Chapter 7: WANs and Remote Connectivity

1. Chapter 7: WANs and Remote Connectivity

2. Objectives • Identify network applications that require WAN technology • Explain various WAN topologies, including their advantages and disadvantages • Describe a variety of WAN transmission and connection methods, including PSTN, ISDN, T-carriers, DSL, broadband cable, and SONET

3. Objectives (continued) • Assess WAN implementation options based on speed, security, and reliability • Understand the hardware and software requirements for remotely connecting to a network

4. WANs and Remote Connectivity • A WAN is a network that connects two or more geographically distinct LANs • Remote connectivity and WANs are significant concerns for organizations attempting to meet the needs of telecommuting workers, global business partners, and Internet-based commerce

5. WAN Essentials • A WAN is a network that traverses some distance and usually connects LANs, whether across the city or across the nation • The internet is the largest WAN in existence today7 • LANs use a building’s internal cabling, such as twisted-pair, that runs from work area to the wall, through plenum areas and to a telecommunications closet

6. WAN Essentials (continued) • WANs typically send data over publicly available communications networks, which are owned by local and long-distance telecommunications carriers • Such carriers, which are privately owned corporations, are also known as network service providers (NSPs)

7. WAN Essentials (continued) • For better throughput, an organization might lease a dedicated line, or a continuously available communications channel, from a telecommunications provider, such as a local telephone company or ISP • A WAN link is a connection between one WAN site (or point) and another site (or point)

9. WAN Topologies • Bus • A WAN in which each site is directly connected to no more than two other sites in a serial fashion is known as a bus topology WAN • A bus topology WAN is similar to a bus topology LAN in that each site depends on every other site in the network to transmit and receive its traffic • The WAN bus topology uses different locations, each one connected to another one through point-to-point links

10. WAN Topologies (continued) • A bus topology WAN is often the best option for organizations with only a few sites and the capability to use dedicated circuits • Bus WAN topologies are suitable for only small WANs • A single failure on a bus topology WAN can take down communications between all sites

12. WAN Topologies (continued) • Ring • In a ring topology WAN, each site is connected to two other sites so that the entire WAN forms a ring pattern • This architecture is similar to the simple ring topology used on a LAN, except that a WAN ring topology connects locations rather than local nodes and in most WANs, a ring topology uses two parallel paths for data

14. WAN Topologies (continued) • A ring topology WAN cannot not be taken down by the loss of one site; instead, if one site fails, data can be rerouted around the WAN in a different direction • WANs that use the ring topology are only practical for connecting fewer than four or five locations

15. WAN Topologies (continued) • Star • The star topology WAN mimics the arrangement of a star topology LAN • A single site acts as the central connection point for several other points

16. WAN Topologies (continued) • If a single connection fails, only one location loses WAN access • When all of its dedicated circuits are functioning, a star WAN provides shorter data paths between any two sites

18. WAN Topologies (continued) • Mesh • A mesh topology WAN incorporates many directly interconnected sites • Because every site is interconnected, data can travel directly from its origin to its destination • Mesh WANs are the most fault-tolerant type of WAN because they provide multiple routes for data to follow between any two points

19. WAN Topologies (continued) • The type of mesh topology in which every WAN site is directly connected to every other site is called a full mesh WAN • Partial mesh WAN are used when only critical WAN sites are directly interconnected and secondary sites are connected through star or ring topologies • Partial mesh WANs are more common in today’s business world than full mesh WANs because they are more economical

21. WAN Topologies (continued) • Tiered • In a tiered topology WAN, sites connected in star or ring formations are interconnected at different levels, with the interconnection points being organized into layers to form hierarchical groupings

22. WAN Topologies (continued) • Tiered systems allow for easy expansion and inclusion of redundant links to support growth • Their enormous flexibility means that creation of tiered WANs requires careful consideration of geography, usage patterns, and growth potential

24. PSTN • Stands for Public Switched Telephone Network • Refers to the network of typical telephone lines and carrier equipment that service most homes • PSTN may also be called plain old telephone service (POTS) • The PSTN comprises the entire telephone system, from the lines that connect homes and businesses to the network centers that connect different regions of a country

25. PSTN (continued) • The PSTN is often used by individuals connecting to a WAN (such as the Internet) via a dial-up connection • A dial-up connection is one in which a user connects, via a modem, to a distant network from a computer and stays connected for a finite period of time

26. PSTN (continued) • A central office is the place where a telephone company terminates lines and switches calls between different locations • The portion of the PSTN that connects your house to the nearest central office is known as the local loop, or the last mile

29. X.25 • X.25 is an analog, packet-switched technology designed for long-distance data transmission • The X.25 standard specifies protocols at the Physical, Data Link, and Network layers of the OSI Model • The X.25 provides excellent flow control and ensures data reliability over long distances by verifying the transmission at every node • X.25 checks for errors and, in the case of an error, either corrects the damaged data or retransmits the original data

30. Frame Relay • An updated, digital version of X.25 that also relies on packet switching • Frame Relay protocols operate at the Data Link layer of the OSI Model and can support multiple different Network and Transport layer protocols • The name is derived from the fact that data is separated into frames, which are then relayed from one node to another without any verification or processing • Frame Relay does not guarantee reliable delivery of data

31. X.25 and Frame Relay • Both X.25 and Frame Relay may be configured as switched virtual circuits (SVCs) or permanent virtual circuits (PVCs) • SVCs are connections that are established when parties need to transmit, then terminated once the transmission is complete • PVCs are connections that are established before data needs to be transmitted and maintained after the transmission is complete and they are not dedicated, individual links • The service provider guarantees a minimum amount of bandwidth, called the committed information rate (CIR)

33. ISDN • Integrated Services Digital Network is an international standard for transmitting digital data over the PSTN • ISDN specifies protocols at the Physical, Data Link, and Transport layers of the OSI Model • ISDN relies on the PSTN for its transmission medium • ISDN is distinguished because it can simultaneously carry as many as two voice calls and one data connection on a single line

34. ISDN (continued) • All ISDN connections are based on two types of channels: B channels and D channels • The B channel is the “bearer” channel, employing circuit-switching techniques to carry voice, video, audio, and other types of data over the ISDN connection • The D channel is the “data” channel, employing packet-switching techniques to carry information about the call, such as session initiation and termination signals, caller identity, call forwarding, and conference calling signals

35. ISDN (continued) • In North America, two types of ISDN connections are commonly used: Basic Rate Interface (BRI) and Primary Rate Interface (PRI) • BRI (Basic Rate Interface) uses two B channels and one D channel • In a process called bonding, these two 64-Kbps B channels can be combined to achieve an effective throughput of 128 Kbps

37. ISDN (continued) • PRI (Primary Rate Interface) uses 23 B channels and one 64-Kbps D channel • PRI is less commonly used by individual subscribers than BRI is, but it may be selected by businesses and other organizations that need more throughput • PRI link can carry voice and data, independently of each other or bonded together

39. T-Carriers • T-carrier standards specify a method of signaling, which means they belong to the Physical layer of the OSI Model • A T-carrier uses time division multiplexing (TDM) over two wire pairs (one for transmitting and one for receiving) to divide a single channel into multiple channels • Each channel may carry data, voice, or video signals • The medium used for T-carrier signaling can be ordinary telephone wire, fiber-optic cable, or wireless links

42. T-Carriers (continued) • Types of T-Carriers • T1 circuit can carry the equivalent of 24 voice or data channels, giving a maximum data throughput of 1.544 Mbps • A T3 circuit can carry the equivalent of 672 voice or data channels, giving a maximum data throughput of 44.736 Mbps

43. T-Carriers (continued) • A fractional T1 lease allows organizations to use only some of the channels on a T1 line and be charged according to the number of channels they use • The signal level refers to the T-carrier’s Physical layer electrical signaling characteristics • DS0 (digital signal, level 0) is the equivalent of one data or voice channel

44. T-Carriers (continued) • T-Carrier Connectivity • Every T-carrier line requires connectivity hardware at both the customer site and the local telecommunications provider’s switching facility • T-carrier lines require specialized connectivity hardware that cannot be used with other WAN transmission methods • T-carrier lines require different media depending on their throughput • Wiring • T1 technology can use unshielded or shielded twisted-pair (UTP or STP) copper wiring • STP is preferable to UTP

45. T-Carriers (continued) • CSU/DSU (Channel Service Unit/Data Service Unit) • The CSU/DSU is the connection point for a T1 line at the customer’s site • The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring • The DSU converts the T-carrier frames into frames the LAN can interpret and vice versa • After being demultiplexed, an incoming T-carrier signal passes on to devices collectively known as terminal equipment

46. T-Carriers (continued) • Terminal Equipment • On a typical T1-connected data network, the terminal equipment will consist of switches, routers, or bridges • Usually, a router or Layer 3 or higher switch is the best option, because these devices can translate between different Layer 3 protocols that might be used on the WAN and LAN • On some implementations, the CSU/DSU is not a separate device, but is integrated with the router or switch as an expansion card

48. DSL • Digital subscriber line (DSL) is a WAN connection method introduced by researchers at Bell Laboratories in the mid-1990s • DSL can span only limited distances without the help of repeaters and is therefore best suited to the local loop portion of a WAN link • DSL can support multiple data and voice channels over a single line • DSL uses advanced data modulation techniques

49. DSL (continued) • Types of DSL • The term xDSL refers to all DSL varieties, of which at least eight currently exist • DSL types can be divided into two categories: asymmetrical and symmetrical • The term downstream refers to data traveling from the carrier’s switching facility to the customer • Upstream refers to data traveling from the customer to the carrier’s switching facility