| A | B |
|---|
| WAN | (wide area network) operates at the physical layer and the data link layer of the OSI reference model. It interconnects LANs (local area networks) that are usually separated by large geographic areas. |
| The major characteristics of WANs are | 1) They operate beyond the local LANs geographic scope. They use the services of carriers such as the Regional Bell Operating Companies (RBOCs) and Sprint and MCI. 2) They use serial connections of various types to access bandwidth over wide-area geographies. 3) By definition, WANs connect devices that are separated by wide geographical areas. |
| WAN devices | 1) routers -- offer many services, including internetworking and WAN interface ports 2) switches -- connect to WAN bandwidth for voice, data, and video communication 3) modems -- interface voice-grade services; channel service units/digital service units (CSU/DSUs) that interface T1/E1 services; and Terminal Adapters/Network Termination 1 (TA/NT1s) that interface Integrated Services Digital Network (ISDN) services 4) communication servers -- concentrate dial-in and dial-out user communication |
| WAN Physical Layer | describes the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Typically, the DCE is the service provider and the DTE is the attached device. |
| WAN physical layer standards | EIA/TIA-232 ; EIA/TIA-449; V.24; V.35; X.21; G.703; EIA-530 |
| WAN encapsulation standards | High-Level Data Link Control (HDLC); Frame Relay; Point-to-Point Protocol (PPP); Simple Data Link Control Protocol (SDLC); Serial Line Interface Protocol (SLIP); Link Access Procedure Balanced (LAPB); Link Access Procedure D-channel (LAPD); Link Access Procedure Frame (LAPF); |
| High-Level Data Link Control (HDLC) | an IEEE standard; 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 |
| Frame Relay | uses high-quality digital facilities; uses simplified framing with no error correction mechanisms, which means it can send Layer 2 information much more rapidly than other WAN protocols |
| Point-to-Point Protocol (PPP) | described by RFC 1661; two standards developed by the IETF; contains a protocol field to identify the network layer protocol |
| Simple Data Link Control Protocol (SDLC) | an IBM-designed WAN data link protocol for System Network Architecture (SNA) environments; largely being replaced by the more versatile HDLC |
| Serial Line Interface Protocol (SLIP) | an extremely popular WAN data link protocol for carrying IP packets; being replaced in many applications by the more versatile PPP |
| Link Access Procedure Balanced (LAPB) | a data link protocol used by X.25; has extensive error checking capabilities |
| Link Access Procedure D-channel (LAPD) | the WAN data link protocol used for signaling and call setup on an ISDN D-channel. Data transmissions take place on the ISDN B channels |
| Link Access Procedure Frame (LAPF) | for Frame-Mode Bearer Services; a WAN data link protocol, similar to LAPD, used with frame relay technologies |
| Circuit-Switched Services | POTS (Plain Old Telephone Service); Narrowband ISDN (Integrated Services Digital Network) |
| Packet-Switched Services | X.25; Frame Relay |
| Cell-Switched Services | ATM (Asynchronous Transfer Mode); SMDS (Switched Multimegabit Data Service) |
| Dedicated Digital Services | T1, T3, E1, E3; xDSL (DSL for Digital Subscriber Line and x for a family of technologies): HDSL -- high-bit-rate DSL; SDSL -- single-line DSL; ADSL -- asymmetric DSL; VDSL -- very-high-bit-rate DSL; RADSL -- rate adaptive DSL; SONET (Synchronous Optical Network) |
| Other WAN Services | dial-up modems; cable modems (shared analog); wireless |
| POTS (Plain Old Telephone Service) | not a computer data service, but included for two reasons: (1) many of its technologies are part of the growing data infrastructure, (2) it is a model of an incredibly reliable, easy-to-use, wide-area communications network; typical medium is twisted-pair copper wire |
| Narrowband ISDN (Integrated Services Digital Network) | a versatile, widespread, historically important technology; was the first all-digital dial-up service; usage varies greatly from country to country; cost is moderate; maximum bandwidth is 128 kbps for the lower cost BRI (Basic Rate Interface) and about 3 Mbps for the PRI (Primary Rate Interface); usage is fairly widespread, though it varies considerably from country to country; typical medium is twisted-pair copper wire |
| 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, which make it reliable but limits its bandwidth; bandwidth may 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; 56kbps and 384kbps 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 |
| ATM (Asynchronous Transfer Mode) | closely related to broadband ISDN; becoming an increasingly important WAN (and even LAN) technology; uses small, fixed length (53 byte) frames 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 (Switched Multimegabit Data Service) | closely related to ATM, and typically used in MANs; maximum bandwidth is 44.736 Mbps; typical media are twisted-pair copper wire and optical fiber; usage not very widespread; cost is relatively high |
| 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 time division multiplexing to "slice up" and assign time slots for data transmission; bandwidth is: 1) T1 -- 1.544 Mbps; 2) T3 -- 44.736 Mbps; 3) E1 -- 2.048 Mbps; 4) E3 -- 34.368 Mbps; 5) other bandwidths are available The media used are typical twisted-pair copper wire and optical fiber. Usage is extremely widespread; cost is moderate. |
| xDSL (DSL for Digital Subscriber Line and x for a family of technologies) | a new and developing WAN technology intended for home use; has a bandwidth which decreases with increasing distance from the phone companies equipment; top speeds of 51.84 Mbps are possible near a phone company office, more common are much lower bandwidths (from 100s of kbps to several Mbps); usage is small but increasing rapidly; cost is moderate and decreasing; x indicates the entire family of DSL technologies, including: 1) HDSL -- high-bit-rate DSL; 2) SDSL -- single-line DSL; 3) ADSL -- asymmetric DSL; 4) VDSL -- very-high-bit-rate DSL; 5) RADSL -- rate adaptive DSL |
| SONET (Synchronous Optical Network) | 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 OC (optical carrier) levels ranging from 51.84 Mbps (OC-1) to 9,952 Mbps (OC-192); can achieve these amazing data rates by using wavelength division multiplexing (WDM), in which lasers are tuned to slightly different colors (wavelengths) in order to send huge amounts of data optically; usage is widespread among Internet backbone entities; cost is expensive (not a technology that connects to your house) |
| dial-up modems (switched analog) | limited in speed, but quite versatile; works with existing phone network; maximum bandwidth approx. 56 kbps; cost is low; usage is still very widespread; typical medium is the twisted-pair phone line |
| cable modems (shared analog) | put data signals on the same cable as television signals; increasing in popularity in regions that have large amounts of existing cable TV coaxial cable (90% of homes in U.S.); maximum bandwidth can be 10 Mbps, though this degrades as more users attach to a given network segment (behaving like an unswitched LAN); cost is relatively low; usage is small but increasing; the medium is coaxial cable. |
| wireless | no medium is required since the signals are electromagnetic waves; there are a variety of wireless WAN links, two of which are: 1) terrestrial -- bandwidths typically in the 11 Mbps range (e.g. microwave); cost is relatively low; line-of-sight is usually required; usage is moderate; 2) satellite -- can serve mobile users (e.g. cellular telephone network) and remote users (too far from any wires or cables); usage is widespread; cost is high |
| Internal configuration components of a router | RAM/DRAM; NVRAM; Flash; ROM; interface |
| RAM/DRAM | Stores routing tables, ARP cache, fast-switching cache, packet buffering (shared RAM), and packet hold queues. RAM also provides temporary and/or running memory for the router's configuration file while the router is powered on. RAM content is lost when you power down or restart. |
| NVRAM | nonvolatile RAM; stores a router's backup/startup configuration file; content remains when you power down or restart. |
| Flash | erasable, reprogrammable ROM; holds the operating system image and microcode; allows you to update software without removing and replacing chips on the processor; content remains when you power down or restart; multiple versions of IOS software can be stored in Flash memory |
| ROM | contains power-on diagnostics, a bootstrap program, and operating system software; software upgrades in ROM require replacing pluggable chips on the CPU |
| interface | network connection through which packets enter and exit a router; it can be on the motherboard or on a separate interface module |
| The Internet | a network of autonomous systems, each of which has routers that typically play one of four roles |
| internal routers | internal to one area |
| area border routers | connect two or more areas |
| backbone routers | primary paths for traffic that is most often sourced from, and destined for, other networks |
| autonomous system (AS) boundary routers | communicate with routers in other autonomous systems |
| two levels of access to commands | user mode & privileged mode |
| user mode | Typical tasks include those that check the router status. In this mode, router configuration changes are not allowed. |
| privileged mode | Typical tasks include those that change the router configuration. |
| Typing a question mark (?) at the user mode prompt or the privileged mode prompt | displays a handy list of commonly used commands |
| To access privileged mode | type enable (or the abbreviation ena). |
| type exit | To logout of the router |
| The ^ | The user interface provides syntax checking by placing a ^ where the error occurred |
| Ctrl-A | return directly to the beginning of the line |
| Ctrl-B | Move back one character |
| Esc-B | Moves back one word |
| Ctrl-E | Move to the end of the command line |
| Ctrl-F | Moves forward one character |
| Esc-F | Moves forward one word |
| Ctrl-P | Recalls previous command |
| Ctrl-N | Recalls most recent command |
| Ctrl-Z | back out of configuration mode |
