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Serial Communication
- sends one data bit at a time across one wire
- parallel communication sends multiple bits over mre wires simultaneously
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Problems with Paralle Communitcation
- Clock Skew
- Cost
- Interference
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Clock Skew
- In a parallel connection, bits may not arrive at the same time(clock skew)
- The recieving end must synchronize itself with the transmitter and then wait until the bits have arrived
- Clock skew slows parallel transmission well below theoretical speed
- Problem with clock skew increases with more parallel wires and longer distances
- Serial connection is however, not affected by clock skew because most serial links do not need clocking
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Cost
- Serial communications use fewer wires, cheaper cables, and fewer connector pins
- Serial communications are considerably cheaper to implement than parallel communication
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Interference
- Parallel wires are physically bundled in a parallel cable, and signals can imprint themselves on each other
- At higher frequencies, crosstalk may cause bytes to be dropped
- Since serial cables have fewer wires, there is less crosstalk, and network devices transmit serial communications at higher, more efficient frequencies
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Serial communication
Due to problems with parallel communication, almost all WAN communications use serial connections
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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(TMD) to maximize the amount of voice traffic carried over a medium
- TDM divides the bandwidth of a single link into seperate 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 mediu by assigning time slots to users
- 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
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Statistical TDM (STDM)
- In TDM, 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 overcame 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
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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
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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 modern 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
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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 encapsultion 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 PPP 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
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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
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