Wide Area Networks (WANs)

Wide Area Networks(WANs) Chapter 7 Updated January 2009 XU Zhengchuan Fudan University

Orientation • Single Networks • Layers 1 and 2 (so OSI standards dominate) • Chapters 4-7: Local to long-distance for single networks • Chapter 4: Wired Ethernet LANs • Chapter 5: Wireless LANs (WLANs) • Chapter 6: Telecommunications (and Internet Access) • Chapter 7: Wide Area Networks (WANs) • Chapter 8: TCP/IP Internetworking • To link multiple single networks

WAN Essentials

Figure 7-1: Wide Area Networks (WANs) • Wide Area Networks (WANs) • Single networks that connect different sites • So Layer 1 and Layer 2 operation • WAN Purposes • Internet access (Chapter 6) • Link sites within the same corporation • Provide remote access to individuals who are off site

Figure 7-1: Wide Area Networks (WANs) • WANs are Characterized by High Cost and Low Speeds • High cost per bit transmitted compared to LANs • Consequently, lower speeds (most commonly 128 kbps to a few megabits per second) • This speed usually is aggregate throughput shared by many users • Much slower than LAN speeds (100 Mbps to 1 Gbps to the desktop)

Figure 7-1: Wide Area Networks (WANs) • Carriers • Beyond their physical premises, companies must use the services of regulated carriers for transmission • Companies do not have rights of way to lay wires beyond their premises • Customers are limited to whatever services the carriers provide • Prices for carrier services change abruptly and without technological reasons • Prices and service availability vary widely from country to country

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Leased Line Networks

Leased Lines: Recap • Leased Line Characteristics • Point-to-point circuits • Always on • High speeds: 64 kbps (rare) to several gigabits per second • Leased for a minimum period of time • Usually offered by telephone companies

Figure 7-2: Leased Line Networks for Voice and Data

Figure 7-3: Full Mesh and Pure Hub-and-Spoke Topologies for Leased Line Data Networks In a full mesh topology, there is a leased line between each pair of sites Highly reliable Highly expensive

Figure 7-3: Full Mesh and Pure Hub-and-Spoke Topologies for Leased Line Data Networks In a pure hub-and-spoke topology, there is only one leased line from the hub site to each other site. Very inexpensive. Very unreliable. Few companies use either of these extreme topologies. They have some backup links.

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Figure 7-4: Leased Line Speeds North American Digital Hierarchy Line Speed Typical Transmission Medium 56 kbps 56 kbps 2-Pair Data-Grade UTP T1 1.544 Mbps 2-Pair Data-Grade UTP 56 kbps leased lines are hardly used today because they are so slow. T1 lines are very widely usedbecause they are in the speed range of greatest corporate demand—128 kbps to a few megabits per second.

Figure 7-4: Leased Line Speeds, Continued North American Digital Hierarchy Line Speed Typical Transmission Medium T1 1.544 Mbps 2-Pair Data-Grade UTP Fractional T1 128 kbps, 256 kbps, 384 kbps, 512 kbps, 768 kbps 2-Pair Data-Grade UTP Bonded T1s (multiple T1s acting as a single line) A few multiples of 1.544 Mbps 2-Pair Data-Grade UTP T1 lines are very widely used. Fractional T1 lines offer lower speeds for companies that need them. Two or three T1 lines can be bonded for higher speeds. T1, Fractional T1, and Bonded T1s are the most widely used leased lines.

Figure 7-4: Leased Line Speeds, Continued North American Digital Hierarchy Line Speed Typical Transmission Medium T1 1.544 Mbps 2-Pair Data-Grade UTP T3 44.736 Mbps Optical Fiber The jump from T1 to T3 speeds is extremely large. Few firms need T3 speeds, and they only needthese speeds for some of their leased lines. Some carriers offer fractional T3 lines to bridge the T1-T3 gap. T3 lines and all faster leased lines use optical fiber.

Figure 7-4: Leased Line Speeds, Continued CEPT Hierarchy Line Speed Typical Transmission Medium 64 kbps 64 kbps 2-Pair Data-Grade UTP E1 2.048 Mbps 2-Pair Data-Grade UTP E3 34.368 Mbps Optical Fiber In Europe, most countries use the CEPT hierarchy E1 lines are slightly faster than T1 lines E3 lines are slightly slower than T3 lines

Figure 7-4: Leased Line Speeds, Continued SONET/SDH Speeds Line Speed (Mbps) Typical Transmission Medium OC3/STM1 155.52 Optical Fiber OC12/STM4 622.08 Optical Fiber OC48/STM16 2,488.32 Optical Fiber OC192/STM64 9,953.28 Optical Fiber OC768/STM256 39,813.12 Optical Fiber For speeds above 50 Mbps, the world uses one technology Called SONET in the United States, SDH in EuropeSONET speeds measured in OC numbers, SDH in STM numbers Speeds are multiples of 51.84 Mbps Used mostly by carriers

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Figure 7-5: Business-Class Symmetric Digital Subscriber Line (DSL) Services HDSL HDSL2 SHDSL Uses Existing 1-Pair Voice-Grade UTP Telephone Access Line to Customer Premises?* Yes* Yes* Yes* Downstream Throughput 768 kbps 1.544 Mbps 384 kbps – 2.3 Mbps Upstream Throughput 768 kbps 1.544 Mbps 384 kbps – 2.3 Mbps *By definition, DSL always uses 1-pair VG UTP Many firms use HDSL and HDSL2 lines instead of T1 and fractional T1 speeds

Figure 7-5: Business-Class Symmetric Digital Subscriber Line (DSL) Services HDSL HDSL2 SHDSL Target Market Businesses Businesses Businesses Symmetrical Throughput? Yes Yes Yes QoS Throughput Guarantees? Yes Yes Yes Businesses need symmetrical throughput and QoS

Public Switched Data Networks (PSDNs)

Figure 7-6: Public Switched Data Networks (PSDNs) • Recap: Leased Line Data Networks • Use many leased lines, which must span long distances between sites • This is very expensive • Company must design and operate its leased line network • Public Switched Data Networks • Carrier does more of the operational and management work • Total cost of technology, service, and management usually lower than leased line networks

Figure 7-7: Public Switched Data Network (PSDN) In Public Switched Data Networks, the PSDN carrier handles all switching. Reduces the load on the network staff. The PSDN central core is shown as a cloud to indicate that the user firm does not have to know how the network operates.

Figure 7-7: Public Switched Data Network (PSDN) In Public Switched Data Networks, the customer needs a single leased line from each site to one of the PSDN carrier’s points of presence (POPs)

Leased Lines in PSDNs • A company has ten sites • It wants to use a PSDN • Will it need leased lines even if it is using a PDSN? • How many leased lines will it need? • Between what two locations will each leased line go?

Figure 7-6: PSDNs • Service Level Agreements (SLAs) • Guarantees for services • Throughput, availability, latency, error rate, etc. • An SLA might guarantee a latency of no more than 100 ms 99.99 percent of the time • SLA guarantees no worse than a certain worst-case level of performance

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Figure 7-8: Virtual Circuit Operation The internal cloud network is a mesh of switches. This creates multiple alternative paths. This gives reliability.

Figure 7-8: Virtual Circuit Operation Mesh switching is slow becauseeach switch must evaluate available alternative paths and select the best one. This creates expensive switching.

Figure 7-8: Virtual Circuit Operation Before communication begins between sites, the PSDN computes a best path called a virtual circuit. All frames travel along this virtual circuit.

Figure 7-8: Virtual Circuit Operation Each frame has a virtual circuit number instead of a destination address. Each switch looks up the VC number in its switching table, sends the frame out the indicated port. VCs greatly reduce switching costs.

Public Switched Data Networks (PSDNs) Frame Relay ATM Metropolitan Area Ethernet Carrier IP Networks

Figure 7-9: Frame Relay • Frame Relay is the Most Popular PSDN Service Today • 56 kbps to 40 Mbps • This fits the range of greatest corporate demand for WAN speed • Usually less expensive than a network of leased lines • Grew rapidly in the 1990s, to be come equal to leased line WANs in terms of market share (about 40%) • Carriers have raised prices, reducing growth

Figure 7.14: Pricing Elements in Frame Relay Service • Frame Relay Pricing • Frame relay access device at site • CSU/DSU at physical layer (converts between internal, PSDN digital formats) • Frame Relay at the Data Link Layer • Leased line from site to POP • Port on the POP • Pay by port speed • Usually the largest price component • Permanent virtual circuits (PVCs) among communicating sites • Other charges

Figure 7-10: Frame Relay Network Elements 1. Access Device Customer Premises A Router or Dedicated Frame Relay Access Device And CSU/DSU Switch POP Customer Premises B Customer Premises C

Figure 7-10: Frame Relay Network Elements Access Device (Frame Relay Access Device) T1 CSU/DSU at Physical Layer Site A T1 Line Frame Relay at Data Link Layer PC Access Device (Router) T3 CSU/DSU at Physical Layer Site B T3 Line ATM etc. at Data Link Layer Server

Figure 7-10: Frame Relay Network Elements • CSU/DSU • Channel service unit (CSU) protects the access line from unapproved voltage levels, etc. coming from the firm. It acts like a fuse in an electrical circuit. • Data service unit (DSU) converts between internal digital format and digital format of access link to Frame Relay network. • May have different baud rate, number of states, voltage levels, etc. DSU

Figure 7-10: Frame Relay Network Elements Customer Premises A 2. Leased Access Line to POP Switch POP Customer Premises B Customer Premises C

Figure 7-10: Frame Relay Network Elements 3. Port Speed Charge at POP Switch Customer Premises A POP has a switch with ports The port speed charge is based on the port speed used The port speed charge usually Is the biggest part of PSDN costs Switch POP Customer Premises B Customer Premises C

Figure 7-10: Frame Relay Network Elements 2. PVCs are multiplexed over a single leased line PVC charges usually are collectively the second-most expensive part of Frame Relay service Customer Premises A 4. PVC Charges PVCs 1&2 Switch POP PVC 1 PVC 2 PVC prices depend on PVC speed PVC 1 PVC 2 PVC 1 Customer Premises B Customer Premises C

Frame Relay Network PVCs • Frame Relay PVC Numbers are called data link control indicators (DLCIs) • Pronounced “Dull’ seas” • Usually 10 bits long • 210 or 1,024 possible PVCs from each site • Multiplexed over the single leased line to the POP • Leased line must be fast enough to handle the combined PVC speeds PVC 1-2 Site 2 PSDN Site 1 POP Site 3 Leased Line PVC 1-3

Figure 7-10: Frame Relay Network Elements Frame Relay networks are managed by the carrier. For management of equipment on the customer premises, there is an extra charge. 5. Management Charges Customer Premises A PVCs 1&2 Switch POP PVC 1 PVC 2 PVC 1 PVC 2 PVC 1 Customer Premises B Customer Premises C

Figure 7.15: Frame Relay Pricing Details • Other Charges • Flat rate versus traffic volume charges • Installation charges • Managed service charges • Service level agreement (SLA) charges • Geographical Scope • Frame Relay systems with broader geographical scope cost more

Figure 7.15: Frame Relay Pricing Details New Not in Book • To Determine Needs for Each Site • Step 1: Determine PVC Needs • Determine needed speed from this site to each other site • You will need a virtual circuit to serve the needed speed to each other site • Sum all the virtual circuit speeds 30 kbps needed 56 kbps PVC Sum 1,056 kbps Site Being Analyzed 800 kbps needed, 1 Mbps PVC

Figure 7.15: Frame Relay Pricing Details New Not in Book • Step 2: Determine Port Speed • You need a port speed equal to or greater than the sum of the PVCs • You can get by with 70% • 70% of 1,056 kbps is 739 kbps • Next-higher port speed may be 1 Mbps • Don’t overdo port speed because port speed is most expensive component

Figure 7.15: Frame Relay Pricing Details New Not in Book • Step 3: Determine Leased Line Speed • Remember that port speed is more expensive than leased line speeds • In general, don’t waste port speed by using a leased line much under its capacity • If port speed is 1 Mbps, leased line should be T1 with 1.544 Mbps capacity

Figure 7.15: Frame Relay Pricing Details New Not in Book • Example • The Situation • Headquarters and two branch offices. • Branches communicate with HQ at 200 kbps • Branches communicate with each other at 40 kbps B1 HQ B2

Figure 7.15: Frame Relay Pricing Details New Not in Book • Example • For HQ • How many connections will HQ need? • What are their speeds? • What will be their PVC speeds (0 kbps, 56 kbps, 256 kbps, 512 kbps, 1 Mbps) • If port speeds are 56 kbps, 256 kbps, 384 kbps, 512 kbps, what port speed will be needed? • What leased line will be needed if speeds are 56 kbps, 256 kbps, 512 kbps, or T1? HQ

Wide Area Networks (WANs)