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WAN Basics

WAN Basics. Agenda. WAN Basics Point-to-Point Protocol (PPP) Integrated Services Digital Network (ISDN) Frame Relay. WAN Basics. What Is a WAN?. A network that serves users across a broad geographic area

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WAN Basics

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  1. WAN Basics

  2. Agenda • WAN Basics • Point-to-Point Protocol (PPP) • Integrated Services Digital Network (ISDN) • Frame Relay

  3. WAN Basics

  4. What Is a WAN? • A network that serves users across a broad geographic area • Often uses transmission devices provided by public carriers (Pacific Bell, AT&T, etc.) • This service is commonly referred to as “plain old telephone service” (POTS) • WANs function at the lower three layers of the OSI reference model • Physical layer, data link layer, and network layer

  5. WAN Devices WAN Switch Switches traffic such as Frame Relay, X.25, and SMDS, and operates at the data link layer Modem Interprets digital and analog signals, enabling data transmission over telephone lines Access Server A concentration point for dial-in and dial-out connections CSU/DSU Adapts a terminal physical interface to a switch interface in a switched-carrier network ISDN Terminal Connects ISDN Basic Rate Interface (BRI) to other interfaces, such as EIA/TIA-232

  6. WAN Terms • Customer premises equipment (CPE) • Demarcation point • Local loop • Central office (CO) • Toll network

  7. WAN Terminating Equipment Physical Cable Types WAN Provider (Carrier) Network EIA/TIA-232 V.35 X.21 Router HSSI To Corporate Network Modem Usually on the Customer’s Premises DCE DTE Data Terminal Equipment Data Circuit-Terminating Equipment The Customer’s Equipment The Service Providers Equipment

  8. Wan Connection Types • Leased line • Circuit switching • Packet switching

  9. Leased Line • One connection per physical interface • Bandwidth: 56 kbps–1.544 Mbps • T1/E1 and fractional T1/E1 • Cost effective at 4–6 hours daily usage • Dedicated connections with predictable throughput • Permanent • Cost varies by distance

  10. Modem Modem WAN Circuit Switching • Dedicated physical circuit established, maintained, and terminated through a carrier network for each communication session • Datagram and data stream transmissions • Operates like a normal telephone call • Example: ISDN

  11. Multiplexing Demultiplexing Modem Modem WAN Packet Switching • Network devices share a point-to-point link to transport packets from a source to a destination across a carrier network • Statistical multiplexing is used to enable devices to share these circuits • Examples: ATM, Frame Relay, SMDS, X.25

  12. WAN Virtual Circuits • A logical circuit ensuring reliable communication between two devices • Switched virtual circuits (SVCs) • Dynamically established on demand • Torn down when transmission is complete • Used when data transmission is sporadic • Permanent virtual circuits (PVCs) • Permanently established • Save bandwidth for cases where certain virtual circuits must exist all the time • Used in Frame Relay, X.25, and ATM

  13. WAN Protocols Network Layer X.25 PLP Data Link Layer LLC Sublayer MAC Sublayer LAPB Frame Relay HDLC PPP SDLC Physical Layer EIA/TIA-232 EIA/TIA-449 V.24 V.35 HSSI G.703 EIA-530 SMDS X.21bis OSI Reference Model WAN Protocols

  14. WAN Protocols SDLC IBM’s SNA data link layer communications protocol HDLC Bit-oriented synchronous data link layer protocol LAPB Data link layer protocol in the X.25 protocol stack PPP Provides router-to-router and host-to-network connections over sync and async circuits X.25 Defines connections for remote terminal access and computer communications in PDNs ISDN Permits telephone networks to carry data, voice, and other source traffic Frame Relay Switched data link layer protocol that handles multiple virtual circuits using HDLC; replacing X.25 due to higher efficiency

  15. Point-to-Point Protocol (PPP)

  16. HDLC

  17. Point-to-Point Protocol (PPP)

  18. LCP Configuration Option • Authentication • PAP (Password Authentication Protocol) • CHAP (Challenge Handshake Authentication Protocol) • Compression • Error detection • Multilink • PPP callback

  19. PPP Session Establishment • Link-establishment phase • Authentication phase • Network layer protocol phase

  20. Integrated Services Digital Network (ISDN)

  21. Integrated Services Digital Network (ISDN) • High bandwidth • Up to 128 Kbps per basic rate interface • Dial on demand • Multiple channels • Fast connection time • Monthly rate plus cost-effective, usage-based billing • Strictly digital LAN Server ISDN BRI2B+D BRI/PRI23B+D30B+D (Europe) Telecommuter/After-Hours, Work-at-Home Company Network

  22. ISDN Terminals and Reference Points

  23. ISDN Protocols • Defined by ITU-T • There are several series: • E  ISDN on the existing telephone network. • I  concepts, aspects, and services. • Q  switching and signaling • Q.921  layer 2 • Q.931  layer 3

  24. ISDN Switch Types

  25. ISDN Basic Rate Interface (BRI) Primary Rate Interface (PRI) • One physical connection to the ISDN network • 23 logical connections (U.S./Canada) • 30 logical connections (Europe) • Used at central site 1.536 Mbps 23B 30B D 144 Kbps 2B D } } 64 Kbps 64 Kbps { 64 Kbps 64 Kbps 16 Kbps • One physical connection to the ISDN network • Two logical connections • Used at remote telecommuter site

  26. Bandwidth-on-Demand Main Office PSTN Remote Site • Adds bandwidth when needed • Configurable thresholds • Controls usage costs Start FileTransfer

  27. Dial Backup Primary X • If a primary link goes down or is too busy • Load balancing • Completely customizable Secondary Autodial

  28. Dial-on-Demand Routing Main Office Remote Site PSTN • Dials connection only when needed • Ideal for low-volume, periodic traffic • Controls usage costs Interesting Traffic

  29. Configuring DDR • Define static routes • Specify the interesting traffic • Configure the dialer information

  30. Configuration Example (1)

  31. Configuration Example (2)

  32. Configuration Example (3)

  33. Frame Relay

  34. X.25 DTE DTE • Very robust protocol for low-quality lines • Packet-switched • Bandwidth: 9.6 kbps–64 kbps • Well-established technology; large installed base • Worldwide availability X.25 DCE DCE

  35. Permanent Virtual Circuit (PVC) Frame Relay • Permanent, not dialup • Multiple connections per physical interface (permanent virtual circuits) • Efficient handling of bursty (peak performance period) data • Guaranteed bandwidth (typical speeds are 56/64 Kbps, 256 Kbps, and 1.544 Mbps)—committed information rate (CIR) • Cost varies greatly by region

  36. Committed Information Rate (CIR) • Access rate  the maximum speed at which the Frame Relay interface can transmit. • CIR  the maximum bandwidth of data guaranteed to be delivered.

  37. Connecting Offices with Frame Relay Frame Relay Peak CIR Traffic { Free If Available { What You Pay For Time

  38. Data Link Connection Identifier (DLCI) • Used to identify virtual circuits. • Locally significant.

  39. Local Management Interface (LMI) • Signaling standard between router and the first Frame Relay switch. • It communicates information about: • Keepalives • Multicasting • Global addressing • Status of virtual circuits • Virtual circuit status: • Active state • Inactive state • Deleted state

  40. Frame Relay Congestion Control • Discard Eligibility (DE) • Forward Explicit Congestion Notification (FECN) • Backward Explicit Congestion Notification (BECN)

  41. WAN Summary • Operate beyond the local LAN’s control • Customers pay telephone service providers for WAN connections such as ISDN, xDSL, Frame Relay, leased line, X.25, etc. • Switching methods include point-to-point, circuit switching, packet switching, dialup, and WAN virtual circuits • Key devices include WAN switches, access servers, modems, and CSU/DSUs • Bandwidth optimization features are essential for controlling WAN costs

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