NT1210 Introduction to Networking. Unit 7:

Transcription

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 t 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 Switched Analog Circuits it for Voice: In early days of telephones, voice call required one analog electrical circuit between two phones Telco installed 2-wire cable into each home: Local loop Other end connected equipment sitting in nearby Telco office: Central Office (CO) When user called number, Telco created electrical circuit from one telephone to other (source to destination) 6

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

8 Introducing Wide Area Networks: Basic Telco Services Circuit Switching Switched Analog Circuits it 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 8

9 Introducing Wide Area Networks: Basic Telco Services Circuit Switching Connecting from a PC to an ISP, Using Modems and an Analog Telco Circuit 9 Figure 7-4

10 Introducing Wide Area Networks: Basic Telco Services Circuit Switching Beginning i mid-20 th 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 10

11 Introducing Wide Area Networks: Basic Telco Services Circuit Switching Digital it Circuits it 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 11 Figure 7-5

12 Introducing Wide Area Networks: Basic Telco Services Circuit Switching Switched Circuits it 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 12 Figure 7-6

13 Introducing Wide Area Networks: Basic Telco Services Circuit Switching To create switched circuits: it Switches and customer devices (telephones and modems) use signaling to setup and tear down circuit Signaling messages allow switches to choose which switch-to-switch links (trunks) to use for particular call 13

14 Introducing Wide Area Networks: Basic Telco Services Circuit Switching Circuit: it 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 14

15 Introducing Wide Area Networks: Basic Telco Services Packet Switching Packet Switching: Telcos next started t offering WAN services using packet switching services General Timeline: Circuit Switching, Digital Circuits, and Packet Switching 15 Figure 7-7

16 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 16

17 Introducing Wide Area Networks: Basic Telco Services Packet Switching Packet Switching Example Example of Packet Switching Service 17 Figure 7-8

18 Introducing Wide Area Networks: Routers Connect LANs to WANs Layer 3 IP Forwarding Logic 18 Figure 7-9

19 Introducing Wide Area Networks: Routers LAN might be simple Ethernet-only t 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 19 Figure 7-10

20 Introducing Wide Area Networks: Routers Encapsulation and De-encapsulation Encapsulation that Happens During the IP Packet Forwarding Process 20 Figure 7-11

21 Introducing Wide Area Networks: Topologies Point-to-Point t P i t 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 21 Figure 7-12

22 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 22 Figure 7-13

23 Introducing Wide Area Networks: Topologies Hub and Spoke Topology example Routers in Enterprise network and how many leased lines required to connect every router to every other router Number of Routers Number of Leased Lines Formula: N(N-1)/ (20 x 19) / (40 x 39) / (60 x 59) / (80 x 79) / (100 x 99) / 2 Number of Leased Lines to Connect Every Pair of Routers 23 Table 7-1

24 Introducing Wide Area Networks: Topologies Multipoint i t 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 24 Figure 7-14

25 Understanding Leased Line WAN Links Customer expects to get basic service of sending bits between two devices Customer buys right to send X number of bits per second constantly between two sites Leased Line in Concept 25 Figure 7-15

26 Understanding Leased Line WAN Links Distance limitations: it ti No single circuit it extends entire distance between two routers Point to point circuits really series of circuits Leased Line: Shorter Electrical Circuits, Knitted Together 26 Figure 7-16

27 Understanding Leased Line WAN Links Telcos in USA offer leased line speeds in multiples l of 64 Kbps: 64, 128, 192, 256, etc., up to 24 times 64 (1.536 Mbps) Speeds above 1.5 Mbps offered in multiples of Mbps up to 28 times (around 43 Mbps) Incremental Up to this Many Speed Value Speed Increment Increments Based on Speed Range DS0 64 Kbps Kbps DS1 (T1) Mbps Mbps Leased Line Speed Options, USA 27 Table 7-2

28 Understanding Leased Line WAN Links Telcos built networks to support digital transmission of voice in mid-20 th century Earlier standards used 64-Kbps building block called digitalit signal level 0 (DS0): Combined bits from slower speed links into single higher speed physical link using multiplexing Bits from 24 DS0s (plus some overhead) combined onto single physical link called digital signal level 1 (DS1, T1) Physical links combined bits from 28 DS1s (plus overhead) to create DS3 (T3) lines Visual Comparison of Speeds: DS0, DS1, DS3 28 Figure 7-17

29 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 29 Figure 7-18

30 Understanding Leased Line WAN Links Customer needs to plan for cabling at end of Telco s leased line cable Example: Customer s s router connects to cable installed by Telco Components and Responsibilities on One Side of a Leased Line 30 Figure 7-19

31 Understanding Leased Line WAN Links Leased line has Channel Services Unit/Data t 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 31 Figure 7-20

32 Understanding Leased Line WAN Links External CSU/DSU takes more planning: Need to know which connectors used by router and CSU/DSU device and then use correct serial cable to connect them Drawings of Some of the Short Serial Cables (Router to CSU/DSU) 32 Figure 7-21

33 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 eeson ete external CSU/DSU 1921 router WIC-1CSU: Photos of Router and Removable WAN Cards 33 Figure 7-22

34 Understanding Leased Line WAN Links Key steps for installing leased lines 1. 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) 2. Install router and serial interface cards in router as needed by leased line 3. If interface card does not have internal CSU/DSU, choose CSU/DSU and matching cable 4. Physically y connect all cables 5. Configure devices (beyond scope of this chapter) 34

35 Break Take 10 35

36 Understanding Leased Line WAN Links: Multiplexing T-Carrier system and multiplexing l i challenge: Too many trunks between sites Example: 3 customers (A, B, C) who each have offices in same two towns about 15 miles apart and each has DS1 or T1 leased line between each site Conceptual View, Three T1 Leased Lines, Three Customers (A, B, C) 36 Figure 7-23

37 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 37 Figure 7-24

38 Understanding Leased Line WAN Links: Multiplexing More efficient i solution: Time Division i i Multiplexing l i (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 38 Figure 7-25

39 Understanding Leased Line WAN Links: Multiplexing Top: 3 routers continually send bits to switch CO1 via 3 separate T1 lines Middle: Switch CO1 combines ( plexes ) bits onto single T3 for transmission Bottom: Switch CO2 demultiplexes incoming signal back into original bit streams, sending bits for customer A s T1 to router A2, and B s and C s Cs likewise A Three Part Drawing of the TDM Example 39 Figure 7-26

40 Understanding Leased Line WAN Links: Multiplexing Link types in T-carrier system (DS0, DS1, DS3) define timing of when TDM switch can send bits over link Example: On customer A s T1 connection, CO1 and CO2 TDM switches give A s traffic turn on T3 on regular basis As long as number of bits allocated to A s traffic totals million every second, TDM switches can send all traffic for A s T1 A View into a Longer Time Period over a T3 Link 40 Figure 7-27

41 Understanding Leased Line WAN Links: Multiplexing TDM switch considers T3 trunk as physical serial link with 28 logical T1 channels Physically, T3 line uses 2-pair cable that sends and receives bits serially (one at a time) at Mbps Logically, TDM switch views T3 as 28 T1 channels numbered 1 through 28 Matching TDM T3 Port and T1 Logical Channel on Both Ends of the T3 Trunk 41 Figure 7-28

42 Understanding Leased Line WAN Links Telco switches and CSU/DSUs view bits as frames A T1 Frame, Shown as Sent Serially Over a T1 42 Figure 7-31

43 Understanding Leased Line WAN Links T1 frames are set of 193 bits that t flow over link T1 equipment uses data in extra 193 rd bit for several functions, including framing (can t be used for customer data bits) 193-Bit T1 Frame 43 Figure 7-30

44 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 44 Figure 7-29

45 Understanding Leased Line WAN Links Example: Serial cable connecting CSU/DSU and router includes wires that set clocking (speed of link) CSU/DSU signals router when to send or receive 1 bit If CSU/DSU configured to make router run at 768 Kbps, CSU/DSU sends clocking signal 768,000 times per second CSU/DSU View of Serial Cable and Router, with Clocking 45 Figure 7-33

46 Understanding Leased Line WAN Links How to use entire T1 speed: Adjust data speed using CSU/DSU CSU/DSU looks at T1 line as 24 separate DS0 channels plus overhead bits (framing) Every 1/8000 th of second, each frame gets to send and receive one byte of data (i.e., each DS0 channel gets 64 Kbps of the capacity of the T1) CSU/DSU Channel View of the T1 Line from Customer to Local CO 46 Figure 7-32

47 Understanding Leased Line WAN Links Customer bought Mbps of T1 between routers (full capacity of 24 DS0 channels) So CSU/DSU must clock router at slightly slower rate of Mbps Full T1 with CSU/DSU: Mbps to the Router, Mbps to the CSU/DSU 47 Figure 7-34

48 Understanding Leased Line WAN Links When taking bits from router, CSU/DSU must fit them into 24 DS0 channels in T1 frame Role of the CSU/DSU in a Full T1 Leased Line 48 Figure 7-35

49 Understanding Leased Line WAN Links Fractional T1: One T1 line divided id d into numerous DS0 channels Example: 256 Kbps (4 times 64-Kbps speed of DS0 channel) uses 4 DS0 channels on T1 line Role of the CSU/DSU in a Fractional T1 Leased Line (256) 49 Figure 7-36

50 Understanding Leased Line WAN Links Type of Line Geography Speed Number of Channels DS0 USA 64 Kbps N/A DS1 (T1) USA Mbps 24 DS0 DS3 (T3) USA Mbps 28 DS1 E0 Europe 64 Kbps N/A E1 Europe Mbps 32* E0 E3 Europe Mbps 16 E1 J0 Japan 64 Kbps N/A J1 Japan Mbps 24 J0 J3 Japan Mbps 20 J1 * 30 E0 channels are available for customer data; 2 E0 channels are for other functions. Summary of Carrier TDM Line Standards 50 Table 7-3

51 Understanding Leased Line WAN Links Data Link Protocols 1. PC1 sends IP packet encapsulated in Ethernet frame to router R1 2. R1 receives frame, removes header/trailer, makes decision to send packet to R2; R1 adds header/trailer based on WAN Data Link protocol 3. R2 receives WAN frame, removes header /trailer, makes decision to send packet to host PC2; R2 adds Ethernet header/trailer so frame arrives at PC2 Encapsulation in a Serial WAN Data Link Protocol between Two Routers 51 Figure 7-37

52 Understanding Leased Line WAN Links High-level l Data Link Control (HDLC) protocol has 2 main purposes: Deliver encapsulated data from sender to correct receiver Perform error detection HDLC Frame Format 52 Figure 7-38

53 Understanding Leased Line WAN Links Field Flag Description 1-byte (7E) that signals beginning of frame Shorthand Similar il to Reminder Ethernet Here comes the frame! Preamble + SFD Address Identifies destination device To there Destination MAC Control Data Defines many subfields used by older devices Data plus all headers from upper layers Old; ignore The actual data payload N/A Data FCS Frame Check Sequence used for error checking (to see if any bits changed during transmission) Check for errors FCS HDLC Header and Trailer Fields 53 Table 7-4

54 Understanding Leased Line WAN Links Point-to-Point t P i t Protocol (PPP): Designed for TCP/IP networks and multiprotocol routers PPP Frame Format 54 Figure 7-39

55 Understanding Packet Switching and Multi- Access WANs Packet switching vs. TDM example: Telco has installed 28 T1 leased lines for customers near two COs Each T1 mapped to one of 28 T1 channels in T3 trunk All 28 channels of T3 used Single T3 Trunk Consumed by Supporting 28 Customer T1s 55 Figure 7-40

56 Understanding Packet Switching and Multi- Access WANs 28 T1 leased lines stretch t from two COs Leased lines run through total of four TDM switches and total of three T3 trunks T1s take up entire capacity on all three TDM trunks Multiple T3 Trunks Completely Used by Supporting 28 Customer T1s 56 Figure 7-41

57 Understanding Packet Switching and Multi- Access WANs How packet switching works: Devices use same TDM physical link types as T-carrier system Use Data Link protocol that identifies destination address using hex code Telco packet switches must know location of each destination, typically using table Packet Switching Concept: Telco Node Receives, Queues, Forwards Packets 57 Figure 7-42

58 Understanding Packet Switching and Multi- Access WANs With packet switching, link capacity between switches used to forward packets as needed or available 58

59 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 59 Figure 7-44

60 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 60

61 Understanding Packet Switching and Multi- Access WANs: Frame Relay Frame Relay terms One Possible Telco Implementation of the Frame Relay Network 61 Figure 7-45

62 Understanding Packet Switching and Multi- Access WANs: Frame Relay Virtual Circuit it (VC): Virtual leased line between CO and customer Telco has to permanently configure VCs so also called Permanent Virtual Circuits (PVCs) Example: R1 is HQ router, Branch are routers at other locations, leased lines connect R1 to each Branch router Four Physical Circuits Between Routers 62 Figure 7-46

63 Understanding Packet Switching and Multi- Access WANs: Frame Relay Same design using PVCs in Frame Relay network Frame Relay Virtual Circuit (VC) Concept; Partial Mesh 63 Figure 7-47

64 Understanding Packet Switching and Multi- Access WANs: Frame Relay Partial and full mesh: Frame Relay full mesh design makes sense when most sites send lots of IP packets to every other site Full mesh: PVC between every pair of Frame Relay routers Full Mesh of Frame Relay PVCs 64 Figure 7-48

65 Understanding Packet Switching and Multi- Access WANs: Frame Relay Every packet switching technology defines protocols used to deliver data Link Access Procedure Frame (LAPF): Frame Relay protocol; often just called Frame Relay LAPF defines header/trailer Header includes LAPF address called Data Link Connection Identifier (DLCI) Frame Relay Header/Trailer: Link Access Procedure Frame (LAPF) 65 Figure 7-49

66 Understanding Packet Switching and Multi- Access WANs: Frame Relay Example: 4 routers where R1 is at HQ with partial mesh of PVCs connected to 3 remote routers (R2, R3, R4) Uses 3 DLCIs that uniquely identifying Frame Relay PVCs to each connection Example: Three DLCIs, Three PVCs, All Using a Single Access Link 66 Figure 7-50

67 Understanding Packet Switching and Multi- Access WANs: Frame Relay Example of DLCI use: R1 receives IP packet from LAN (not shown) and decides to forward packet to router R2 Example: Frame Relay Frame with DLCI 102 (R2) 67 Figure 7-51

68 Understanding Packet Switching and Multi- Access WANs Packet Switching Services: SONET speeds (Rounded) Line Speed (in Name Mbps) OC-1 52 OC OC OC OC OC OC SONET Optical Carrier (OC) Names and (Rounded) Line Speeds 68 Table 7-5

69 Understanding Packet Switching and Multi- Access WANs Timeline for WAN technologies (dates are general and only meant for comparing some of technologies) Comparison Timeline of WAN Technologies Mentioned in this Chapter 69 Figure 7-52

70 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 s perspective. Illustrated the reasons why IP routers work well at forwarding data between different types of LANs and WANs. Drew common WAN topologies. 70

71 Summary, This Chapter Drew and contrasted the different customer-site 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. 71

72 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. 72

73 Questions? Comments? 73