Chapter 7: WANs and Remote Connectivity

Chapter 7: WANs and Remote Connectivity Network+ Guide to Networks Third Edition

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 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

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

Chapter 7: WANs and Remote Connectivity