WAN Chapter 2

• Serial communication sent one data bit at a time across one wire
• Parallel communication sends multiple bits over more wires simultaneously

• Clock Skew
• Cost
• Interference

• 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

• 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 causes bytes to be dropped

Due to the problems with parallel communication, almost all WAN communications uses 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 (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

• 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

• 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 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.

• 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.

• 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