Chapter Seven

1. Chapter Seven WANs and Remote Connectivity The chapter is meant as only a brief introduction to some of the concepts and terminology of Wide Area Networks (WANs).

2. WANs – Wide Area Networks

3. WAN – Wide Area Networks • Major characteristics of WANs: • They connect LANs that are separated by wide geographical areas. They span a large geographic area, such as a state, province or country. • They use the services of carriers such as the Regional Bell Operating Companies (RBOCs), Sprint, MCI, and others. • They use serial connections of various types to access bandwidth over large geographic areas.

4. WAN Devices • Routers • offer many services, including internetworking and WAN interface ports. • Switches • in the WAN provide connectivity for voice, data, and video communication. • Modems • include 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. • Communication servers • concentrate dial-in and dial-out user communication.

5. WAN Essentials • WAN link • Connection between one WAN site and another site • A WAN link is typically described as point-to-point • Dedicated line • Continuously available link that is leased through another carrier

6. WAN Essentials Figure 7-1: Differences in LAN and WAN connectivity

7. Need for WANs • Why do we need WANs? • Any networks with multiple sites scattered over a wide geographical need a way to exchange data between those sites. • Although all networks need a WAN for exchanging data, they may not need the same kind of WANs. • Depends on traffic load, budget, geographical distance, etc.

8. The WAN interface Your Ethernet LAN connects to your router. But what does you router connect to? ? WAN 802.3 LAN Cat5 UTP

9. RBOCs Regional Bells Provide Media Regional Bells usually provide the media for Wide-Area Network connectivity. However, some organizations own their own WAN links. SF NY

11. WAN Technologies • Many types of WAN Technologies exists. They differ in terms of speed, reliability, cost, distance covered, and security • PSTN • X.25 & Frame Relay • ISDN • T-Carriers • DSL • Cable • SONET

12. PSTN • Sometimes, it's hard to think of the telephone system in use today as a network, but it is the first and most comprehensive WAN built to date. It has been only recently (within the last 20-30 years) that we've used the phone system to move data, as well as voice traffic. • The telephone network, also called the Public Switched Telephone Network (or PSTN), serves as the basis for all WAN connections. • Also called plain old telephone service (POTS)

13. PSTN • A dial-up connection uses a PSTN or other line to access remote servers via modems at both the source and destination • The Federal Communications Commission (FCC) sets standards and policy for telecommunications transmission equipment in the United States (set the 56 Kbps limitation to avoid crosstalk.) • The place where two telephone systems meet is the point of presence(POP)

14. PSTN Figure 7-2: A typical PSTN connection to the Internet

15. X.25 and Frame Relay • X.25 • Analog, packet-switched LAN technology optimized for long-distance data transmission. • Developed as a more reliable alternative to the voice telephone system for connecting mainframe computers and remote terminals. • Ensures reliability over long distances lines by verifying the transmission at every node. • Verification renders X.25 slow & unsuitable for time sensitive applications such as audio & video. • Never widely adopted in US. • Bandwidth 64 Kbps – 2.048 Mbps.

16. X.25 and Frame Relay • Frame Relay • Updated, digital version of X.25 that also relies on packet switching. • Name derived from fact that data is separated into frames which are relayed from one node to another without any verification or error detection. • Bandwidth 1.544 Mbps – 45 Mbps • Reliable long-distance WAN connections.

17. X.25 and Frame Relay X.25 & Frame Relay can be configured as • SVCs (switched virtual circuits) • Connections established when parties need to transmit, then dismantled once the transmission is complete • PVCs (private virtual circuits) • Connections established before data needs to be transmitted and maintained after transmission is complete • CIR (committed information rate) • Guaranteed minimum amount of bandwidth selected when leasing a frame relay circuit

18. X.25 and Frame Relay Figure 7-3: A WAN using frame relay

19. ISDN (Integrated Services Digital Network) • Digital phone service was introduced to the public in 1962. Most consumers didn't have any contact with it, however, because it wasn't available as a last mile service. • The phone company used digital phone service to move voice traffic around the phone network, before converting it back to analog signals on delivery. • Although ISDN has been around for nearly 30 years, it's only been in the recent past that we've heard much about it.

20. ISDN • All ISDN connections are based on two types of channels: • The B channel is the “bearer” channel • Carries voice, video, audio data. • Max bandwidth 64 Kbps • The D channel is the “data” channel • Carries information such as call setup & termination • Max bandwidth 16 Kbps • 2 types of ISDN • BRI – Basic Rate Interface • PRI – Primary Rate Interface

21. BRI (Basic Rate Interface) • ISDN using two 64-Kbps bearer (B) channels and one 16-Kbps data (D) channel, as indicated by the following notation: • 2B+D • Through bonding, the two 64-Kbps channels can be combined to achieve an effective throughput of 128-Kbps

22. BRI (Basic Rate Interface) • The Network Termination 1 (NT1) device connects twisted-pair wiring at customer’s building with ISDN terminal equipment (TE) via RJ-11 or RJ-45 data jacks • A terminal adapter (TA) converts digital signals into analog signals for use with ISDN phones and other analog devices Figure 7-4: A BRI link

23. PRI (Primary Rate Interface) • Type of ISDN using 23 B channels and one 64-Kbps D channel, as represented by the following notation: • 23B+D • PRI links use same kind of equipment as BRI links, but require the services of an extra network termination device—called a Network Termination 2 (NT2)—to handle multiple ISDN lines

24. PRI (Primary Rate Interface) • It is only feasible to use ISDN for the local loop portion of a WAN link, because ISDN can only span 18,000 feet before repeater equipment is need to boost the signal. • ISDN resembles T1 service because they're based on the same carrier system and the data on these two systems is encoded using the same encoding schemes. Figure 7-5: A PRI link

25. T-Carriers • Broadband • Group of network connection types or transmission technologies generally capable of exceeding 1.544 Mbps throughput • The most common T carriers are T1 (1.544 Mbps) and T3 (45 Mbps). • The "T" stands for terrestrial (as opposed to satellite transmissions), and the 1 is an abbreviation for the 1.544-Mbps signal rate.

26. Types of T-Carriers • The most common T-carrier implementations are T1 and T3 • Signal level • ANSI standard for T-carrier technology that refers to its Physical layer electrical signaling characteristics • DSO (digital signal, level 0) • Equivalent of one data or voice channel • Fractional T1 • Arrangement allowing an organization to use only some channels on a T1 line, paying for what they use

27. Types of T-Carriers Figure 7-1: Carrier specifications

28. T-Carrier Connectivity • Wiring • Can use unshielded or shielded twisted-pair copper wiring • CSU/DSU (Channel Service Unit/Data Service Unit) • CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring • DSU converts the digital signal used by bridges, routers, and multiplexers into the digital signal sent via the cabling Figure 7-6: A CSU/DSU connecting a T1

29. T-Carrier Connectivity • Multiplexer • Device that combines multiple voice or data channels on one line Figure 7-7: Typical use of a multiplexer on a T1-connected data network

30. T-Carrier Connectivity • Routers and bridges • On a typical T1-connected data network, terminal equipment will consist of bridges, routers or a combination of the two Figure 7-8: A router on a T1-connected network

31. DSL • Digital Subscriber Lines • Uses advanced data modulation techniques to achieve extraordinary throughput over regular phone lines • Like ISDN, DSL can span only limited distances without the help of repeaters

32. Types of DSL • Term xDSL refers to all DSL varieties, of which at least eight currently exist • DSL types can be divided into two categories: • Asymmetrical • Symmetrical • To understand the difference between these two categories, you must understand the concept of downstream and upstream data transmission

33. Types of DSL Table 7-2: Comparison of DSL types

34. DSL Connectivity • DSL connectivity, like ISDN, depends on the PSTN • Inside carrier’s POP, a device called a DSL access multiplexer (DSLAM) aggregates multiple DSL subscriber lines and connects them to a larger carrier or to the Internet backbone Figure 7-9: A DSL connection

35. DSL Connectivity • Once inside the customer’s home or office, the DSL line must pass through a DSL modem Figure 7-10: A DSL modem

36. Cable • Cable connections require that the customer use a special cable modem, a device that modulates and demodulates signals for transmission and reception via cable wiring Figure 7-11: A cable modem

37. Cable • Hybrid fiber-coax (HFC) • Very expensive fiber-optic link that can support high frequencies • HFC upgrades to existing cable wiring are required before current TV cable systems can serve as WAN links • Cable drop • Fiber-optic or coaxial cable connecting a neighborhood cable node to a customer’s house • Head-end • Cable company’s central office, which connects cable wiring to many nodes before it reaches customers’ sites

38. Cable Figure 7-12: Cable infrastructure

39. SONET (Synchronous Optical Network) • Can provide data transfer rates from 64 Kbps to 39.8 Gbps using the same TDM technique used by T-carriers • Known internationally as SDH (Synchronous Digital Hierarchy) • SONET is self-healing Figure 7-13: SONET technology on a long-distance WAN

40. SONET (Synchronous Optical Network) Table 7-3: SONET OC levels

41. WAN Implementation:Speed Table 7-4a: A comparison of WAN technology transmission speeds

42. WAN Implementation:Speed Table 7-4b: A comparison of WAN technology transmission speeds

43. WAN Implementation:Reliability • WAN implementations can roughly be divided as follows: • Not very reliable, suited to individual or unimportant transmissions: • PSTN dial-up • Sufficiently reliable, suited for day-to-day transmissions: • ISDN, T1, fractional T1, T3, DSL, cable, X.25, and frame relay • Very reliable, suited to mission-critical applications: • SONET

44. WAN Implementation:Security • Among other things, consider the following issues: • WAN security depends in part on the encryption measures each carrier provides for its lines • Enforce password-based authorization for LAN and WAN access and teach users how to choose difficult-to-decrypt passwords • Take the time to develop, publish, and enforce a security policy for users in your organization • Maintain restricted access to network equipment rooms and data centers

45. WAN Implementation:Virtual Private Networks (VPNs) • VPNs are wide area networks logically defined over public transmission systems that serve an organization’s users, but isolate that organization’s traffic from other users on the same public lines Figure 7-14: An example of a VPN

46. Remote Connectivity • Remote access methods: • Direct dial to the LAN • The computer dialing into the LAN becomes a remote node on the network • Direct dial to a workstation • Software running on both remote user’s computer and LAN computer allows remote user to “take over” the LAN workstation, a solution known as remote control • Internet/Web interface • Through a browser, a user at home or on the road connects to a LAN whose files are made visible to the Web through Web server software

47. Remote Connectivity • ICA (Independent Computing Architecture) client • Remote access client developed by Citrix Systems, Inc. • Enables remote users to use virtually any LAN application over any type of connection, public or private • Remote Access Service (RAS) • One of the simplest dial-in servers • This software is included with Windows 2000 Server

48. Dial-Up Networking • Refers to the process of dialing into a LAN’s (private) access server or to an ISP’s (public) access server to log onto a network Figure 7-15: Choosing a network connection type

49. SLIP and PPP • Serial Line Internet Protocol (SLIP) • Communications protocol enabling a workstation to connect to a server using a serial connection • Can carry only IP packets • Supports only asynchronous transmission • Point-to-Point Protocol • Communications protocol enabling a workstation to connect to a server using a serial connection • Can carry many different types of Network layer packets • Supports both asynchronous and synchronous transmission