WAN Technologies

1. WAN Technologies CCNA4: Module 2; 2.1-2.3

2. WAN Technologies Overview

3. WAN Services • Examples of WAN Services that will be discussed include…. • Dedicated Links • Fractional T1, T1, E1, T3, E3 • DSL Varieties • Circuit-switched Links • Analog dialup • ISDN • Packet-switched Links • X.25 • Frame Relay • Cell-switched Links • ATM

4. WAN Physical Structure and Terminology • CO Switch—the WAN provider’s Central Office. • Local Loop—is the “last mile” from the toll network to the customer premise. • Demarc—is the legally defined end of the WAN provider’s responsibility and the beginning of the customer’s responsibility. • CPE—or Customer Premises Equipment can be leased equipment from the WAN provider or wholly owned by the customer.

5. DTE/DCE Interface • The transfer of responsibility normally occurs at the DCE/DTE Interface • DCE (also called Data Circuit-Terminating Equipment or Data Communications Equipment) prepares customer data for the appropriate physical layer signaling standard in the WAN provider’s toll network. • DTE or Data Termination Equipment, sends the customer’s data to the DCE.

6. WAN Encapsulations • You should be very familiar with the LAN frame format for Ethernet. • WANs encapsulated frame formats are usually based on the HDLC standard. • Generically, most WAN encapsulations include the HDLC frame fields Address, Control, and Protocol. • For point-to-point links, the Address field will be empty or all 1s. However, Frame Relay and others use an address. • The Control field identifies the type of frame: • Information—data frames • Supervisory—link maintenance frames • Unnumbered—link setup frames • The Protocol field specifies what upper layer protocol is encapsulated.

7. WAN Link Options

8. WAN Link Options Overview • Below are the most common WAN Link Options that will be introduced in this presentation.

9. Dedicated Services • Dedicated services are distinguished by their “always on”, point-to-point connectivity.

10. Dedicated Services: Leased Lines • Also called a “Leased Line”, dedicated services offer full time, “always on” connectivity. • Used when a permanent, point-to-point link is needed to connect two remote locations. • Cost is based on distance and bandwidth • Speeds vary from fractional T1 (a ¼ T1 would be 386 kbps) to OC-48 at 2.4 Gbps.

11. Leased Line Types & Bandwidth

12. Dedicated Services: DSL • DSL provides analog phone and digital data service over the same local loop connection. • DSL is a broadband access method. • Voice below 4Khz • Data above 4Khz • DSL is listed as “dedicated” because the data portion of the link is “always on”.

13. DSL Varieties • DSL comes is two basic types: • ADSL is Asynchronous, meaning download is faster than upload. • SDSL is Synchronous, meaning download and upload speeds are the same. • All varieties of DSL can be classified as either ADSL or SDSL.

14. Circuit-Switched Services • Circuit-switched services are “on demand” services that only activate when needed.

15. Circuit-Switched Services: PSTN • The Public Switched Telephone Network (PSTN) is circuit-switched. • Also know as POTS (Plain Old Telephone Network) • When you make a phone call, a path is established through the phone company’s network before you even hear the first ring. • This path is maintained until you hang up.

16. Circuit-Switched Services: Analog Dialup • Analog dialup provides a low-cost WAN solution when temporary, low bandwidth access is needed or when no other WAN service is available. • Low speeds; typically around 33 kbps. • Good for small amounts of data sent intermittently. • For example, nightly backups or transmitting reports. • Requires a modem to modulate/demodulate between a computer’s digital signal and local loop’s analog signal.

17. Circuit-Switched Services: ISDN • Integrated Services Digital Network • Your DTE device must be ISDN capable, as is the router on the right side of the graphic. • If not, then you must purchase a terminal adapter to connect to the ISDN local loop, as shown in the network on the left.

18. Circuit-Switched Services: ISDN • ISDN comes in two basic sizes. • Basic Rate Interface (BRI) is 2 64 kbps bearer channels (B channels) plus a 16 kbps signaling or delta channel (D channel) • Primary Rate Interface (PRI) is 23 64 kbps B channels and one 64 kbps D channel • Also called a “digital T1” because the speed of PRI is the same as a T1: 1.544 Mbps.

19. Circuit-Switched Services: ISDN • ISDN as a home connectivity option has declined recently due to the popularity of DSL and cable. However... • DSL and cable saturation has not yet reached all remote locations that need corporate access via VPN. • ISDN is often selected to provide a fault tolerant solution without the added expense of another Frame Relay link. • ISDN can be dynamically activated when traffic reaches a configured level on the Frame Relay link. • ISDN can also be activated if the Frame Relay link goes down.

20. Packet-Switched Services • Packet-switched services are a lower cost alternative to both dedicated and circuit-switched services.

21. Packet-Switched Services: X.25 and Frame Relay • Packet-switched networks (PSN) allow multiple users to share links between devices in the WAN cloud. • Circuit-switched connections are dedicated bandwidth. • If the users are not consuming all the bandwidth, then it is wasted. • Packet-switched means that each packet is routed independently of all other packets. • The best route to the destination is evaluated for each packet.

22. WAN Virtual Circuits • Virtual Circuits use an addressing scheme that identifies end points with the closest service provider’s switch. • The path data takes through the network is dynamically determined based on current traffic. • In the graphic, one router is running four virtual circuits over the same physical, access link. • This is done by using logical subinterfaces—a topic discussed later in the course.

23. WAN Virtual Circuits • A Virtual Circuit is a logical path between end points that appears to the user to be an actual, physical path. There are two types of VCs. • Permanent Virtual Circuits (PVCs) are permanently established circuits with one mode—data transfer; similar to a leased line. • Switched Virtual Circuits (SVCs) are WAN paths to the destination established and terminated on demand; similar to a telephone call. • Virtual Circuits are established only between the access device (DTE) and the access switch (DCE). • The path through the service provider’s network is NOT established, but determined for each packet.

24. Packet-Switched Services: X.25 • X.25 was developed as an option to expensive leased lines. • Data transfers are limited to 48 kbps. • Very reliable because it has been thoroughly debugged. • Perfect for POS systems to verify credit card sales. • Also used for other types of data that do not require much bandwidth. • Customers can request either PVCs (“always on”) or SVCs (“on demand”).

25. Packet-Switched Services: Frame Relay • Frame Relay came about as a solution to X.25’s weaknesses. • X.25 has high latency because each packet is... • evaluated at the network layer. • stored in the switch to check it for errors. • Frame Relay improves X.25 by... • allowing processing only at the data-link layer. • providing a connectionless transfer with no error checking. • This is possible because today’s networks are physically much more reliable than the networks initially used by X.25.

26. Cell-Switched Services • Cell-switched services use a packet that is always the same size to provide faster switching.

27. Cell-Switched Services: ATM • Asynchronous Transfer Mode (ATM) • Cell-based technology that segments data into 48 byte cells with a 5 byte header (53 bytes). • Ideal for video since data does not have to wait for a large packet to be assembled and transmitted. • However, can be up to 20% less efficient than Frame Relay because of the overhead of the 5 byte header.

28. Designing with the Three Layers

29. The Three Layer Hierarchical Model • Core Layer: fast switching; redundant paths • Distribution Layer: policy-based connectivity • Access Layer: local and remote user access

30. Benefits of Hierarchical Design • Scalability • allows for future growth without sacrificing control or functionality • Ease of Implementation • logically constructed layers specify the functions of each layer • Ease of troubleshooting • well-defined functions at each layer aid in the isolation of problems • Predictability • behavior of functional layers can be estimated and planned for • Protocol support • allows easier implementation of future technologies because the network has been logically constructed • Manageability • All the above aids net. admin. in overall management of the network

31. Core Layer Functions • Fast Switching of Frames to Remote Sites. • Redundant paths to guard against a single point of failure. • Additional Core devices can be added to increase redundancy • Load sharing across redundant links. • Efficient use of bandwidth by implementing scalable routing protocols and route summarization. • Little or no security to decrease latency.

32. Distribution Layer Functions • Policy-Based connectivity with ACLs and other security measures • Boundary definition & packet manipulation • Protects the Core Layer from the Access Layer • Controls access to services of the core layer and other distribution layer routers • VLAN routing and address aggregation

33. Access Layer Functions • Local and Remote User Access to the Network • Isolation of Broadcast Traffic • Shared and Switched Bandwidth • MAC-layer filtering • Microsegmentation • VPN Access

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