CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 56 pptx

WAN Characteristics 519 Figure 10-4 illustrates some of the common data link encapsulations associated with synchronous serial lines. Figure 10-4 Data Link Encapsulations Figure 10-5 illustrates the synchronous serial communications model where services offered to the router are made available through a modem or a CSU/DSU. Figure 10-5 CSU/DSU WAN Connection Options You can use different WAN connection options to interconnect LANs. The following subsections give a brief description of the most common WAN connection services: ■ Circuit-switched services ■ Packet-switched services ■ Cell-switched services ■ Dedicated digital services ■ Dialup, cable, and wireless services Figure 10-6 depicts the different WAN connection services. HDLC PPP Frame Relay ISDN chpt_10.fm Page 519 Tuesday, May 27, 2003 8:57 AM 520 Chapter 10: WANs and Routers Figure 10-6 WAN Connection Services Circuit-Switched Services Circuit switching is a WAN switching method in which a dedicated physical circuit through a carrier network is established, maintained, and terminated for each communi- cation session. ISDN is an example of a circuit-switched WAN technology. Figure 10-7 illustrates a network topology that supports circuit switching. Figure 10-7 Circuit Switching Table 10-3 describes the characteristics of two most common circuit-switched services—plain old telephone service (POTS) and narrowband Integrated Services Digital Network (ISDN). chpt_10.fm Page 520 Tuesday, May 27, 2003 8:57 AM WAN Characteristics 521 POTS and ISDN are dialup services, which means that when the call is made, an end- to-end physical path is set up and the bandwidth is reserved end to end. Both POTS and ISDN use straight time-division multiplexing (TDM), which is sometimes referred to as synchronous transfer mode (STM). Packet-Switched Services Packet-switched services route small units of data called packets through a network based on the destination address contained within each packet. Figure 10-8 shows an example network topology that supports packet-switched services. Figure 10-8 Packet Switching Table 10-3 Circuit-Switched Services Circuit-Switched Service Characteristics POTS Not a computer data service, but included because many of its technologies are part of the growing data infrastructure, and it is an incredibly reliable, easy-to- use, wide-area communications network. Typical medium is twisted-pair copper wire. Narrowband ISDN A versatile, widespread, historically important technology. Was the first all-digital dialup service. Cost is moderate. Maximum bandwidth is 128 kbps for the lower cost Basic Rate Interface (BRI) and about 3 Mbps for the Primary Rate Interface (PRI). Usage is fairly widespread, though it varies considerably from country to country. Typical medium is twisted-pair copper wire. CSU/DSU CSU/DSU CSU/DSU VC Synchronous Serial Synchronous Serial chpt_10.fm Page 521 Tuesday, May 27, 2003 8:57 AM 522 Chapter 10: WANs and Routers Table 10-4 describes the characteristics of two most common packet-switched services—X.25 and Frame Relay. X.25 is both connection-oriented and reliable at both Layers 2 and 3, which is a big part of why it is so much slower than Frame Relay. Frame Relay is typically regarded as a much faster, sleeker version of X.25; it has no defined Layer 3 protocol, nor is it reliable. However, it is connection-oriented. Circuit-switched services use time-division multiplexing (TDM) and are said to be syn- chronous (they use STM), whereas packet-switched services use statistical TDM and are sometimes said to be asynchronous (similar to ATM). Table 10-4 Packet-Switched Services Packet-Switched Service Characteristics X.25 An older technology, but still widely used. Has extensive error-checking capabilities from the days when WAN links were more prone to errors. This makes it reliable but limits its bandwidth. Bandwidth can be as high as 2 Mbps. Usage is fairly extensive. Cost is moderate. Typical medium is twisted-pair copper wire. Frame Relay A packet-switched version of narrowband ISDN. Has become an extremely popular WAN technology in its own right. More efficient than X.25, but with similar services. Maximum bandwidth is 44.736 Mbps. 56 kbps and 384 kbps are extremely popular in the U.S. Usage is widespread. Cost is moderate to low. Typical media include twisted-pair copper wire and optical fiber. chpt_10.fm Page 522 Tuesday, May 27, 2003 8:57 AM WAN Characteristics 523 Cell-Switched Services Cell-switched services provide a dedicated-connection switching technology that organizes digital data into cell units and transmits them over a physical medium using digital signal technology. Table 10-5 describes the characteristics of two most common cell-switched services— Asynchronous Transfer Mode (ATM) and Switched Multimegabit Data Service (SMDS). Dedicated Digital Services Dedicated digital services also provide circuit-switched services but the connection is “always-up.” Table 10-6 describes the characteristics of the most common dedicated digital services— T1, T3, E1, E3; digital subscriber line (xDSL); and Synchronous Optical Network (SONET). Table 10-5 Cell-Switched Services Cell-Switched Service Characteristics ATM Closely related to broadband ISDN. Is becoming an increasingly important WAN technology. Uses small, fixed 53-byte length cells to carry data. Maximum bandwidth is currently 622 Mbps, though higher speeds are being developed. Typical media are twisted-pair copper wire and optical fiber. Usage is widespread and increasing. Cost is high. SMDS Closely related to ATM, and typically used in metropolitan-area networks (MANs). Maximum bandwidth is 44.736 Mbps. Typical media are twisted-pair copper wire and optical fiber. Usage is not very widespread. Cost is relatively high. chpt_10.fm Page 523 Tuesday, May 27, 2003 8:57 AM 524 Chapter 10: WANs and Routers Table 10-6 Dedicated Digital Services Dedicated Digital Service Characteristics T1, T3, E1, E3 The T series of services in the U.S. and the E series of services in Europe are extremely important WAN technologies. They use TDM to slice up and assign time slots for data transmission. Bandwidths for the T and E series lines are 1.544 Mbps for T1, 44.736 Mbps for T3, 2.048 Mbps for E1, and 34.368 Mbps for E3. Other bandwidths are also available. The media used are typically twisted-pair copper wire and opti- cal fiber. Usage is extremely widespread. Cost is moderate. xDSL A new and developing family of WAN technologies intended primarily for home use. xDSL indicates the entire family of DSL technologies, including high-bit-rate DSL (HDSL), single-line DSL (SDSL), asymmetric DSL (ADSL), and very-high-data-rate DSL (VDSL). Bandwidth decreases with increasing distance from the phone company’s equipment. Top speeds of 51.84 Mbps are possible near a phone company office; however, most bandwidths are much lower from hun- dreds of kbps to several Mbps. Cost is moderate and decreasing. SONET A family of very high-speed physical layer technologies. Designed for optical fiber, but can also run on copper cables. Has a series of data rates available with special designations. Implemented at different Optical Carrier (OC) levels, ranging from 51.84 Mbps (OC-1) to 9,952 Mbps (OC-192). Can achieve high data rates by using wavelength division multi- plexing (WDM). WDM is when lasers are tuned to slightly different colors, or wavelengths, to send huge amounts of data optically. Usage is widespread among Internet backbone entities. The cost is expensive. This is not a technology that connects to homes. chpt_10.fm Page 524 Tuesday, May 27, 2003 8:57 AM WAN Characteristics 525 Dialup, Cable, and Wireless Services Table 10-7 describes the characteristics of other WAN services that do not fall into any of the previously covered categories of WAN technologies. Among these miscellaneous technologies are dialup modems, cable modems, and terrestrial and satellite wireless. Figure 10-9 illustrates different WAN technologies connected by routers. Table 10-7 Dialup, Cable, and Wireless Services WAN Service Characteristics Dialup modem (switched analog) Limited in speed, but quite versatile. Works with existing phone network. Maximum bandwidth approximately 56 kbps. Cost is low. Usage is still very widespread. Typical medium is the twisted-pair phone line. Cable modem (shared analog) Puts data signals on the same cable as television signals. Increasing in popularity in regions that have large amounts of existing cable TV coaxial cable. Ninety percent of homes in United States have existing cable TV coaxial cable. Maximum bandwidth can be 10 Mbps, though this decreases as more users attach to a given network segment. Behaves like an unswitched (shared) LAN. Cost is relatively low. The medium is coaxial cable. Wireless Wireless requires no medium since the signals are electromag- netic waves that are transmitted through the air. A variety of wireless WAN links exist, including the following: Terrestrial has bandwidths typically in the 11 Mbps range (for example, microwave). Cost is relatively low, line of sight (LOS) is usually required, and usage is moderate. Satellite can serve mobile users such as those in a cellular tele- phone network and remote users that are too far from any wires or cables. Usage is widespread, and the cost is high. chpt_10.fm Page 525 Tuesday, May 27, 2003 8:57 AM 526 Chapter 10: WANs and Routers Figure 10-9 WAN Technologies Connected by Routers Comparison of WAN Technologies Table 10-8 shows an overview of the previously discussed WAN technologies. Table 10-8 WAN Technologies WAN Acronym WAN Name Maximum Bandwidth Comments POTS plain old telephone service 4 kHz analog Standard for reliability ISDN Integrated Services Digital Network 128 kbps Data and voice together X.25 X.25 2 Mbps Old reliable workhorse Frame Relay Frame Relay Up to 44.736 Mbps New workhorse ATM Asynchronous Transfer Mode 622 Mbps High-powered networks SMDS Switched Multi- megabit Data Service 1.544 Mbps and 44.736 Mbps MAN variant of ATM T1, T3 T1, T3 1.544 Mbps and 44.736 Mbps Widely used tele- communications chpt_10.fm Page 526 Tuesday, May 27, 2003 8:57 AM WAN Characteristics 527 xDSL digital subscriber line 384 kbps Technology over phone lines SONET Synchronous Opti- cal Network 9,992 Mbps Fast optical fiber transmission Dialup modem modem 56 kbps Mature technology using phone lines Cable modem cable modem 10 Mbps Technology using cable TV Terrestrial wireless wireless 11 Mbps Microwave and laser links Satellite wireless wireless 2 Mbps Microwave and laser links More Information: Costs of the WAN Tempering the various performance criteria of the WAN is cost. The costs of owning and oper- ating a WAN include the initial startup costs, as well as the monthly recurring expenses. Not surprisingly, the larger and more powerful network components are much more expensive than smaller, less-robust components. Therefore, designing a WAN becomes an economic exercise in which a careful balance of performance and cost is achieved. Achieving this balance can be painful. No one wants to design a WAN that disappoints the users with its performance, but no one wants to design a WAN that blows the budget, either! Fortunately, the following suggestions can help guide the design of a WAN that satisfies exist- ing requirements, provides flexibility for future growth, and doesn’t exceed the budget: ■ The capital investments in routers and other network hardware become a fixed part of the network. After the hardware components are placed in operation, the logistics of replacing hardware become quite complicated. And, depending on your depreciation schedule for capital equipment, you might find yourself obligated to use the hardware for five or more years! It might behoove you to purchase a larger router that is relatively low in port den- sity. You can add hardware (memory, CPUs, and interfaces) in the future as the need for them arises. This method allows future expansion at modest incremental costs and little (if any) operational downtime. ■ The transmission facilities are relatively easy to replace with other transmission facilities. They are an expense item, not a capital investment, so there is no depreciation expense to retire. These can be replaced with other facilities as often as your lease agreement with the carrier permits. Therefore, you might want to explore your options for meeting perfor- mance requirements with the various available transmission facilities and technologies. Table 10-8 WAN Technologies (Continued) WAN Acronym WAN Name Maximum Bandwidth Comments chpt_10.fm Page 527 Tuesday, May 27, 2003 8:57 AM 528 Chapter 10: WANs and Routers More Information: Resource Utilization Rates The degree to which the various physical resources of the WAN are being utilized is also a good indicator of how well, or how poorly, the WAN is performing relative to the performance requirements. Two main categories of resource utilization rates should be monitored carefully: ■ Router CPU and memory utilization rates ■ Transmission facility utilization rates Router Physical Resource Rates Routers are one of the most vital components of any WAN. And, unlike the transmission facili- ties, they are outside the view of the telecommunications carrier. Therefore, they are distinctly the responsibility of the customer. Fortunately, routers are intelligent devices that contain their own CPU and memory. These physical resources are indispensable in the calculation of WAN routes and the forwarding of packets. They can also be used to monitor the performance of the router. If either CPU or memory utilization rates approach 100 percent, performance suffers. Numer- ous conditions can result in either utilization rate temporarily spiking upward with subsequent performance degradation. One example is a sudden increase in transmissions from the LAN to the WAN. LANs can operate at speeds of up to 1 Gbps, but they usually operate only at 10, 16, or 100 Mbps. Any of these speeds is a gross mismatch against the typical WAN transmission facility, which offers a paltry 1.544 Mbps of bandwidth. This mismatch in bandwidth must be buffered by the router’s memory. It won’t take long for a router to become resource constricted given a sustained period of heavy LAN transmissions. If such situations are rarely experienced, they should be considered aberrations. Aberrations should be monitored, but they shouldn’t drive physical upgrades. If these resource constrictions recur or constitute a trend, however, something needs to be done. Usually this requires an upgrade, either to the next larger router or via an expansion of memory. If a router is chronically at or near 100 percent of capacity with its memory, it is time to purchase additional memory. However, responding to chronically high CPU utilization rates might not be as simple as a memory upgrade. Really, only three options exist for improving high CPU utilization rates: ■ If possible, add another CPU to the router. ■ Upgrade to a more powerful router. ■ Investigate the WAN’s traffic patterns to see if the load on the problematic router can be reduced. Manipulating traffic patterns is really only a viable option in larger WANs with complex topolo- gies that can afford route redundancy. Transmission Facility Rates Transmission facilities can also be monitored for utilization. Typically, this utilization rate is expressed in terms of the percentage of consumed bandwidth. If you are using a T1, for exam- ple, a given sample might indicate that 30 percent of its 1.544 Mbps of available bandwidth is currently being utilized. chpt_10.fm Page 528 Tuesday, May 27, 2003 8:57 AM . chpt _10 .fm Page 5 21 Tuesday, May 27 , 20 03 8:57 AM 522 Chapter 10 : WANs and Routers Table 10 -4 describes the characteristics of two most common packet-switched services—X .25 and Frame Relay. X .25 . high. chpt _10 .fm Page 523 Tuesday, May 27 , 20 03 8:57 AM 524 Chapter 10 : WANs and Routers Table 10 -6 Dedicated Digital Services Dedicated Digital Service Characteristics T1, T3, E1, E3 The T. facilities and technologies. Table 10 -8 WAN Technologies (Continued) WAN Acronym WAN Name Maximum Bandwidth Comments chpt _10 .fm Page 527 Tuesday, May 27 , 20 03 8:57 AM 528 Chapter 10 : WANs and Routers More