Point-to-Point ProtocolAccessing the WAN – Chapter 2

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Describe the fundamental concepts of point-to-point serial communication. – Describe key PPP concepts. – Configure PPP encapsulation. – Explain and configure PAP and CHAP authentication.

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Nội dung Text: Point-to-Point ProtocolAccessing the WAN – Chapter 2

Point-to-Point Protocol Accessing the WAN – Chapter 2 1 ITE I Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy 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. 2 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy 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. 3 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy 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. 4 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy 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. 5 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy 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. 6 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy 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. 7 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy 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. 8 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy 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). 9 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy 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, SDH - Synchronous Digital Hierarchy and optically modulates the signal, sending it out using a light emitting device over fiber with a bit rate equal to SONET - Synchronous optical networking (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. 10 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy TDM Examples - T-Carrier Hierarchy DS0: The original unit used in multiplexing telephone calls is 64 kb/s, which represents one phone call. T1: In North America, 24 DS0 units are multiplexed using TDM into a higher bit-rate signal with an aggregate speed of 1.544 Mb/s for transmission over T1 lines. –While it is common to refer to a 1.544 Mb/s transmission as a T1, it is more correct to refer to it as DS1. –T-carrier refers to the bundling of DS0s. –A T1 = 24 DSOs, –A T1C = 48 DSOs (or 2 T1s), and so on. E1: Outside North America, 32 DS0 units are multiplexed for E1 transmission at 2.048 Mb/s. 11 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy Demarcation Point The demarcation point marks the point where your network interfaces with the network owned by another organization. –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 service unit/data service unit (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-232 serial interface. 12 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
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 a form acceptable to the WAN service provider transmission link. •This signal is received at the remote DCE, which decodes the signal back into a sequence of bits. •The remote DCE then signals this sequence to the remote DTE. The connection between the two DCE devices is the WAN service provider transmission network. Cisco Internal T1 CSU/DSU WIC-1DSU-T1 13 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy DTE and DCE DTE and DCE Cable Standards –Originally, the concept of DCEs and DTEs was based on two types of equipment: terminal equipment that generated or received data, and communication equipment that only relayed data. –We are left with two different types of cables: •one for connecting a DTE to a DCE, •another for connecting two DTEs directly to each other. The DTE/DCE interface standard defines the following specifications: –Mechanical/physical - Number 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-232 standard only defined the connection of DTEs with DCEs, which were modems. –A null modem is a communication method to directly connect two DTEs, such as a computer, terminal, or printer, using a RS-232 serial cable. With a null modem connection, the transmit (Tx) and receive (Rx) lines are crosslinked. 14 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
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 serial transition cable is available with the connector appropriate for the standard that is to be used. The Smart Serial Connector –To support higher port densities in a smaller form factor, Cisco has introduced a Smart Serial cable. •The router interface end of the Smart Serial cable is a 26-pin connector that is significantly more compact than the DB-60 connector. The Router-to-Router –When using a null modem, keep in mind that synchronous connections require a clock signal. –When using a null modem cable in a router-to- router connection, one of the serial interfaces must be configured as the DCE end to provide the clock signal for the connection. 15 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy DTE and DCE: Parallel to Serial Conversion The terms DTE and DCE are relative with respect to what part of a network you are observing. –RS-232C 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-232C interface that a computer uses to exchange data with a modem or other serial device. •The DCE is the RS-232C interface that a modem or other serial device uses in exchanging data with the computer. Your PC also has a Universal Asynchronous 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-232C 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. 16 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy WAN Encapsulation Protocols On WAN connection, data is encapsulated into frames before crossing the WAN link. The protocol depends on the WAN technology and communicating equipment: –HDLC - The default encapsulation type on point-to-point 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 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. With SLIP, you have to know –X.25/Link Access Procedure, Balanced (LAPB) - X.25 specifies the IP address assigned to you LAPB, a data link layer protocol. by your service provider. You •X.25 is a predecessor to Frame Relay. also need to know the IP –Frame Relay - Frame Relay eliminates some of the time- address of the remote system consuming processes (such as error correction and flow control) employed in X.25. you will be dialing into. You may also need to configure such –ATM - The cell relay in which devices send multiple service types (voice, video, or data) in fixed-length (53-byte) cells. details as MTU (maximum •Fixed-length cells allow processing to occur in hardware, thereby transmission unit), MRU reducing transit delays. (maximum receive unit), etc. 17 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy HDLC Encapsulation HDLC is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). –HDLC was developed from the Synchronous Data Link Control (SDLC) standard proposed in the 1970s. –HDLC provides both connection-oriented and connectionless service. –HDLC defines a Layer 2 framing structure that allows for flow control and error control through the use of acknowledgments. –HDLC uses a frame delimiter, or flag, to mark the beginning and the end of each frame. Cisco has developed an extension to the HLDC protocol to solve the inability to provide multiprotocol support. –Cisco HLDC (also referred to as cHDLC) is proprietary –Cisco HDLC frames contain a field for identifying the network protocol being encapsulated. 18 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy HLDC Frame Types Flag - The flag field initiates and terminates error checking. –The frame always starts and ends with an 8-bit flag field. The bit pattern is 01111110. Address - The address field contains the HDLC address of the secondary station. –This address can contain a specific address, a group address, or a broadcast address. Control - HDLC defines three types of frames, each with a different control field format: –Information (I) frame: I-frames carry upper layer information and some control information. –Supervisory (S) frame: S-frames provide control information. –Unnumbered (U) frame: U-frames support control purposes and are not sequenced. Protocol - (only in Cisco HDLC) It specifies the protocol type encapsulated within the frame (e.g. 0x0800 for IP). Data - The data field contains a path information unit (PIU) or exchange identification (XID) information. Frame check sequence (FCS) - The FCS precedes the ending flag delimiter and is usually a cyclic redundancy check (CRC) calculation remainder. 19 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public
Cisco Thai Nguyen Networking Academy Configuring HDLC Encapsulation Cisco HDLC is the default encapsulation method used by Cisco devices on synchronous serial lines. –You use Cisco HDLC as a point-to-point protocol on leased lines between two Cisco devices. –If the default encapsulation method has been changed, use the encapsulation hdlc command in privileged mode to re-enable HDLC. If you are connecting to a non-Cisco device, use synchronous PPP. There are two steps to enable HDLC 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" 20 ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public