Serial communication sent one data bit at a time across one wire
Parallel communication sends multiple bits over more wires simultaneously
Problems with Parallel Connection
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
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
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
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.
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.
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.
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