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After reading this you should be able to

  • Identify a variety of uses for WANs
  • Explain different WAN topologies, including their advantages and disadvantages
  • Compare the characteristics of WAN technologies, including their switching type, throughput, media, security, and reliability
  • Describe several WAN transmission and connection methods, including PSTN, ISDN, T-carriers, DSL, broadband cable, ATM and SONET
  • Describe multiple methods for remotely connecting to a network

WAN Essentials

  • A WAN is a network that traverses some distance and usually connects LANs
  • The internet is an example of a WAN (although not a typical one)
  • For every business need, a few (or possibly only one appropriate) WAN connection  type exists
  • LANs and WANs have several fundamental properties in common – typically from Layer 3 of the OSI model and higher
  • LANs and WANs typically differ at Layers 1 and 2 of the ODI model in access methods, topologies and sometimes media
  • a WAN ink is a connection between one WAN site and another site
  • Most WAN links are point-to-point

WAN Topologies

  • WAN topologies resemble LAN topologies but their details differ because of the distance they cover
  • The following sections describes different WAN topologies and special considerations for using each

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
  • Each site depends on every other site in the network to transmit and receive its traffic
  • LANs use computers with shared access to one cable, whereas the WAN bus topology uses different locations, each one connected to another one through point-to-point links
  • A bus topology WAN is often the best option for organizations with only few sites and the capability to use dedicated circuits (T1, DSL & ISDN connections)
  • BUS topology model does not scale well so only suitable for small WANs
  • A single failure on a bus topology WAN can take down communications between all sites

Ring

  • Each site is connected to two other sites so that the entire WAN forms a ring pattern]
  • Difference between LAN & WAN ring topologies is with the WAN it connects locations rather than local nodes
  • On most modern WANs, a ring topology WAN has redundant rings to carry data thus it cannot be taken down by the loss of one site
  • Expanding ring configured WANs can be difficult, and it is more expensive than expanding bus topology WANs
  • WANs that use ring topology are only practical for connecting fewer than four or five locations

Star

  • Star topology WANs mimic the arrangement of a star topology LAN
  • If a single connection fails, only one location loses WAN access
  • Extending a star WAN is relatively simple and less costly than extending a bus or ring topology WAN
  • The greatest drawback of a star WAN is that failure at the central connection point can bring down the entire WAN

Mesh

  • A mesh topology WAN incorporates many directly interconnected sites
  • 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
  • The major drawback of a full mesh WAN is cost
  • A more affordable solution is a partial mesh WAN

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 grouping
  • Variations on this topology abound, and flexibility makes the tiered approach quite practical
  • The enormous flexibility of the tiered topology requires careful consideration of geography, usage patterns, and growth potential
  • Tiered systems allow for easy expansion and inclusion of redundant links to support growth

PSTN (Public Switched Telephone Network)

  • Refers to the network of lines and carrier equipment that provides telephone service to most homes and businesses
  • It is sometimes also called POTS (Plain old Telephone Service)
  • Originally PSTN carried only analog traffic, today PSTN uses digital transmission
  • The portion of the PSTN that connect any residence or business to the nearest CO is known as the local loop (or the last mile)
  • The local loop is the portion of the PSTN most likely still to consist of copper wire
  • The advantages to using the PSTN for an internet connection is ubiquity, ease of use and low cost (phone lines are accessible nearly everywhere and affordable)
  • PSTN offers only marginal security because of the many points it can be intercepted
  • PSTN is not limited to servicing workstation dial-up WAN connections, there are more sophisticated WAN technologies that also rely on the public telephone network

X.25 and Frame Relay

X.25

  • X.25 is an analog packet switched technology designed for long distance data transmission
  • Original standard specified a maximum of 64-Kbps throughput but has been updated to include maximum throughput of 2.048 Mbps
  • Originally developed as a more reliable alternative to the voice telephone system for connecting mainframe computers and remote terminals – later adopted for connecting  clients and servers over WANS
  • Specifies protocols at the Physical, Data Link, and Network Layers of the OSI model
  • Provides excellent flow control and ensures data reliability over long distances by verifying transmission at every node
  • The verification method makes it comparatively slow and unsuitable for time sensitive applications
  • Not popular in the US, but popular elsewhere and for a long time the dominant packet-switching technology used on WANs around the world

Frame Relay

  • An updated digital version of X.25 that also relies on packet switching
  • protocols operate at the Data Link layer
  • 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
  • Important difference between frame relay and X.25 is that frame relay does not guarantee reliable delivery of data, frame relay simply checks for errors and leaves error correct up to higher level layer
  • Offers throughputs between 64 Kbps and 45 Mbps

Both Frame Relay and X.25

  • Rely on virtual circuits which are connection between network nodes that although based on potentially disparate links, logically appear to be direct, dedicated links between those nodes
  • One advantage of virtual circuits is their configurable use of limited bandwidth which can make them more efficient
  • A virtual circuit uses a channel only when it needs to transmit data – leaving the channel available for use by other virtual circuits
  • X.25 & Frame Relay may be configured as SVCs (switched virtual circuits) or PVCs (permanent virtual circuits)
  • SVCs are connections that are established when parties need to transmit, then terminated after the transmission is complete
  • PVCs are connections that are established before data needs to be transmitted and maintained after the transmission is complete
  • Frame relays & X.25 are a fairly old technology an have been replaced by newer technologies in most of the world

PVCs are not dedicated, individual links. When leasing X.25 or frame relay circuit from your local carrier, your contract reflects the endpoints you specify and the amount of bandwidth you require between those endpoints. The service provider guarantee's a minimum amount of bandwidth called the CIR (committed information rate). PVC links are best suited to frequent and consistent data transmission. The advantage of leasing a frame relay circuit over leasing a dedicated service is that you pay for only the amount of bandwidth required.

ISDN (Integrated Services Digital Network)

  • An international standard
  • Specifies protocols at the Physical, Data Link, and Transport layers of the OSI model
  • Handle signaling, framing, connection setup and termination, routing, flow controls, and error detection and correction
  • Relies on PSTN for transmission medium
  • Connections can be either dialup or dedicated
  • ISDN can simultaneously carry as many as two voice calls and one data connection on a single line
  • All ISDN connections are based on two types of channels: B channels and D channels.
  • B channel is the “bearer” channel, employing circuit switching techniques to carry voice, video, and audio
  • A single B channel has a maximum throughput 64 Kbps depending on the type of ISDN connection
  • The number of B channels in a single ISDN connection may vary
  • The D channel is the data channel
  • Each ISDN uses only one D channel with a maximum throughput of either 16 or 64 Kbps depending on the type of ISDN connection
  • You get two types of ISDN, PRI (Primary Rate Interface) and BRI (Basic Rate Interface) with BRI being the more common one for home use
  • ISDN has a limit of 18000 linear feat before a repeater is required thus it is only feasible for the local loop portion of the WAN link

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 TDM (time division multiplexing) over two wire pairs (one for transmitting and one for receiving) to divide a single channel into multiple channels
  • Multiplexing allows a single T1 circuit to carry 24 channels, each capable of 64 Kbps thus a T1 connection has a maximum capacity of 1.544 Mbps
  • Each channel may carry data, voice, or video signals
  • T-carrier medium for signaling can be ordinary telephone wire, fiber-optic cable, or wireless links

Types of T-Carriers

  • A number of T-carrier varieties are available including T1’2, T2’s, T3’s etc.
  • The speed of a T-carrier depends on its signal level
  • DS0 (digital signal, level 0) is the equivalent of one data or voice channel – all other signal levels are multiples of DS0

T-Carrier Connectivity

  • 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
  • T1 technology can use UTP or STO copper wiring (plain telephone wire, coax, microwave or fiber-optic)
  • The faster the connection, the better the wiring (i.e. a T3 connection needs fiber-optic cabling or microwave)
  • Smart jacks are required at the terminating point. They also function as a monitoring point to the connection
  • CSU/DSU (Channel Service Unit / Data Service Unit) – although separate devices they are typically combined on a single electronic card.
  • CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring
  • DSU converts the T-carrier frames into frames the LAN can interpret and vice versa
  • A DSU usually incorporates a multiplexer
  • Terminal Equipment consists of switches, routers, or bridges. the router switch accepts incoming signals from a CSU/DSU and, if necessary, translates Network layer protocols, then directs data to its destination exactly as it does on any LAN.

DSL (Digital Subscriber Line)

  • Operates over the PSTN and competes directly with ISDN & T1 services
  • DSL can span only limited distances without the help of repeaters thus best suited to the local loop portion of a WAN link
  • Can support multiple data and voice channels over a single line
  • A DSL connection may use a modulation technique based on amplitude or phase modulation
  • The details of DSL modulation are beyond the scope of this course
  • The type of modulation used by a DSL version affect its throughput and the distance it signals can travel before requiring a repeater

Types of DSL

  • xDSL refers to all DSL varieties – at least 8 currently exist
  • ADSL (Asymmetric DSL)
  • G.Lite (a version of ADSL)
  • HDSL (High Bit-Rate DSL)
  • SDSL (Symmetric or Single line DSL)
  • VDSL (Very high bit rate DSL)
  • SHDSL (Single line high bit rate DSL)

DSL varieties can be broken into two categories

  1. Asymmetrical – offers more throughput in one direction than in the other (typically faster downstream than upstream connection)
  2. Symmetrical – offers equal capacity for data traveling upstream and downstream

Broadband Cable

  • Broadband cable or cable modem access is based on the coaxial cable wiring used for TV signals
  • Broadband is asymmetrical in nature
  • Operates at the Physical and Data Link layer of the OSI model

ATM (Asynchronous Transfer Mode)

  • ATM is a third WAN technology that functions in the Data Link layer.
  • In asynchronous communications, a node can transmit at any instant, and the destination node must accept the transmission as it comes
  • ATM may run over fiber optic cable, Cat 5 or higher UTP or STP cable
  • ATM specifies Data Link layer framing techniques at a fixed packet size
  • In ATM, a packet is called a cell and always consists of 48 bytes of data plus a 5 byte header (53 bytes in total)
  • On a ATM network, switches determine the optimal path between the sender and receiver, then establish this path before the network transmits data
  • ATM is a connection oriented technology
  • ATM’s throughput rivals any other described in this chapter
  • ATM is relatively expensive, and is rarely used on small LANs
  • Gigabit Ethernet has replaced ATM on many networks
  • Where ATM is still used, it’s often deployed over the popular SONET WAN technology

SONET (Synchronous Optical Network)

  • High-bandwidth WAN signaling technique
  • Specifies framing and multiplexing techniques at the Physical layer of the OSI model
  • It has 4 key strengths
  1. It can integrate many other WAN technologies
  2. It offers fast data transfer rates
  3. It allows for simple link additions and removals
  4. It provides a high degree of fault tolerance
  • The word synchronous as used in the name of this technology means that data being transmitted and received by nodes must conform to a timing scheme
  • Possibly the most important factor for SONET is that it provides interoperability
  • The data rate of a particular SONET ring is indicated by its OC (Optical Carrier) level
  • SONET technology it typically not implemented by small or medium sized businesses because of its high cost

Remote Connectivity

  • Many remote access methods exist, and they vary according to the type of transmission technology, clients, hosts, and software they can or must use

Dial-up Networking

  • Refers to dialing directly into a private network’s ISP’s remote access server to log on to a network
  • To accept client connections, the remote access server is attached to a group of modems, all of which are associated with one phone number
  • When a connection is made, the remote access server presents the remote user with a prompt for his credentials

RAS (Remote Access Server)

  • A remote client attempting to connect to a LAN or WAN requires a server to accept its connection and grant it privileges to the network’s resources
  • Many types of remote access servers exist
  • RRAS (Routing and Remote Access Service) is Microsoft’s remote access software available with Windows Server
  • RRAS enables a computer to accept multiple remote client connections over any type of transmission path
  • Remote access servers depend on several types of protocols to communicate with clients as described in the next section

Remote Access Protocols

  • To exchange data, remote access servers and clients require special protocols
  • SLIP (Serial Line Internet Protocol) and PPP (Point-to-Point Protocol) are two protocols that enable a workstation to connect to another computer using a serial connection
  • Such protocols are necessary to transport Network layer traffic over serial interfaces, which belong to the Data Link layer of the OSI model
  • Both SLIP and PPP encapsulate higher-layer networking protocols such as TCP and IP in their lower-layer data frames
  • SLIP is an earlier and much simpler version of the protocol than PPP
  • SLIP can carry only IP packets whereas PPP can carry many different types of Network layer packets
  • SLIP requires significantly more setup than PPP
  • SLIP supports only asynchronous data transmission, PPP supports both asynchronous and synchronous transmission
  • When PPP is used over an Ethernet network it is known as PPPoE (PPP over Ethernet)

Remote Virtual Computing

  • Remote virtual computing allows a user on one computer to control another computer across a network connection
  • Many types of remote virtual computing software exist including Remote Desktop, VNC or ICA

VPNs (Virtual Private Networks)

  • VPNs are wide area networks that are logically defined over public transmission systems
  • VPNs provide a way of constructing a convenient and relatively inexpensive WAN
  • Two important considerations when designing a VPN are interoperability and security
  • VPN protocols encapsulate higher-layer protocols in a process known as tunneling
  • Two major types of tunneling protocols are used on contemporary VPN’s: PPTP or L2TP
  • PPTP (Point to Point Tunneling Protocol) was developed by Microsoft that expands on PPP
  • L2TP (Layer 2 Tunneling Protocol) was developed by Cisco
Posted on Monday, January 23, 2012 6:27 AM UNISA COS 2626 Networks | Back to top


Comments on this post: Computer Networks UNISA - Chap 7 – WANS and Remote Connectivity

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Dear admin,

I can't understand the portion of comparing the characteristics of WAN technologies, including their switching type, throughput, media, security, and reliability. Pls let me know. With thanks
Left by Sandar on May 27, 2014 12:19 PM

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