© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public ITE I Chapter 6 1 Point-to-Point Protocol Accessing the WAN – Chapter 2 Cisco Thai Nguyen Networking Academy © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public ITE 1 Chapter 6 2 Objectives In this chapter, you will learn to: – Describe the fundamental concepts of point-to-point serial communication. – Describe key PPP concepts. – Configure PPP encapsulation. – Explain and configure PAP and CHAP authentication. Cisco Thai Nguyen Networking Academy © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public ITE 1 Chapter 6 3 How Does Serial Communication Work? Most PCs have both serial and parallel ports. Computers use of relatively short parallel connections between interior components, but use a serial bus to convert signals for most external communications. –With a serial connection, information is sent across 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. –A parallel connection sends the bits over more wires simultaneously. In the 25-pin parallel port on your PC, there are 8 data wires to carry 8 bits simultaneously. •The parallel link theoretically transfers data eight times faster than a serial connection. In reality, it is often the case that serial links can be clocked considerably faster than parallel links, and they achieve a higher data rate –Two factors that affect parallel communications: clock skew and crosstalk interference. Cisco Thai Nguyen Networking Academy © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public ITE 1 Chapter 6 4 Parallel connection: Clock Skew & Interference In a parallel connection, it is wrong to assume that the 8 bits leaving the sender at the same time arrive at the receiver at the same time. Clock Skew –Some of the bits get there later than others. This is known as clock skew. –Overcoming clock skew is not trivial. The receiving end must synchronize itself with the transmitter and then wait until all the bits have arrived. The process of reading, waiting, waiting adds time to the transmission. –This is not a factor with serial links, because most serial links do not need clocking. Interference –Parallel wires are physically bundled in a parallel cable. The possibility of crosstalk across the wires requires more processing. –Since serial cables have fewer wires, there is less crosstalk, and network devices transmit serial communications at higher, more efficient frequencies. Cisco Thai Nguyen Networking Academy © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public ITE 1 Chapter 6 5 Serial Communication Standards In a serial communication process. –Data is encapsulated by the sending router. –The frame is sent on a physical medium to the WAN. –There are various ways to traverse the WAN, –The receiving router uses the same communications protocol to de-encapsulate the frame when it arrives. There are three key serial communication standards affecting LAN-to-WAN connections: –RS-232 - Most serial ports on personal computers conform to the RS-232C standards. •Both 9-pin and 25-pin connectors are used. •It be used for device, including modems, mice, and printers. –V.35 – It is used for modem-to-multiplexer communication. •V.35 is used by routers and DSUs that connect to T1 carriers. –HSSI - A High-Speed Serial Interface (HSSI) supports transmission rates up to 52 Mb/s. •HSSI is used to connect routers on LANs with WANs over high- speed lines such as T3 lines. Cisco Thai Nguyen Networking Academy © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public ITE 1 Chapter 6 6 Serial Communication: RS-232 While this course does not examine the details of V.35 and HSSI pinning schemes, a quick look at a 9-pin RS-232 connector used to connect a PC to a modem helps illustrate the concept. –Pin 1 - Data Carrier Detect (DCD) indicates that the carrier for the transmit data is ON. –Pin 2 - The receive pin (RXD) carries data from the serial device to the computer. –Pin 3 - The transmit pin (TxD) carries data from the computer to the serial device. –Pin 4 - Data Terminal Ready (DTR) indicates to the modem that the computer is ready to transmit. –Pin 5 - Ground –Pin 6 - Data Set Ready (DSR) is similar to DTR. It indicates that the Dataset is ON. –Pin 7 - The RTS pin requests clearance to send data to a modem –Pin 8 - The serial device uses the Clear to Send (CTS) pin to acknowledge the RTS signal of the computer. In most situations, RTS and CTS are constantly ON throughout the communication session. –Pin 9 - An auto answer modem uses the Ring Indicator (RI) to signal receipt of a telephone ring signal. Cisco Thai Nguyen Networking Academy © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public ITE 1 Chapter 6 7 Time Division Multiplexing (TDM) Bell Laboratories invented TDM to maximize the amount of voice traffic carried over a medium. Compare TDM to a train with 32 railroad cars. –Each car is owned by a different freight company, and every day the train leaves with the 32 cars attached. –If the companies has cargo to send, the car is loaded. –If the company has nothing to send, the car remains empty but stays on the train. –Shipping empty containers is not very efficient. –TDM shares this inefficiency when traffic is intermittent, because the time slot is still allocated even when the channel has no data to transmit. Cisco Thai Nguyen Networking Academy © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public ITE 1 Chapter 6 8 Time Division Multiplexing (TDM) 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. –TDM is a physical layer concept. It has no regard of the information that is being multiplexed. The multiplexer (MUX) accepts input from attached devices in a round-robin fashion and transmits the data in a never-ending pattern. –The MUX puts each segment into a single channel by inserting each segment into a timeslot. –A MUX at the receiving end separate data streams based only on the timing of the arrival of each bit. – A technique called bit interleaving keeps track of the sequence of the bits so that they can be efficiently reassembled into their original form upon receipt. Cisco Thai Nguyen Networking Academy © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public ITE 1 Chapter 6 9 Statistical Time Division Multiplexing Statistical time-division multiplexing (STDM) was developed to overcome this inefficiency. –STDM uses a variable time slot length allowing channels to compete for any free slot space. –It employs a buffer memory that temporarily stores the data during periods of peak traffic. –STDM does not waste high-speed line time with inactive channels using this scheme. –STDM requires each transmission to carry identification information (a channel identifier). Cisco Thai Nguyen Networking Academy © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public ITE 1 Chapter 6 10 TDM Examples - ISDN and SONET An example of a technology that uses synchronous TDM is 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. On a larger scale, the industry uses the SONET or SDH for optical transport of TDM data. –SONET, used in North America, and SDH, used elsewhere, for synchronous TDM over fiber. –SONET/SDH takes n bit streams, multiplexes them, and optically modulates the signal, sending it out using a light emitting device over fiber with a bit rate equal to (incoming bit rate) x n. Thus traffic arriving at the SONET multiplexer from four places at 2.5 Gb/s goes out as a single stream at 4 x 2.5 Gb/s, or 10 Gb/s. SDH - Synchronous Digital Hierarchy SONET - Synchronous optical networking [...]... observing –RS -23 2C is the recommended standard (RS) describing the physical interface and protocol for relatively low-speed, serial data communication between computers and related devices The DTE is the RS -23 2C interface that a computer uses to exchange data with a modem or other serial device The DCE is the RS -23 2C interface that a modem or other serial device uses in exchanging data with the computer... Receiver/Transmitter (UART) chip on the motherboard The UART is the DTE agent of your PC and communicates with the modem or other serial device, which, in accordance with the RS -23 2C standard, has a complementary interface called the DCE interface The data in your PC flows along parallel circuits, the UART chip converts the groups of bits in parallel to a serial stream of bits ITE 1 Chapter 6 © 20 06 Cisco Systems,... –This is the interface between customer-premises equipment (CPE) and network service provider equipment The demarcation point is the point in the network where the responsibility of the service provider ends The example presents an ISDN scenario –In the United States, a service provider provides the local loop into the customer premises, The customer provides the active equipment such as the channel... transmission rate other than those imposed by the particular DTE/DCE interface in use Most of the work done by PPP is at the data link and network layers by the LCP and NCPs The LCP sets up the PPP connection and its parameters The NCPs handle higher layer protocol configurations, and the LCP terminates the PPP connection ITE 1 Chapter 6 © 20 06 Cisco Systems, Inc All rights reserved Cisco Public 28 Cisco Thai... Layer The LCP sits on top of the physical layer and has a role in establishing, configuring, and testing the datalink connection The LCP establishes the point-to-point link The LCP also negotiates and sets up control options on the WAN data link, which are handled by the NCPs The LCP provides automatic configuration of the interfaces at each end, including: –Handling varying limits on packet size –Detecting... of pins and connector type –Electrical - Defines voltage levels for 0 and 1 –Functional - Specifies the functions that are performed by assigning meanings to each of the signaling lines in the interface –Procedural - Specifies the sequence for transmitting data The Serial Cables The original RS -23 2 standard only defined the connection of DTEs with DCEs, which were modems –A null modem is a communication... RS -23 2 serial cable With a null modem connection, the transmit (Tx) and receive (Rx) lines are crosslinked ITE 1 Chapter 6 © 20 06 Cisco Systems, Inc All rights reserved Cisco Public 14 Cisco Thai Nguyen Networking Academy DTE and DCE The DB-60 Connector The cable for the DTE to DCE connection is a shielded serial cable The router end of the serial cable may be a DB-60 connector The other end of the. .. Public 12 Cisco Thai Nguyen Networking Academy DTE and DCE In order to be connecting to the WAN, a serial connection has a DTE device at one end of the connection and a DCE device at the other end The DTE, which is generally a router The DTE could also be a terminal, computer, printer, or fax machine The DCE, commonly a modem or CSU/DSU, is the device used to convert the user data from the DTE into... (CSU/DSU) on which the local loop is terminated The customer is responsible for maintaining, replacing, or repairing the equipment –In other countries, the network terminating unit (NTU) is provided and managed by the service provider The customer connects a CPE device, such as a router or frame relay access device, to the NTU using a V.35 or RS -23 2 serial interface ITE 1 Chapter 6 © 20 06 Cisco Systems,... encapsulation: –Step 1 Enter the interface configuration mode of the serial interface –Step 2 Enter the encapsulation hdlc command to specify the encapsulation protocol on the interface The output of the show interfaces serial command displays information specific to serial interfaces When HDLC is configured, "Encapsulation HDLC" ITE 1 Chapter 6 © 20 06 Cisco Systems, Inc All rights reserved Cisco Public 20 Cisco . router. The frame is sent on a physical medium to the WAN. –There are various ways to traverse the WAN, The receiving router uses the same communications protocol to de-encapsulate the frame. meanings to each of the signaling lines in the interface –Procedural - Specifies the sequence for transmitting data The Serial Cables The original RS -23 2 standard only defined the connection. There are three key serial communication standards affecting LAN-to -WAN connections: –RS -23 2 - Most serial ports on personal computers conform to the RS -23 2C standards. •Both 9-pin and 25 -pin