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BT0072 Computer Networks -1


Write about ISO OSI model.

The ISO (International Standards Organization) has created a layered model called the OSI (Open Systems Interconnect) model to describe defined layers in a network operating system. The purpose of the layers is to provide clearly defined functions to improve internetwork connectivity between “computer” manufacturing companies. Each layer has a standard defined input and a standard defined output. Understanding the function of each layer is instrumental in understanding data communication within networks whether Local, Metropolitan or Wide.

There are 7 Layers of the OSI model:

7. Application Layer (Top Layer)

6. Presentation Layer

5. Session Layer

4. Transport Layer

3. Network Layer

2. Data Link Layer

1. Physical Layer (Bottom Layer

1 Application layer:

This layer provides a means for the user to access information on the network through an application. Many user applications that need to communicate over the network interact with the Application layer protocol directly. The user applications are not part of OSI Application layer, use the networking services offered by the networking protocol suite. Application layer functions typically include identifying communication partners, and determining availability of required resources. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

2 Presentation layer:

Presentation layer converts local host computer data representations into a standard network format for transmission on the network. On the receiving side, it changes the network format into the appropriate host computer’s format so that data can be utilized independent of the host computer. ASCII and EBCDIC conversions, cryptography, and the like are handled here.
Examples of Presentation layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes.

Presentation layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding.

3. Session layer:

The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session-layer implementations include AppleTalk’s Zone Information Protocol (ZIP), and Decent Phase Session Control Protocol (SCP).

4.Transport layer:

Transport layer is responsible for providing reliable service between the hosts. Upper layer datagrams are broken down into manageable datagrams and then appropriate header information (such as sequence number, port number, etc.) is added to the datagram before passing it on to the Network layer. Two frequently used transport protocols are the TCP (Transmission Control Protocol) and the UDP (User Datagram Protocol).

Important features of Transport layer:

Transport layer ensures reliable service.

Breaks the message (from sessions layer) into smaller datagrams, and appends appropriate unit header information.

Responsible for communicating with the Session layer

Important features of TCP/UDP:

TCP/IP widely used protocol for Transport/Network layers

TCP: (Transport Control Protocol) TCP ensures that a packet has reached its intended destination by using an acknowledgement. If not, it retransmits the lost messages. Hence, TCP is called a connection oriented protocol.

UDP (Universal Data gram Protocol): UDP simply transmits packets over the internet. It does not wait for an acknowledgement. It is the responsibility of upper layer protocols to ensure that the information had reached the intended partner(s). Hence, UDP is often called connectionless protocol.

Application programs that do not need connection-oriented protocol generally use UDP.

5. Network layer:

Network layer is responsible for the routing of packets through the entire network. The layer uses logical addressing for this purpose. Note that the physical address (like MAC address) keeps changing from hop to hop when a packet travels from source to destination. As a result, an address that doesn’t change is required to ensure continuity between hops. This is nothing but logical address. For IP networks, IP address is the logical address; and for Novell network, IPX address is the logical address, and so on. This layer also provides for congestion control, and accounting information for the network. IP (Internet Protocol) is an example of a network layer protocol.

6. Data link layer:

Data link layer provides delivery of information frames between communicating partners. This layer is responsible for flow regulation, error detection and correction, and framing of bits for transmission. The network data frame is made up of checksum, source address, destination address, and the data itself. The largest frame size that can be sent is known as the maximum transmission Unit (MTU).

Important features of Data link layer:

Assembles bits into frames, making them ready for transmission over the network.

Provides error detection, and correction to transmitted frames. If the checksum is not correct, it asks for retransmission. (Send a control message).

Consists of two sub layers:

Logical Link Control (LLC): Defines how data is transferred over the cable and provides data link service to the higher layers.

Medium Access Control (MAC): Controls media access by regulating the communicating nodes using pre-defined set of rules. (i.e. Token passing, Ethernet [CSMA/CD] all have MAC sub-layer protocol).

Different Data link layer protocols define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to logical addressing) defines how devices are addressed at the data link layer. The protocols used in Data link layer are SLIP, PPP, and CSLP.

7. Physical layer:

This is the bottom-most layer of the OSI model. The Physical layer handles the bit-level communications across the physical medium. The physical medium could be made up of wired electrical signals, or light, or radio (wireless) signals. Physical layer specifications define characteristics such as media, data rates, maximum transmission distances, and physical connectors.

Some of the important standards that deal with physical layer specifications are:

RS-232(for serial communication lines), X.21, EIA 232, and G730.

Physical layer and Data link layer implementations can be categorized as either LAN or WAN specifications.

2.Describe the architecture and usage of ISDN.

Integrated Services Digital Network (ISDN) is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network.

ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kilobit/s. A major market application for ISDN in some countries is Internet access, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions. ISDN B-channels can be bonded to achieve a greater data rate, typically 3 or 4 BRIs (6 to 8 64 kbit/s channels) are bonded.

ISDN should not be mistaken for its use with a specific protocol, such as Q.931 whereby ISDN is employed as the network, data-link and physical layers in the context of the OSI model. In a broad sense ISDN can be considered a suite of digital services existing on layers 1, 2, and 3 of the OSI model. ISDN is designed to provide access to voice and data services simultaneously.

Integrated services refers to ISDN’s ability to deliver at minimum two simultaneous connections, in any combination of data, voice, video, and fax, over a single line. Multiple devices can be attached to the line, and used as needed. That means an ISDN line can take care of most people’s complete communications needs at a much higher transmission rate, without forcing the purchase of multiple analog phone line.

The entry level interface to ISDN is the Basic(s) Rate Interface (BRI), a 128 kbit/s service delivered over a pair of standard telephone copper wires. The 144 kbit/s rate is broken down into two 64 kbit/s bearer channels (‘B’ channels) and one 16 kbit/s signaling channel (‘D’ channel or delta channel)The other ISDN service available is the Primary Rate Interface (PRI), which is carried over an E1 (2048 kbit/s) in most parts of the world. An E1 is 30 ‘B’ channels of 64 kbit/s, one ‘D’ channel of 64 kbit/s and a timing and alarm channel of 64 kbit/s. In North America PRI service is delivered on one or more T1s (sometimes referred to as 23B+D) of 1544 kbit/s (24 channels). A T1 has 23 ‘B’ channels and 1 ‘D’ channel for signalling (Japan uses a circuit called a J1, which is similar to a T1).The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-Channels can also be used to carry data, since they are nothing more than digital channels.Bharat Sanchar Nigam Limited, the State owned and largest communication service provider, offers both ISDN BRI and PRI services across the country. With the introduction of broadband technology, the load on bandwidth is being absorbed by ADSL. ISDN continues to be an important backup network for point-to-point leased line customers such as banks, Eseva Centers [1], Life Insurance Corporation of India, and SBI ATMs.

3. Discuss various LAN protocols.

Local Area Network (LAN) is a data communications network connecting terminals, computers and printers within a building or other geographically limited areas. These devices could be connected through wired cables or wireless links. Ethernet, Token Ring and Wireless LAN using IEEE 802.11 are examples of standard LAN technologies.

1.Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA/CD to physically monitor the traffic on the line at participating stations. If no transmission is taking place at the time, the particular station can transmit. If two stations attempt to transmit simultaneously, this causes a collision, which is detected by all participating stations. After a random time interval, the stations that collided attempt to transmit again. If another collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as exponential back off.

CSMA/CD is a type of contention protocol. Networks using the CSMA/CD procedure are simple to implement but do not have deterministic transmission characteristics. The CSMA/CD method is internationally standardized in IEEE 802.3 and ISO 8802.3.

1-Persistent CSMA

Sense the channel.If busy, keep listening to the channel and transmit immediately when the channel becomes idle.If idle, transmit a packet immediately.If collision occurs,Wait a random amount of time and start over again. The protocol is called 1-persistent because the host transmits with a probability of 1 whenever it finds the channel idle.

Non-Persistent CSMA

Sense the channel .If busy, wait a random amount of time and sense the channel again If idle, transmit a packet immediately if collision occurs wait a random amount of time and start all over again

Trade of B and C become ready in the middle of A’s transmission, 1-Persistent: B and C collide Non-Persistent: B and C probably do not collide If only B becomes ready in the middle of A’s transmission, 1-Persistent: B succeeds as soon as A ends Non-Persistent: B may have to wait off between 1- and Non-Persistent CSMA

P-Persistent CSMA

Optimal strategy: use P-Persistent CSMA Assume channels are slotted One slot = contention period (i.e., one round trip propagation delay) 1. Sense the channel If channel is idle, transmit a packet with probability p if a packet was transmitted, go to step 2 if a packet was not transmitted, wait one slot and go to step 1 If channel is busy, wait one slot and go to step 1. 2. Detect collisions If a collision occurs, wait a random amount of time and go to step 1 Consider p-persistent CSMA with p=0.5 When a host senses an idle channel, it will only send a packet with 50% probability If it does not send, it tries again in the next slot.

4. Explain the concept of framing in Data Link Layer and its importance in data


The Data Link Layer is concerned with local delivery of frames between devices on the same LAN. Data Link frames, as these protocol data units are called, do not cross the boundaries of a local network. Inter-network routing and global addressing are higher layer functions, allowing Data Link protocols to focus on local delivery, addressing, and media arbitration. In this way, the Data Link layer is analogous to a neighborhood traffic cop; it endeavors to arbitrate between parties contending for access to a medium.

When devices attempt to use a medium simultaneously, frame collisions occur. Data Link protocols specify how devices detect and recover from such collisions, but it does not prevent them from happening.

Delivery of frames by layer 2 devices is affected through the use of unambiguous hardware addresses. A frame’s header contains source and destination addresses that indicate which device originated the frame and which device is expected to receive and process it. In contrast to the hierarchical and routable addresses of the network layer, layer 2 addresses are flat, meaning that no part of the address can be used to identify the logical or physical group to which the address belongs.

When a frame needed to be sent, the bridge could look up the destination MAC address in the bridge table, and know which port should be sent out. The capability to send data to only the correct host was a huge advance in switching because collisions became much less likely. If the destination MAC address wasn’t found in the bridge table, the switch would simply flood it out all ports. That’s the only way to find where a host actually lives for the first time, so as you can see, flooding is an important concept in switching. It turns out to be quite necessary in routing, too.

5. Discuss the IEEE 802.11 Standard.

In 1997 the IEEE adopted IEEE Std. 802.11-1997, the first wireless LAN (WLAN) standard. This standard defines the media access control (MAC) and physical (PHY) layers for a LAN with wireless connectivity. It addresses local area networking where the connected devices communicate over the air to other devices that are within close proximity to each other. This paper provides an overview of the 802.11 architecture and the different topologies incorporated to accomodate the unique characteristics of the IEEE 802.11 wireless LAN standard.

The standard is similar in most respects to the IEEE 802.3 Ethernet standard. Specifically, the 802.11 standard addresses:

Functions required for an 802.11 compliant device to operate either in a peer-to-peer fashion or integrated with an existing wired LAN

Operation of the 802.11 device within possibly overlapping 802.11 wireless LANs and the mobility of this device between multiple wireless LANs

MAC level access control and data delivery services to allow upper layers of the 802.11 network

Several physical layer signaling techniques and interfaces

Privacy and security of user data being transferred over the wireless media

The difference between a portable and mobile station is that a portable station moves from point to point but is only used at a fixed point. Mobile stations access the LAN during movement.

When two or more stations come together to communicate with each other, they form a Basic Service Set (BSS). The minimum BSS consists of two stations. 802.11 LANs use the BSS as the standard building block. A BSS that stands alone and is not connected to a base is called an Independent Basic Service Set (IBSS) or is referred to as an Ad-Hoc Network. An ad-hoc network is a network where stations communicate only peer to peer. There is no base and no one gives permission to talk. Mostly these networks are spontaneous and can be set up rapidly. Ad-Hoc or IBSS networks are characteristically limited both temporally and spatially.

Extended Service Set (ESS)

Station Services the 802.11 standard defines services for providing functions among stations. Station services are implemented within all stations on an 802.11 WLAN (including access points). The main thrust behind station services is to provide security and data delivery services for the WLAN.

Authentication -Because wireless LANs have limited physical security to prevent unauthorized access, 802.11 defines authentication services to control access to the WLAN. The goal of authentication service is to provide access control equal to a wired LAN.
The authentication service provides a mechanism for one station to identify another station. Without this proof of identity, the station is not allowed to use the WLAN for data delivery. All 802.11 stations, whether they are part of an independent BSS or ESS network, must use the authentication service prior to communicating with another station.
IEEE 802.11 defines two types of authentication services.

6. Write about TCP/IP Protocol suite.

The Transmission Control Protocol (TCP) is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite (the other being Internet Protocol, or IP), so the entire suite is commonly referred to as TCP/IP. Whereas IP handles lower-level transmissions from computer to computer as a message makes its way across the Internet, TCP operates at a higher level, concerned only with the two end systems, for example a Web browser and a Web server. In particular, TCP provides reliable, ordered delivery of a stream of bytes from a program on one computer to another program on another computer. Besides the Web, other common applications of TCP include e-mail and file transfer. Among other management tasks, TCP controls segment size, flow control, and data exchange rate

The TCP/IP model consists of four layers (RFC 1122). From lowest to highest, these are the Link Layer, the Internet Layer, the Transport Layer, and the Application Layer

TCP provides a communication service at an intermediate level between an application program and the Internet Protocol (IP). That is, when an application program desires to send a large chunk of data across the Internet using IP, instead of breaking the data into IP-sized pieces and issuing a series of IP requests, the software can issue a single request to TCP and let TCP handle the IP details.

IP works by exchanging pieces of information called packets. A packet is a sequence of bytes and consists of a header followed by a body. The header describes the packet’s destination and, optionally, the routers to use for forwarding until it arrives at its final destination. The body contains the data IP is transmitting.

TCP is used extensively by many of the Internet’s most popular applications, including the World Wide Web (WWW), E-mail, File Transfer Protocol, Secure Shell, peer-to-peer file sharing, and some streaming media applications.

TCP consists of a set of rules: for the protocol, that are used with the Internet Protocol, and for the IP, to send data “in a form of message units” between computers over the Internet. At the same time that IP takes care of handling the actual delivery of the data, TCP takes care of keeping track of the individual units of data transmission, called segments, that a message is divided into for efficient routing through the network. For example, when an HTML file is sent from a Web server, the TCP software layer of that server divides the sequence of bytes of the file into segments and forwards them individually to the IP software layer (Internet Layer). The Internet Layer encapsulates each TCP segment into an IP packet by adding a header that includes (among other data) the destination IP address. Even though every packet has the same destination address, they can be routed on different paths through the network. When the client program on the destination computer receives them, the TCP layer (Transport Layer) reassembles the individual segments and ensures they are correctly ordered and error free as it streams them to an application.


7. Discuss various transmission and switching techniques.

Different types of switching techniques are employed to provide communication between two computers. These are : circuit switching, message switching and packet switching.
Circuit Switching
In this technique, first the complete physical connection between two computers is established and then data are transmitted from the source computer to the destination computer. That is, when a computer places a telephone call, the switching equipment within the telephone system seeks out a physical copper path all the way from sender telephone to the receiver’s telephone. The important property of this switching technique is to setup an end-to-end path (connection) between computer before any data can be sent.
Message Switching
In this technique, the source computer sends data or the message to the switching office first, which stores the data in its buffer. It then looks for a free link to another switching office and then sends the data to this office. This process is continued until the data are delivered to the destination computers. Owing to its working principle, it is also known as store and forward. That is, store first (in switching office), forward later, one jump at a time.
Packet Switching
With message switching, there is no limit on block size, in contrast, packet switching places a tight upper limit on block size. A fixed size of packet which can be transmitted across the network is specified. Another point of its difference from message switching is that data packets are stored on the disk in message switching whereas in packet switching, all the packets of fixed size are stored in main memory. This improves the performance as the access time (time taken to access a data packet) is reduced, thus, the throughput (measure of performance) of the network is improved.

Data transmission, digital transmission or digital communications is the physical transfer of data (a digital bit stream) over a point-to-point or point-to-multipoint communication channels. Examples of such channels are copper wires, optical fibres, wireless communication channels, and storage media. The data is often represented as an electro-magnetic signal, such as an electrical voltage, radiowave, microwave or infra-red signal.

While analog communications is the transfer of continuously varying information signal, digital communications is the transfer of discrete messages. The messages are either represented by a sequence of pulses by means of a line code (baseband transmission), or by a limited set of continuously varying wave forms (passband transmission), using a digital modulation method. According to the most common definition of digital signal, both baseband and passband signals representing bit-streams are considered as digital transmission, while an alternative definition only considers the baseband signal as digital, and the passband transmission as a form of digital-to-analog conversion.

Data transmitted may be digital messages originating from a data source, for example a computer or a keyboard. It may also be an analog signal such as a phone call or a video signal, digitized into a bit-stream for example using pulse-code modulation (PCM) or more advanced source coding (data compression) schemes. This source coding and decoding is carried out by codec equipment.

Asynchronous transmission uses start and stop bits to signify the beginning bit[] ASCII character would actually be transmitted using 10 bits e.g.: A “0100 0001” would become “1 0100 0001 0”. The extra one (or zero depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data is sent intermittently as opposed to in a solid stream. In the previous example the start and stop bits are in bold. The start and stop bits must be of opposite polarity. This allows the receiver to recognize when the second packet of information is being sent.

Synchronous transmission uses no start and stop bits but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signal(s) built into each component[vague]. A continual stream of data is then sent between the two nodes. Due to there being no start and stop bits the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in protocol (computing) for sending/receiving data, so some bytes could become corrupted (by losing bits)[ Ways to get around this problem include re-synchronization of the clocks and use of check digits to ensure the byte is correctly interpreted and received.

8. Discuss IEEE 802 standards for LANs.

IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks. More specifically, the IEEE 802 standards are restricted to networks carrying variable-size packets. (By contrast, in cell-based networks data is transmitted in short, uniformly sized units called cells. Isochronous networks, where data is transmitted as a steady stream of octets, or groups of octets, at regular time intervals, are also out of the scope of this standard.) The number 802 was simply the next free number IEEE could assign, though “802” is sometimes associated with the date the first meeting was held — February 1980. The services and protocols specified in IEEE 802 map to the lower two layers (Data Link and Physical) of the seven-layer OSI networking reference model. In fact, IEEE 802 splits the OSI Data Link Layer into two sub-layers named Logical Link Control (LLC) and Media Access Control (MAC) , so that the layers can be listed like this: Data link layer LLC Sublayer MAC Sublayer Physical layer

The IEEE 802 family of standards is maintained by the IEEE 802 LAN/MAN Standards Committee (LMSC). The most widely used standards are for the Ethernet family, Token Ring, Wireless LAN, Bridging and Virtual Bridged LANs. An individual Working Group provides the focus for each area.

9. Discuss the concept of Error Detection and Correction techniques.

In information theory and coding theory with applications in computer science and telecommunication, error detection and correction or error control are techniques that enable reliable delivery of digital data over unreliable communication channels. Many communication channels are subject to channel noise, and thus errors may be introduced during transmission from the source to a receiver. Error detection techniques allow detecting such errors, while error correction enables reconstruction of the original data.

The general definitions of the terms are as follows:

Error detection is the detection of errors caused by noise or other impairments during transmission from the transmitter to the receiver.[1]

Error correction is the detection of errors and reconstruction of the original, error-free data.

Error correction may generally be realized in two different ways:

Automatic repeat request (ARQ) (sometimes also referred to as backward error correction): This is an error control technique whereby an error detection scheme is combined with requests for retransmission of erroneous data. Every block of data received is checked using the error detection code used, and if the check fails, retransmission of the data is requested – this may be done repeatedly, until the data can be verified.

Forward error correction (FEC): The sender encodes the data using an error-correcting code (ECC) prior to transmission. The additional information (redundancy) added by the code is used by the receiver to recover the original data. In general, the reconstructed data is what is deemed the "most likely" original data.

ARQ and FEC may be combined, such that minor errors are corrected without retransmission, and major errors are corrected via a request for retransmission: this is called hybrid automatic repeat-request (HARQ).

A repetition code is a coding scheme that repeats the bits across a channel to achieve error-free communication. Given a stream of data to be transmitted, the data is divided into blocks of bits. Each block is transmitted some predetermined number of times. For example, to send the bit pattern "1011", the four-bit block can be repeated three times, thus producing "1011 1011 1011". However, if this twelve-bit pattern was received as "1010 1011 1011" – where the first block is unlike the other two – it can be determined that an error has occurred.

A parity bit is a bit that is added to a group of source bits to ensure that the number of set bits (i.e., bits with value 1) in the outcome is even or odd. It is a very simple scheme that can be used to detect single or any other odd number (i.e., three, five, etc.) of errors in the output. An even number of flipped bits will make the parity bit appear correct even though the data is erroneous

A checksum of a message is a modular arithmetic sum of message code words of a fixed word length (e.g., byte values). The sum may be negated by means of a one's-complement prior to transmission to detect errors resulting in all-zero messages.

A cyclic redundancy check (CRC) is a single-burst-error-detecting cyclic code and non-secure hash function designed to detect accidental changes to digital data in computer networks. It is characterized by specification of a so-called generator polynomial, which is used as the divisor in a polynomial long division over a finite field, taking the input data as the dividend, and where the remainder becomes the result.


The Internet

In a typical TCP/IP stack, error control is performed at multiple levels:

Deep-space telecommunications

Satellite broadcasting (DVB)

The demand for satellite transponder bandwidth continues to grow, fueled by the desire to deliver television (including new channels and High Definition TV) and IP data. Transponder availability and bandwidth constraints have limited this growth, because transponder capacity is determined by the selected modulation scheme and Forward error correction (FEC) rate.

Data storage

Error detection and correction codes are often used to improve the reliability of data storage media.

A "parity track" was present on the first magnetic tape data storage in 1951. The "Optimal Rectangular Code" used in group code recording tapes not only detects but also corrects single-bit errors.

10. Write about the Point to Point Protocol.

In networking, the Point-to-Point Protocol, or PPP, is a data link protocol commonly used to establish a direct connection between two networking nodes. It can provide connection authentication, transmission encryption privacy, and compression.PPP is used over many types of physical networks including serial cable, phone line, trunk line, cellular telephone, specialized radio links, and fiber optic links such as SONET. Most Internet service providers (ISPs) use PPP for customer dial-up access to the Internet. Two encapsulated forms of PPP, Point-to-Point Protocol over Ethernet (PPPoE) and Point-to-Point Protocol over ATM (PPPoA), are used by Internet Service Providers (ISPs) to connect Digital Subscriber Line (DSL) Internet service.PPP is commonly used as a data link layer protocol for connection over synchronous and asynchronous circuits, where it has largely superseded the older, non-standard Serial Line Internet Protocol (SLIP) and telephone company mandated standards (such as Link Access Protocol, Balanced (LAPB) in the X.25 protocol suite). PPP was designed to work with numerous network layer protocols, including Internet Protocol (IP), Novell's Internetwork Packet Exchange (IPX), NBF and AppleTalk.PPP is also used over broadband connections. RFC 2516 describes Point-to-Point Protocol over Ethernet (PPPoE), a method for transmitting PPP over Ethernet that is sometimes used with DSL. RFC 2364 describes Point-to-Point Protocol over ATM (PPPoA), a method for transmitting PPP over ATM Adaptation Layer 5 (AAL5), which is also sometimes used with DSL.The previous section introduced the use of LCP options to meet specific WAN connection requirements. PPP may include the following LCP options:

Authentication - Peer routers exchange authentication messages. Two authentication choices are Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). Authentication is explained in the next section.

Compression - Increases the effective throughput on PPP connections by reducing the amount of data in the frame that must travel across the link. The protocol decompresses the frame at its destination. Two compression protocols available in Cisco routers are Stacker and Predictor.

Error detection - Identifies fault conditions. The Quality and Magic Number options help ensure a reliable, loop-free data link. The Magic Number field helps in detecting links that are in a looped-back condition. Until the Magic-Number Configuration Option has been successfully negotiated, the Magic-Number must be transmitted as zero. Magic numbers are generated randomly at each end of the connection.

Multilink - Cisco IOS Release 11.1 and later supports multilink PPP. This alternative provides load balancing over the router interfaces that PPP uses. Multilink PPP (also referred to as MLPPP, MP, MPPP, MLP, or Multilink) provides a method for spreading traffic across multiple distinct PPP connections.

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