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WANs

WANs. Semester 4, Chapter 2 Allan Johnson. Table of Contents. WAN Technology & Devices. Go There!. WANs & The OSI Model. Go There!. WAN Encapsulation Formats. Go There!. WAN Link Options. Go There!. WAN Technology & Devices. Table of Contents. WAN Services.

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WANs

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  1. WANs Semester 4, Chapter 2 Allan Johnson

  2. Table of Contents • WAN Technology & Devices Go There! • WANs & The OSI Model Go There! • WAN Encapsulation Formats Go There! • WAN Link Options Go There!

  3. WAN Technology & Devices Table of Contents

  4. WAN Services • A WAN is the communications network that connects LANs through a WAN Service Provider (e.g. RBOC) • WANs operate at the first three layers of the OSI, but focus mainly on the physical and data link layers. • Examples of WAN Services include…. • Call-setup • SS7 used for telephone calls • ISDN • A circuit-switched service using TDM, which allocates bandwidth to different sources on the same media • Frame Relay • A packet-switched service using statistical multiplexing

  5. WAN Physical Structure Toll Network – collection of switches/trunks in WAN cloud CO Switch – nearest point of presence for the WSP’s service Local loop – extends from the CPE (at the demarc) to the CO CPE – devices located on premises, either owned or leased

  6. DTE/DCE Interface • The transfer of responsibility for service between the ISP and the customer occurs at the DTE/DCE interface. • The customer’s gateway router is always the DTE. The ISP’s modem or CSU/DSU is the DCE and sets the clock rate. • In our standard lab, we must make one router on our WAN links be the DCE since we are not attaching to an ISP

  7. WAN Virtual Circuits • Switched Virtual Circuits (SVCs) are WAN paths to the destination established and terminated on demand. • Three phases: • Circuit establishment – creates the virtual circuit • Data transfer – sending and receiving user data • Circuit termination – tearing down the virtual circuit • Telephone service and ATM use SVCs • Increased use of bandwidth but decreased cost • Permanent Virtual Circuits (PVCs) are permanently established circuits with one mode: data transfer • X.25 and Frame Relay use PVCs • Decreased use of bandwidth but increased cost

  8. Link Types & Bandwidth

  9. WAN Devices • In order to connect to a leased line, the customer must have… • Access to the service provider’s circuit • An appropriate router port available • An a CSU/DSU, modem or ISDN Terminal Adapter

  10. Routers • Routers offer many services including: • Internetworking • WAN serial interfaces • Routers can operate as... • Internal Routers • Backbone Routers • Area Border Routers • Autonomous System Boundary Routers

  11. Router Router CSU/DSU CSU/DSU Modems • Also called CSU/DSUs (channel service units/digital service units) • Interface with voice-grade connection in order to convert analog signal to digital. • Represents the DCE side of the DTE/DCE connection. Leased line

  12. WANs & The OSI Model Table of Contents

  13. WAN Standards • What layers of the OSI model do WAN standards describe? • Physical and Data Link Layers

  14. WAN Physical Layer • Protocols that describe how to provide electrical, mechanical, operational, and functional connections for WAN services. • These services are most often obtained from WAN service providers such as RBOCs, alternate carriers, post-telephone, and telegraph (PTT) agencies. • Describes the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE).

  15. WAN Physical Layer • Typically, the DCE is the service provider and the DTE is the attached device. • In this model, the services offered to the DTE are made available through a modem or a CSU/DSU.

  16. WAN Physical Layer • Several physical layer standards specifying this interface between the DTE & DCE are... • EIA/TIA-232 • EIA/TIA-449 • V.24 • V.35 • X.21 • G.703 • EIA-530

  17. WAN Data-Link Layer • WAN data link protocols describe how frames are carried between systems on a single data link. • They include protocols designed to operate over dedicated point-to-point, multipoint, and multi-access switched services. • WAN standards are defined and managed by a number of recognized authorities, including the following agencies: ITU-T, ISO, IETF, & EIA

  18. WAN Encapsulation Formats Table of Contents

  19. WAN Data-Link Encapsulations • PPP (Point-to-Point Protocol) • Developed by IETF; replacing SLIP • Contains a field to identify the network layer protocol • PPP can check for link quality during connection establishment • Supports PAP (Password Authentication Protocol) & CHAP (Challenge Handshake Authentication Protocol)

  20. WAN Data-Link Encapsulations • High-Level Data Link Control (HDLC) • Cisco default encapsulation; typically used between routers running Cisco IOS; replacing SDLC • Streamlined: no windowing or flow control • may not be compatible with different vendors because of the way each vendor has chosen to implement it. • HDLC supports both point-to-point and multipoint configurations with minimal overhead

  21. WAN Data-Link Encapsulations • Frame Relay • uses high-quality digital facilities; • uses simplified framing with no error correction mechanisms (connectionless!!); • it can send Layer 2 information much more rapidly than other WAN protocols

  22. WAN Link Options Table of Contents

  23. WAN Link Options Switched Dedicated T1, E1, T3, E3 xDSL SONET Packet/Cell Switched Circuit Switched X.25 Frame Relay ATM (cell) SMDS (cell) POTS ISDN

  24. WAN Link Options Switched Dedicated T1, E1, T3, E3 xDSL SONET Packet/Cell Switched Circuit Switched X.25 Frame Relay ATM (cell) SMDS (cell) POTS ISDN

  25. Dedicated Digital Services • T series in U.S. and E series in Europe • Uses time division multiplexing to “slice up” data and assign time slots for transmissions • T1 = 1.544 Mbps • T3 = 44.736 Mbps • E1 = 2.048 Mbps • E3 = 34.368 Mbps Dedicated Digital Services provide full-time connectivity through a point-to-point link Uses twisted pair & fiber Extremely popular Moderate cost

  26. Dedicated Digital Services • Digital Subscriber Lines (xDSL); the x stands for a family of technologies • WAN Technology for home use; decreasing bandwidth with increasing distance from the phone companies CO. • Data rates as high as 51.84 Mbps but more common to be in the 100s of Kbps • Varieties include HDSL, SDSL, ADSL, & VDSL • Moderate expense and getting cheaper

  27. Dedicated Digital Services • Synchronous Optical Network (SONET) • Specialized high bandwidth technology for use at various Optical Carrier speeds (OC) ranging from 51.84 Mbps (OC-1) to 9,952 Mbps (OC-192) • Uses lasers to divide the wavelength of the light into sections that can carry large amounts of data (Wave Division Multiplexing) • Very expensive; used by large ISPs and other Internet backbone entities.

  28. WAN Link Options Switched Dedicated T1, E1, T3, E3 xDSL SONET Packet/Cell Switched Circuit Switched X.25 Frame Relay ATM (cell) SMDS (cell) POTS ISDN

  29. Circuit Switched Services • Plain Old Telephone System (POTS) • Not a computer data service but... • POTS is an important component of our communication infrastructure and • It is still the standard for designing reliable networks

  30. B D B Circuit Switched Services • Integrated Services Digital Network (ISDN) • Historically important--first dial-up digital service • Provides both voice & data transmission over existing phone lines • Cost is moderate; max. bandwidth = 128 kbps for BRI (Basic Rate Interface) • 2 B channels @ 64kps and 1 D channel @ 16kps • B channels are voice/data channels; D for signaling

  31. WAN Link Options Switched Dedicated T1, E1, T3, E3 xDSL SONET Packet/Cell Switched Circuit Switched X.25 Frame Relay ATM (cell) SMDS (cell) POTS ISDN

  32. Packet Switched Services • X.25 (Connection-oriented) • Older WAN technology developed in 1970s • Reliable--X.25 has been extensively debugged and is now very stable--literally no errors in modern X.25 networks • Store & Forward--Since X.25 stores the whole frame to error check it before forwarding it on to the destination, it has an inherent delay (unlike Frame Relay) and requires large, expensive memory buffering capabilities.

  33. Packet Switched Services • Frame Relay (Connectionless) • More efficient and much faster than X.25 • Packet switched version of ISDN (which is circuit switched); data rates up to 44.736Mbps with 56kbps and 384kbps being the most popular • Used mostly to forward LAN IP and IPX packets but can be used to forward other types of traffic • Primary competitive advantage is its low cost

  34. Cell Switched Services • Asynchronous Transfer Mode (ATM) • Relatively new WAN Technology related to broadband ISDN; max. bandwidth = 622 Mbps • Developed in order to provide one technology for both WANs and LANs to transport data, video, and voice. (High Cost) • Key Benefits: • One network for all traffic--voice, data, video • Compatible with current wiring infrastructure (cable plant) • Very flexible and scalable • Simplifies network management

  35. Cell Switched Services • Switched Multimegabit Data Service (SMDS) • Closely related to ATM; SMDS is the MAN (Metropolitan Area Network) implementation of ATM • High Cost with max. bandwidth 44.736 Mbps

  36. WAN Link Options Review

  37. Table of Contents End Slide Show

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