WANs and Remote Connectivity

1
Chapter 7

• WANs and Remote Connectivity

2
WAN Essentials

• A WAN traverses a large geographic area
• A WAN link is a connection from one site to
  another and is described as point-to-point
• Carry primarily digital signals
• Uses different transmission systems, topologies
  and media
• Data usually sent over public communication links
• Data lines may be leased to provide a dedicated
  line
• Typically use a mesh or partial mesh topology
• See examples where a WAN is needed p. 269

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WAN Transmission Methods

• PSTN
• ISDN
• xDSL
• Cable

4
Public Switched Telephone Network (PSTN)

• Refer to regular telephone lines or POTS
• Most now use digital transmission through fiber
  optics, twisted pair, and microwave
• Most dial-up connections use this medium
• A dial-up connection needs a modem that does
  conversion from digital to analog and vice versa
• You cannot be guaranteed the quality of the
  connection (corruption of data during
  transmission, speed of transmission)
• Most significant limiting factor is its capacity
  (lt 56 Kbps)

5
PSTN Continued

• See Figure 7-2, p. 272
• PSTN uses circuit switching - uses a dedicated
  point-to-point connection
• Not very secure there are many points where
  transmission can be intercepted
• Data can be encrypted to make it more secure

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Integrated Services Digital Network (ISDN)

• An international standard for transmitting data
  using digital signals
• Uses the public telephone lines and dial-up
  connections
• Differs from PSTN in that it relies exclusively
  on digital connections and that it can carry
  data and voice simultaneously
• Began being offered in the mid 1980s
• Has not caught on as quickly as predicted
• There are other competing transmission methods

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ISDN Continued

• Two types of channels
• B channel circuit switched channel that carries
  data or voice. Max of 56Kbps
• D channel packet switched channel that carries
  information about the call. Max of 16Kbps. Only
  one D channel is used
• Basic Rate ISDN uses 2 B channels and 1 D
  channel. Max transmission rate of 128 Kbps
• Primary Rate ISDN uses 23 B channels and 1 D
  channel. Max transmission rate of 1.544 Mbps

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Digital Subscriber Lines (xDSL)

• A new transmission technology that competes with
  ISDN
• Uses advanced modulation techniques over regular
  phone lines
• To support high throughput the phone company must
  have modern switching equipment
• Provides data and voice over a single line
• DSL uses a connection that is dedicated (you
  dont have to dial a number like you do with
  ISDN)
• The x in xDSL refers to the particular version, a
  number of which are available

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xDSL Continued

• Downstream data traveling from the carriers
  point of presence (POP) to the customer
• Upstream data traveling from customer to
  carriers POP.
• A technology that offers more throughput in one
  direction than another is said to be asymmetrical
• The types of DSL vary in terms of their symmetry,
  line capacity, and maximum line length
• Typical speeds of 1.544 Mbps and maximum distance
  of 18,000 feet
• See Table 7-1, p. 277

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Cable

• Uses existing coaxial cable for TV
• Asymmetric technology
• 3-10 Mbps downstream and 2Mbps upstream due to
  its shared nature
• Cable modem required that connects to NIC
• Provides a dedicated connection that does not
  require dialing
• Issue with security and speed (See p. 278)

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T-Carriers

• Permanent dedicated lines that are leased from a
  public telecommunications company
• Use time division multiplexing to divide a single
  channel into multiple channels
• T1 line has 24 channels, each capable of 64Kbps
  throughput for a total of 1.544 Mbps
• Devices called multiplexers are needed at the
  sending and receiving ends
• T3 line has 672 channels, each capable of 64Kbps
  for a total throughput of 44.736 Mbps

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T1 Line

• Commonly used by businesses to connect sites that
  are far apart
• T1 costs as much as 4000 to install plus an
  additional 1000-2000 per month usage fee. The
  longer the run of the T1 line, the higher the fee
• Some applications require multiple T1 lines for
  added throughput
• Preferable to use shielded twisted pair
• Multiple T1 lines require coaxial cable or fiber
  optics

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T3 Line

• Very expensive - 8,000 to install
• Monthly fee depends upon average throughput. If
  you are using most of the bandwidth of the line,
  the monthly fee can be in the tens of thousands
  of dollars
• Used by the most data-intensive businesses
• Require coaxial cable or fiber optics

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T-Carrier Connectivity Devices

• Every T-carrier requires connectivity devices at
  both ends of the T line
• The cost of these devices is not included in
  above approximate installation fees
• CSU/DSU needed on each end of the T connection
• CSU provides termination of the digital signal
  and handles error correction and line monitoring
• DSU converts digital signal from local computer
  network into digital signal appropriate for
  transmission over T connection
• See Figure 7-7, p. 282 for CSU/DSU layout

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FDDI Standard

• Originally specified by ANSI in the mid-1980 and
  later refined by ISO
• Token ring standard
• Uses a dual fiber-optics ring that operates at
  100Mbps
• Was the first technology to support the 100 Mbps
  rate
• Used to support network backbones
• Can span distances as far as 62 miles
• More expensive than fast Ethernet
• Second ring acts as backup

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X.25 and Frame Relay

• X.25 is an analog, packet switched technology
  standardized in the mid 1970s for long-distance
  data transmission
• Frame Relay is an updated digital version of X.25
  using packet switching
• Frame Relay offers a maximum of 1.544 Mbps
  throughput
• When you lease as X.25 or Frame Relay from your
  local carrier, you do not get a dedicated line
• You are just guaranteed a minimum bandwidth for
  use

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ATM

• A new WAN transmission method that is being used
  increasingly in network backbones and WANS and
  uses packet switching
• Achieves transfer rates of 25-622 Mbps but
  depends upon the type of T-connector used
• A fixed packets size, called a cell, consists of
  48 bytes of data plus a 5-byte header
• The ATM cell is a fraction of the size of an
  Ethernet frame
• Uses digital signals over fiber-optics
• Very expensive and lack of well-defined standard

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SONET

• Similar to FDDI in that two rings are used
• Uses T carriers
• Interoperates well with ISDN and ATM technology
• Used to link WANs between N. America, Europe, and
  Asia because it can work with different standards
  used in these countries
• Very expensive

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Establishing Remote Connectivity

• Three methods for accessing a network from a
  remote location
• Direct dial to LAN Requires a dial-in server on
  your LAN. Most complex method to configure, but
  it can provide the best security
• Direct dial to a workstation Remote user dials
  into a workstation directly attached to the LAN.
  Only one connection to the LAN at a time is
  allowed
• Web interface Files are made available through
  Web server software. Security cannot be
  controlled as well as with other methods. Any
  number of remote users can access the LAN at the
  same time.