WAN Chapter 2

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lunasanity
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167487
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WAN Chapter 2
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2012-08-26 08:17:59
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WAN Chapter
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WAN Chapter 2
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  1. Serial Communications
    • Serial communication sent one data bit at a time across one wire
    • Parallel communication sends multiple bits over more wires simultaneously
  2. Problems with Parallel Connection
    • Clock Skew
    • Cost
    • Interference
  3. Clock Skew
    • In a parallel connection, bits may not arrive at the same time(clock skew)
    • The receiving end must synchronize itself with the transmitter and then wait until all the bits have arrived
    • Clock skew slows parallel transmission well below theoretical speed
    • Problem with clock skew increases with more parallel wires and longer distance
    • Serial connection is however, not affected by clock skew because most serial links do not need clocking
  4. Cost
    • Serial communications use fewer wires, cheaper cables, and fewer connector pins
    • Serial communications are considerably cheaper to implement than parallel communication
  5. Interference
    • Parallel wires are physically bundled in a parallel cable, and signals can imprint themselves on each other 
    • At higher frequencies, crosstalk may causes bytes to be dropped
  6. Serial communication
    Due to the problems with parallel communication, almost all WAN communications uses serial connections
  7. Time Division Multiplexing(TDM)
    • Before multiplexing, each telephone call required its own physical link. This was an expensive and unscalable solution
    • Bell Laboratories invented time-division multiplexing (TDM) to maximize the amount of voice traffic carried over a medium
    • TDM divides the bandwidth of a single link into separate channels or time slots
    • TDM transmits two or more channels over the same link by allocating a different time interval (time slot) for the transmission of each channel
    • In effect, the channels take turns using the link
    • TDM shares available transmission time on a medium by assigning timeslots to usersThe multiplexer (MUX) accepts input from attached devices in a round-robin fashion and transmits the data
    • The multiplexer (MUX) accepts input from attached devices in a round-robin fashion and transmits the data
    • T1/E1 and ISDN telephone lines are common examples of synchronous TDM
  8. Statistical TDM (STDM)
    • In TDM, the the time slot is still allocated even when the device has no data to transmit
    • Hence TDM may be inefficient if the traffic is intermittent STDM overcome this inefficiency by using a variable time slot length allowing devices to compete for any free slot space.
    • This requires each transmission to carry identification information
  9. TDM Example - ISDN
    • ISDN basic rate (BRI) has three channels consisting of two 64 kb/s B-channels (B1 and B2), and a 16 kb/s D-channel
    • The TDM has nine timeslots, which are repeated in the sequence shown in the figure.
  10. Data Terminal Equipment (DTE) and  Data Communications Equipment (DCE)
    • DTE is the Customer Premises Equipment (CPE)
    • DTE is generally a router, but can also be a terminal, computer, printer, or fax machine if they connect directly to the service provider network.
    • The DCE, commonly a modem or CSU/DSU, is the device used to convert the user data from the DTE into a form acceptable to the WAN service provider transmission link.
  11. WAN Encapsulation Protocols
    • Data is encapsulated into frames before crossing the WAN link.
    • The choice of encapsulation protocol depends on the WAN technology and the communicating equipment
    • HDLC - The default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections when the link uses two Cisco devices. 
    • PPP - Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP works with several Network layer protocols, such as IP and IPX. PPP also has built-in security mechanisms such as PAP and CHAP. 
    • Serial Line Internet Protocol (SLIP) - A standard protocol for point-to-point serial connections using TCP/IP. SLIP has been largely displaced by PPP.
  12. HDLC Encapsulation
    • HDLC is a synchronous Data Link layer protocol
    • HDLC uses synchronous serial transmission to provide error-free communication between two points
    • HDLC defines a Layer 2 framing structure that allows for flow control and error control through the use of acknowledgments
    • Cisco has developed an extension to the HDLC protocol to provide multiprotocol support
    • Cisco HDLC(cHDLC) frames contain a field for identifying the network protocol being encapsulated

    • Flag - The frame always starts and ends with an 8-bit flag field. The bit pattern is 01111110. When frames are transmitted consecutively, the end flag of the first frame is used as the start flag of the next frame
    • Address - The address field contains the HDLC address of the secondary station
    • Control - T

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