1. Chapter 7 WANs and Remote Connectivity

2. WAN Essentials A WAN traverses a large geographic area A WAN link is a connection from one site to another and is described as point-to-point Carry primarily digital signals Uses different transmission systems, topologies and media Data usually sent over public communication links Data lines may be leased to provide a dedicated line Typically use a mesh or partial mesh topology See examples where a WAN is needed – p. 269

3. WAN Transmission Methods PSTN ISDN xDSL Cable

4. Public Switched Telephone Network (PSTN) Refer to regular telephone lines or POTS Most now use digital transmission through fiber optics, twisted pair, and microwave Most dial-up connections use this medium A dial-up connection needs a modem that does conversion from digital to analog and vice versa You cannot be guaranteed the quality of the connection (corruption of data during transmission, speed of transmission) Most significant limiting factor is its capacity (<= 56 Kbps)

5. PSTN – Continued See Figure 7-2, p. 272 PSTN uses circuit switching - uses a dedicated point-to-point connection Not very secure – there are many points where transmission can be intercepted Data can be encrypted to make it more secure

6. Integrated Services Digital Network (ISDN) An international standard for transmitting data using digital signals Uses the public telephone lines and dial-up connections Differs from PSTN in that it relies exclusively on digital connections and that it can carry data and voice simultaneously Began being offered in the mid 1980s Has not caught on as quickly as predicted There are other competing transmission methods

7. ISDN – Continued Two types of channels –B channel – circuit switched channel that carries data or voice. Max of 56Kbps –D channel – packet switched channel that carries information about the call. Max of 16Kbps. Only one D channel is used Basic Rate ISDN – uses 2 B channels and 1 D channel. Max transmission rate of 128 Kbps Primary Rate ISDN – uses 23 B channels and 1 D channel. Max transmission rate of 1.544 Mbps

8. Digital Subscriber Lines (xDSL) A new transmission technology that competes with ISDN Uses advanced modulation techniques over regular phone lines To support high throughput the phone company must have modern switching equipment Provides data and voice over a single line DSL uses a connection that is dedicated (you don’t have to dial a number like you do with ISDN) The x in xDSL refers to the particular version, a number of which are available

9. xDSL – Continued Downstream – data traveling from the carrier’s point of presence (POP) to the customer Upstream – data traveling from customer to carrier’s POP. A technology that offers more throughput in one direction than another is said to be asymmetrical The types of DSL vary in terms of their symmetry, line capacity, and maximum line length Typical speeds of 1.544 Mbps and maximum distance of 18,000 feet See Table 7-1, p. 277

10. Cable Uses existing coaxial cable for TV Asymmetric technology 3-10 Mbps downstream and 2Mbps upstream due to its shared nature Cable modem required that connects to NIC Provides a dedicated connection that does not require dialing Issue with security and speed (See p. 278)

11. T-Carriers Permanent dedicated lines that are leased from a public telecommunications company Use time division multiplexing to divide a single channel into multiple channels T1 line has 24 channels, each capable of 64Kbps throughput for a total of 1.544 Mbps Devices called multiplexers are needed at the sending and receiving ends T3 line has 672 channels, each capable of 64Kbps for a total throughput of 44.736 Mbps

12. T1 Line Commonly used by businesses to connect sites that are far apart T1 costs as much as $4000 to install plus an additional $1000-$2000 per month usage fee. The longer the run of the T1 line, the higher the fee Some applications require multiple T1 lines for added throughput Preferable to use shielded twisted pair Multiple T1 lines require coaxial cable or fiber optics

13. T3 Line Very expensive - $8,000 to install Monthly fee depends upon average throughput. If you are using most of the bandwidth of the line, the monthly fee can be in the tens of thousands of dollars Used by the most data-intensive businesses Require coaxial cable or fiber optics

14. T-Carrier Connectivity Devices Every T-carrier requires connectivity devices at both ends of the T line The cost of these devices is not included in above approximate installation fees CSU/DSU needed on each end of the T connection CSU provides termination of the digital signal and handles error correction and line monitoring DSU converts digital signal from local computer network into digital signal appropriate for transmission over T connection See Figure 7-7, p. 282 for CSU/DSU layout

15. FDDI Standard Originally specified by ANSI in the mid-1980 and later refined by ISO Token ring standard Uses a dual fiber-optics ring that operates at 100Mbps Was the first technology to support the 100 Mbps rate Used to support network backbones Can span distances as far as 62 miles More expensive than fast Ethernet Second ring acts as backup

16. X.25 and Frame Relay X.25 is an analog, packet switched technology standardized in the mid 1970s for long-distance data transmission Frame Relay is an updated digital version of X.25 using packet switching Frame Relay offers a maximum of 1.544 Mbps throughput When you lease as X.25 or Frame Relay from your local carrier, you do not get a dedicated line You are just guaranteed a minimum bandwidth for use

17. ATM A new WAN transmission method that is being used increasingly in network backbones and WANS and uses packet switching Achieves transfer rates of 25-622 Mbps but depends upon the type of T-connector used A fixed packets size, called a cell, consists of 48 bytes of data plus a 5-byte header The ATM cell is a fraction of the size of an Ethernet frame Uses digital signals over fiber-optics Very expensive and lack of well-defined standard

18. SONET Similar to FDDI in that two rings are used Uses T carriers Interoperates well with ISDN and ATM technology Used to link WANs between N. America, Europe, and Asia because it can work with different standards used in these countries Very expensive

19. Establishing Remote Connectivity Three methods for accessing a network from a remote location –Direct dial to LAN – Requires a dial-in server on your LAN. Most complex method to configure, but it can provide the best security –Direct dial to a workstation – Remote user dials into a workstation directly attached to the LAN. Only one connection to the LAN at a time is allowed –Web interface – Files are made available through Web server software. Security cannot be controlled as well as with other methods. Any number of remote users can access the LAN at the same time.