30. Understanding Leased Line WAN Links: Multiplexing
More efficient solution: Time Division Multiplexing (TDM) uses TDM switches and one T3 trunk
Telco connects cable using T3 card in each TDM switch to use T3 link ( Mbps—28 times T1 speed)
CO Switches Multiplexing T1 Bits onto Faster T3 Circuit 30
Figure 7-25

31. Understanding Leased Line WAN Links
Customer buys T1 line at each site with full T1 speed (1.536 Mbps)
What happens if customer router can only transmit at 768 Kbps?
Speed Differences on a 768-Kbps Leased Line WAN 31
Figure 7-29

32. Understanding Leased Line WAN Links
Type of Line
Geography
Speed
Number of Channels
DS0
USA
64 Kbps
N/A
DS1 (T1)
1.544 Mbps
24 DS0
DS3 (T3)
Mbps
28 DS1 E0
Europe E1
2.048 Mbps
32* E0 E3
Mbps
16 E1 J0
Japan J1
24 J0 J3
Mbps
20 J1
* 30 E0 channels are available for customer data; 2 E0 channels are for other functions.
Summary of Carrier TDM Line Standards 32
Table 7-3

33. Understanding Packet Switching and Multi-Access WANs
With packet switching, link capacity between switches used to forward packets as needed or available
The Telco, acting as a packet switching service, expects that over time, the total of all bits from all routers will not exceed a T3’s worth of bits.
Router A1 sends its next message over its T1 link connects to Packet Switch 1.
The header of the packet identifies router A2 as the destination.
Packet Switch 1 looks at the destination address, and decides to send the packet out the T3 link (port 2) on the right.
The same packet crosses the T3 link.
The packet arrives at the second packet switch, which matches its forwarding table, sending the packet out port 4 towards router A2 33

34. Understanding Packet Switching and Multi-Access WANs: Frame Relay
Frame Relay: Allows any device connected to network to communicate with any other network and details of Frame Relay design do not matter
Typical Drawing of a Frame Relay Design, One Customer, Ignoring Details 34
Figure 7-44

35. Understanding Packet Switching and Multi-Access WANs: Frame Relay
Frame Relay physical links: Edge between customer site and Frame Relay network
Point of Presence (PoP): Where Telco devices/cables interface with customer premises
DTE (Data Terminal Equipment): Customer device (e.g., router)
Frame Relay switch: Telco device that forwards customer frames (also called DCE [Data Communications Equipment])
Access link: Physical link between DTE and DCE
DLCI: Data Link Control Identifier, used instead of IP address 35

36. Understanding Packet Switching and Multi-Access WANs: Frame Relay
Frame Relay terms
One Possible Telco Implementation of the Frame Relay Network 36
Figure 7-45

37. Understanding Packet Switching and Multi-Access WANs
Packet Switching Services: SONET speeds
Name
(Rounded) Line Speed (in Mbps)
OC-1 52
OC-3
155
OC-12
622
OC-24
1244
OC-48
2488
OC-96
4976
OC-192
9952
SONET Optical Carrier (OC) Names and (Rounded) Line Speeds 37
Table 7-5

38. Summary, This Chapter…
Compared switched circuits as used for a typical home telephone call with two computers sending data over a similar switched circuit using modems.
Explained the basic differences between a circuit switching WAN service and a packet switching WAN service from the customer’s perspective.
Illustrated the reasons why IP routers work well at forwarding data between different types of LANs and WANs.
Drew common WAN topologies. 38

39. Summary, This Chapter…
Drew and contrasted the different customer-site cabling for a leased line WAN installed between two routers.
Listed the types of physical links in the US T-carrier hierarchy, their approximate speeds, and the specific number of slowed-speed channels that fit in the next higher-speed line.
Explained how Telcos use CSU/DSUs to match a leased line speed to a physical DS1 line, using an example of a 768 Kbps fractional T1 leased line between two routers.
Compared and contrast the HDLC and PPP standards. 39

40. Summary, This Chapter…
Explained the differences between packet switching and circuit switching from the Telco perspective.
Used an example network, explain how with Frame Relay, a router can have one physical link connected to the WAN, but send data to many other destination routers.
Listed the other WAN packet switching services, and show whether they were introduced before or after Frame Relay. 40

41. Questions? Comments? 41

42. Unit 7 Assignment Unit 7 Assignment 1: Wide Area Networks Review
Complete the multiple-choice questions
Complete the Define Key Terms table and the List the Words Inside the Acronyms table.
Reading Assignment. Read Chapter 8

43. Unit 7 Lab Complete all Labs in Chapter 7 of the lab book.
Lab should be completed in class.
Uncompleted Lab must be submitted in the next class.

44. Research Project
Unit 7 Research Project 1: Chapter 8 Mind Maps (NT1210 Graded Assignments)