WAN Chapter 2

• sends one data bit at a time across one wire
• parallel communication sends multiple bits over mre wires simultaneously

• Clock Skew
• Cost
• Interference

• 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

• Serial communications use fewer wires, cheaper cables, and fewer connector pins
• Serial communications are considerably cheaper to implement than parallel communication

• 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

Due to problems with parallel communication, almost all WAN communications use serial connections

• 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

• 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

• 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
• 

• 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

• 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

• 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