Chapter 7: WANs and Remote Connectivity

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

Objectives (continued)

Assess WAN implementation options based on speed, security, and reliability

Understand the hardware and software requirements for remotely connecting to a network

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

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

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)

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)

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

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

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


• 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


• 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


• 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


• 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


• 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


• 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


• 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

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

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

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

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

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

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)

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

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

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

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

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

Network + 41

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


• 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


• 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


• 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


• 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

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

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

Network + 50

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

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

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

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

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

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

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

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

Network + 62

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

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

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)

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

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


• 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


• 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


• 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

• 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


• 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

Virtual Private Networks (VPN)

• 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

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

Virtual Private Networks (VPN) (continued)

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

Summary (continued)

• WAN implementation options based on speed, security, and reliability

• Hardware and software requirements for remotely connecting to a network

Chapter 7: WANs and Remote Connectivity

Documents

Transcript of Chapter 7: WANs and Remote Connectivity