• Objectives• Identify a variety of

• Objectives
• Identify a variety of uses for WANs
• Explain different WAN topologies, including their advantages and disadvantages
• Compare the characteristics of WAN technologies, including their switching type, throughput, media, security, and reliability
• Describe several WAN transmission and connection methods, including PSTN, ISDN, T-carriers, DSL, broadband cable, broadband over powerline, ATM, and SONET
• WAN Essentials
• WAN
– Network traversing some distance, connecting LANs
– Transmission methods depend on business needs
• WAN and LAN common properties
– Client-host resource sharing
– Layer 3 and higher protocols
– Packet-switched digitized data
• WAN Essentials (cont’d.)
• WAN and LAN differences
– Layers 1 and 2 access methods, topologies, media
– LAN wiring: privately owned
– WAN wiring: public through NSPs (network service providers)
• Examples: AT&T, Verizon, Sprint
• WAN site
– Individual geographic locations connected by WAN
• WAN link
– WAN site to WAN site connection

• WAN Topologies
• Differences from LAN topologies
– Distance covered, number of users, traffic
– Connect sites via dedicated, high-speed links
• Use different connectivity devices
• WAN connections
– Require Layer 3 devices
• Routers
– Cannot carry nonroutable protocols
• Bus
• Bus topology WAN
– Each site connects serially to two sites maximum
– Network site dependent on every other site to transmit and receive traffic
– Different locations connected to another through point-to-point links
• Best use
– Organizations requiring small WAN, dedicated circuits
• Drawback
– Not scalable
• Ring
• Ring topology WAN
– Each site connected to two other sites
– Forms ring pattern
– Connects locations
– Relies on redundant rings
• Data rerouted upon site failure
– Expansion
• Difficult, expensive
• Best use
– Connecting maximum five locations
• Star
• Star topology WAN
– Single site central connection point
– Separate data routes between any two sites
• Advantages
– Single connection failure affects one location
– Shorter data paths between any two sites
– Expansion: simple, less costly
• Drawback
– Central site failure can bring down entire WAN
• Mesh
• Mesh topology WAN
– Incorporates many directly interconnected sites
– Data travels directly from origin to destination
– Routers can redirect data easily, quickly
• Most fault-tolerant WAN type
• Full-mesh WAN
– Every WAN site directly connected to every other site
– Drawback: cost
• Partial-mesh WAN
– Less costly
• Tiered
• Tiered topology WAN
– Sites connected in star or ring formations
• Interconnected at different levels
– Interconnection points organized into layers
• Form hierarchical groupings
• Flexibility
– Allows many variations, practicality
– Requires careful considerations
• Geography, usage patterns, growth potential
• PSTN
• PSTN (Public Switched Telephone Network)
– Network of lines, carrier equipment providing telephone service
– POTS (plain old telephone service)
– Encompasses entire telephone system
– Originally: analog traffic
– Today: digital data, computer controlled switching
• Dial-up connection
– Modem connects computer to distant network
• Uses PSTN line
• PSTN (cont’d.)
• PSTN elements
– Cannot handle digital transmission
• Requires modem
• Signal travels path between modems
– Over carrier’s network
• Includes CO (central office), remote switching facility
• Signal converts back to digital pulses
• CO (central office)
– Where telephone company terminates lines
– Switches calls between different locations
• PSTN (cont’d.)
• Local loop (last mile)
– Portion connecting residence, business to nearest CO
– May be digital or analog
• Digital local loop
– Fiber to the home (fiber to the premises)
• Passive optical network (PON)
– Carrier uses fiber-optic cabling to connect with multiple endpoints
• PSTN (cont’d.)
• Optical line terminal
– Single endpoint at carrier’s central office in a PON
– Device with multiple optical ports
• Optical network unit
– Distributes signals to multiple endpoints using fiber-optic cable
• Or copper or coax cable
• X.25 and Frame Relay
• X.25 ITU standard
– Analog, packet-switching technology
• Designed for long distance
– Original standard: mid 1970s
• Mainframe to remote computers: 64 Kbps throughput
– Update: 1992
• 2.048 Mbps throughput
• Client, servers over WANs
– Verifies transmission at every node
• Excellent flow control, ensures data reliability
• Slow, unreliable for time-sensitive applications
• X.25 and Frame Relay (cont’d.)
• Frame relay
– Updated X.25: digital, packet-switching
– Protocols operate at Data Link layer
• Supports multiple Network, Transport layer protocols
• Both perform error checking
– Frame relay: no reliable data delivery guarantee
– X.25: errors fixed or retransmitted
• Throughput
– X.25: 64 Kbps to 45 Mbps
– Frame relay: customer chooses
• X.25 and Frame Relay (cont’d.)
• Both use virtual circuits
– Node connections with disparate physical links
• Logically appear direct
– Advantage: efficient bandwidth use
• Both configurable as SVCs (switched virtual circuits)
– Connection established for transmission, terminated when complete
• Both configurable as PVCs (permanent virtual circuits)
– Connection established before transmission, remains after transmission
• X.25 and Frame Relay (cont’d.)
• PVCs
– Not dedicated, individual links
• X.25 or frame relay lease contract
– Specify endpoints, bandwidth
– CIR (committed information rate)
• Minimum bandwidth guaranteed by carrier
• PVC lease
– Share bandwidth with other X.25, frame relay users
• X.25 and Frame Relay (cont’d.)
• Frame relay lease advantage
– Pay for bandwidth required
– Less expensive technology
– Long-established worldwide standard
• Frame relay and X.25 disadvantage
– Throughput variability on shared lines
• Frame relay and X.25 easily upgrade to T-carrier dedicated lines
– Same connectivity equipment
• ISDN
• Standard for transmitting digital data over PSTN
• Gained popularity: 1990s
– Connecting WAN locations
• Exchanges data, voice signals
• Protocols at Physical, Data Link, Transport layers
– Signaling, framing, connection setup and termination, routing, flow control, error detection and correction