Serial PPP

Computer make use of relatively short parallel connections between interior components, but use a serial bus to convert signals for most external communications

Serial : sends information accross one wire, one data bit at a time. the 9-pin serial connector on most PCs uses two loops of wire, one in each direction for data communication, plus additional wires to control the flow of information.
Parallel : sends the bits over more wire simultaneosly. 25-pin parallel on your PC, eight data-carrying wires to carry 8 bits simultaneously.

theoritically parallel is faster 8 times than serial connections, in reality there are some problems with parallel connection such as:
Clock Skew : all the bits in parallel connections don't leave the transmitter and received at the receiver at the same time. some bits get there later. the receiver must wait until the bits have arrived and synchronize itself with the transmitter.
Cross Talk : because paralles use more wires, there is much probability of having a corrupted data through cross talks.

three key serial communication standard:
RS-232 - uses 9 pins. RJ-45 conform to the RS-232 standard.
RS-232 pins:
pin 1 - Data Carrier Detect (DCD)
pin 2 - Receive pin (RxD). used to carries data from the serial device to the computer
pin 3 - Transmit pin (TxD). used to carries data from the computer to the serial device
pin 4 - Data Terminal Ready (DTR) indicates the modem that the computer is ready
pin 5 - ground
pin 6 - Data Set Ready (DTS) indicates that the dataset is on
pin 7 - Request to Send (RTS) requests clearance to send data to a modem
pin 8 - Clear to Send (CTS) acknoledge the RTS signal of the computer.
pin 9 - Ring Indicater (RI) receipt of a telephone ring signal
V.35 - typically used for modem-to-multiplexer communication. ITU standard for high-speed, synchronous data exchange combines the bandwidth of several telephone circuits. in US V.35 is the interface standard used by most routers and DSUs that connect to T1 carriers.
HSSI - High-Speed Serial Interface (HSSI) support transmission rates up to 52Mb/s. commonly used for connecting routers on LANs with WANs over high-speed lines such as T3 lines.

TDM. slices time into smaller intervals so that the link carries the bits from multiple input sources.
T1/E1 and ISDN telephone lines are common example of synchronous TDM

Statistical time-division multiplexing (STDM). channels compete for free slot (int TDM when a channel has nothing to send then the slot is empty, wasting resources). STDM requires each transmission to carry identification information.
SONET is an example of STDM

Telco (24 Ds0 is multiplexed int one Ds1/T1)
Ds0 - 64 kb/s
Ds1 - 1.544 mb/s (24 DS0)
Ds2 - 6.312 mb/s (96 Ds0, 2T1c - T1c = 2 T1)
Ds3 - 45 Mb/s (7 T2)

DTE : end of CPE, usually a router
DCE : end of the WAN provider's side. usually a modem or CSU/DSU. the device that convert user data received from DTE to the form acceptable to the WAN. responsible for providing clocking signal.

there are times when you want to connect two DTE devices (such as routers, computers, terminals or printers) directly without the DCE. a special cable called null modem eliminates the needs of a DCE. with a null modem connection, Tx and Rx lines are crosslinked (crosslinked at the DCE cable). remember that one of the device must provide the clocking signal, one of the device must be the DCE (DCE is the device with the female interface connector).

Universal Asynchronous Receiver/Transmitter (UART) is a chip on a PC motherboard that converts the parallel stream bits to the serial stream bits and vice-versa to support communication with external device such as modem. because mose data flow in a PC is in parallel form.

cisco has introduced a smart serial cable (serial interfaces on most cisco routers). smart serial interface is smaller (thinner) which has 26-pin than the DB-60 connector.

WAN Encapsulation
on each wan connection, data is encapsulated. the encapsulation depends on the technology used for wan connection
HDLC - the default encapsulation for point-to-point, dedicated links and circuit-swicthed connections when the link uses two cisco devices.
PPP - provides router-to-router and host-to-host networkc connections over synchronous and asynchronous circuits. PPP can work with several layer 3 protocol (IP, IPX) and has some authentication mechanism (CHAP, PAP).
SLIP - Standard Line Internet Protocol. standard point-to-point protocol for serial connections using TCP/IP. SLIP has been largely replaced by PPP.
X.25 - uses LAPB (Link Access Procedure, Balanced) encapsulation.
Frame Relay - industry standard form packet switched wan connection. successor of X.25
ATM - international standard for cell relay. data is sent on fixed length. ATM takes advanages of high-speed transmission, such as E2, SONET and T3.

HDLC - bit-oriented synchronous data link layer protocol developed by the ISO (current standard for HDLC is ISO 13239). HDLC defines layer 2 framing standard that allows flow control and error control through the use of acknowledgements (sequence number).
When you want to transmit frames over synchronous or asynchronous links, you must remember that those links have no mechanism to mark the beginnings or ends of frames. HDLC uses a frame delimiter, or flag, to mark the beginning and the end of each frame.
You use Cisco HDLC as a point-to-point protocol on leased lines between two Cisco devices. If you are connecting to a non-Cisco device, use synchronous PPP.