1. Chapter 7: WANs and Remote Connectivity
Network+ Guide to Networks
Third Edition
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
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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
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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
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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
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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)
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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)
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8. 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
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9. 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
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10. 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
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11. 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
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12. 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
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13. 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
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14. 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
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15. 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
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16. 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
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17. 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
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18. 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
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19. 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
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20. 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
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21. 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
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22. 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
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23. 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)
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24. 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
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25. 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
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26. 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
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27. 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
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28. 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
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29. 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 Mbps
A T3 circuit can carry the equivalent of 672 voice or data channels, giving a maximum data throughput of Mbps
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30. 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
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31. 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
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32. 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
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33. 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
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34. 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
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35. 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
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36. DSL (continued)
A technology that offers more throughput in one direction than in the other is considered asymmetrical
In asymmetrical communications, downstream throughput is higher than upstream throughput
Symmetrical technology provides equal capacity for data traveling both upstream and downstream
Symmetrical transmission is suited to users who both upload and download significant amounts of data
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37. DSL (continued) DSL Connectivity
A DSL modem is a device that modulates outgoing signals and demodulates incoming DSL signals
The DSL modem may be external to the computer and connect to a computer’s Ethernet NIC via an RJ-45,USB, or wireless interface
DSL access multiplexer (DSLAM) aggregates multiple DSL subscriber lines and connects them to a larger carrier or to the Internet backbone
DSL is not available in all areas of the United States
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38. 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
Cable modems operate at the Physical and Data Link layer of the OSI Model, and therefore do not manipulate higher-layer protocols such as IP or IPX
To provide Internet access through its network, the cable company must upgrade its existing equipment to support bidirectional, digital communications
The cable company’s network wiring must be replaced with hybrid fiber-coax (HFC), an expensive fiber-optic link that can support high frequencies
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39. Cable (continued)
Either fiber-optic or coaxial cable may connect the node to the customer’s business or residence via a connection known as a cable drop
These nodes then connect to the cable company’s central office, which is known as its head-end
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40. SONET
SONET (Synchronous Optical Network) is a high-bandwidth WAN signaling technique
SONET specifies framing and multiplexing techniques at the Physical layer of the OSI Model
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41. SONET (continued)
Its four key strengths are that it: can integrate many other WAN technologies; offers fast data transfer rates; allows for simple link additions and removals; and provides a high degree of fault tolerance
The word synchronous means that data being transmitted and received by nodes must conform to a timing scheme
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42. SONET (continued) SONET provides interoperability
SONET is often used to aggregate multiple T1s, T3s, or ISDN lines
SONET is also used as the underlying technology for ATM transmission
Internationally, SONET is known as SDH (Synchronous Digital Hierarchy)
SONET’s extraordinary fault tolerance results from its use of a double-ring topology over fiber-optic cable
Self-healing is a characteristic of dual-ring topologies that allows them to automatically reroute traffic along the backup ring if the primary ring becomes severed
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43. SONET (continued)
Most SONET multiplexers allow for easy additions or removals of connections to the SONET ring, which makes this technology easily adaptable to growing and changing networks
The data rate of a particular SONET ring is indicated by its Optical Carrier (OC) level
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44. WAN Implementation Reliability
A WAN’s reliability depends partly on the transmission medium it uses and partly on its topology and transmission methods
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
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45. WAN Implementation (continued)
Security
Fiber optic media are the most secure transmission media
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
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46. Remote Connectivity
As a remote user, you must connect to a LAN via remote access, which can be accomplished in one of several ways: dial-up networking, remote control, terminal services, Web portals, or a virtual private network (VPN)
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47. Remote Connectivity (continued)
Dial-up Networking
Dial-up networking refers to dialing into a private network’s or ISP’s remote access server to log on to a network
The remote access server (a server designed to accept incoming client connections) is attached to a group of modems, all of which are associated with one phone number
An advantage to using this remote access option are that the technology is well-understood and its software comes with virtually every operating system
Dialing into a remote access server can be slow because it relies on the PSTN
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48. Remote Connectivity (continued)
One well known program used to provide remote access is the Microsoft Routing and Remote Access Service (RRAS), which is available with the Windows Server 2003 network operating system
The Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP) are two protocols that enable a workstation to connect to another computer using a serial connection
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49. Remote Connectivity (continued)
Remote Control
Remote control allows the remote user to “take over” a computer that’s directly connected to the LAN
Remote control is not difficult to configure, but suffers from the same slow throughput as dialing into a remote access server
Another disadvantage to this solution is that it allows only one connection to the LAN at any given time
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50. Remote Connectivity (continued)
Terminal Services
In terminal services, multiple remote computers can connect to a terminal server on the LAN
A terminal server is a computer that runs specialized software that allows it to act as a host and supply applications and resource sharing to remote clients
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51. Remote Connectivity (continued)
Many companies have created software to supply terminal services
Microsoft’s version of this solution is called Terminal Services
Citrix System, Inc.’s version is Metaframe and remote workstations rely on software known as an ICA (Independent Computing Architecture) client
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52. Remote Connectivity (continued)
Web Portals
A Web portal is simply a secure, Web-based interface to an application
Any type of Internet connection is sufficient for using Web portals
On the host side, a Web server supplies the application to multiple users upon request
The use of Web portals calls for secure transmission protocols
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53. Virtual Private Networks (VPN)
Virtual private networks (VPNs) are wide area networks logically defined over public transmission systems
To allow access to only authorized users, traffic on a VPN is isolated from other traffic on the same public lines
The software required to establish VPNs is usually inexpensive, and in some cases is being included with other widely used software
RRAS allows you to create a simple VPN by turning a Windows server into an access server and allowing clients to dial into it
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54. Virtual Private Networks (VPN) (continued)
Two important considerations when designing a VPN are interoperability and security
To make sure a VPN can carry all types of data in a private manner over any kind of connection, special VPN protocols encapsulate higher-layer protocols in a process known as tunneling
A VPN tunneling protocol operates at the Data Link layer and encapsulates Network layer packets
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55. Virtual Private Networks (VPN) (continued)
Two major types of tunneling protocols are used on contemporary VPNs: PPTP or L2TP
PPTP (Point-to-Point Tunneling Protocol) is a protocol developed by Microsoft that expands on PPP by encapsulating it so that any type of PPP data can traverse the Internet masked as an IP or IPX transmission
Another VPN tunneling protocol is L2TP (Layer 2 Tunneling Protocol), based on technology developed by Cisco and standardized by the IETF
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56. Summary Network applications that require WAN technology
WAN topologies, including their advantages and disadvantages
WAN transmission and connection methods, including PSTN, ISDN, T-carriers, DSL, broadband cable, and SONET
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57. Summary (continued)
WAN implementation options based on speed, security, and reliability
Hardware and software requirements for remotely connecting to a network
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