1) Identify and describe the functions of each of the seven layers of the OSI (Open Systems Interconnection) reference model.

The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating networking systems. Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. Physical-layer implementations can be categorised as either LAN or WAN specifications.

The data link layer provides reliable transit of data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to networking addressing) defines how devices are addressed at the data link layer. Network topology consists of the data-link layer specifications that often define how devices are to be physically connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.

The institute of Electrical and Electronics Engineers (IEEE) has subdivided the data-link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). The Logical Link Control sublayer of the data-link layer manages communications between devices over a single link of a network. LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection orientated services used by high-layer protocols. IEEE 802.2 defines a number of fields in data-link layer frames that enable multiple higher-layer protocols to share a single physical data link. The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one another at the data link layer.

The network layer provides routing and related functions that enable multiple data links to be combined into an internetwork. This is accomplished by the logical addressing (as opposed to the physical addressing) of devices. The network layer supports both connection-orientated and connectionless service from higher-layer protocols. Network-layer protocols typically are routing protocols, but other types of protocols are implemented at the network layer as well.

Some common routing protocols include Border Gateway Protocol (BGP), an Internet interdomain routing protocol; Open Shortest Path First (OSPF), a link-state, interior gateway protocol developed for use in TCP/IP networks; and Routing Information Protocol (RIP), a distance vector, internet routing protocol that uses hop count as its metric. Routing protocols affect the network layer and how it operates BUT routing protocols are actually much higher up the OSI model (typically they are considered applications).

The transport layer implements reliable internetwork data transport services that are transparent to upper layers. Transport-layer functions typically include flow control, multiplexing, virtual circuit management, and error checking and recovery.

Flow control manages data management between devices so that the transmitting device does not send more data than the receiving device can process. Multiplexing enables data from several applications to be transmitted onto a single physical link. Virtual circuits are established, maintained, and terminated by the transport layer. Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves taking an action, such as requesting that data be retransmitted, to resolve any errors that occur.

Some transport-layer implementations include Transport Control Protocol, Name Binding Protocol, and OSI transport protocols. Transport Control Protocol (TCP) is the protocol in the TCP/IP suite that provides reliable transmission of data. Name Binding Protocol (NBP) is the protocol that associates Appletalk names with addresses. OSI transport protocols are a series of transport protocols in the OSI protocol suite.

The session layer establishes, manages, and terminates communication sessions between presentation layer entities. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are co-ordinated by protocols implemented at the session layer. Some examples of session-layer implementations include Zone Information Protocol (ZIP), the Appletalk protocol that co-ordinates the name binding process; and Session Control Protocol (SCP), the DECnet Phase IV session-layer protocol.

The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application of one system will be readable by the application of another system. Some 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.

Common data representation formats, or the use of standard image, sound, and video formats, enable the interchange of application data between different types of computer systems. Conversion schemes are used to exchange information with systems by using different text and data representations, such as EBCDIC and ASCII. Standard data compression schemes enable data that is compressed at the source device to be properly decompressed at the destination. Standard data encryption schemes enable data encrypted at the source device to be properly deciphered at the destination.

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

Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF is a standard for compressing and coding graphic images. JPEG is another compression and coding standard for graphic images, and TIFF is a standard coding format for graphic images.

The application layer is the OSI layer closest to the end user, which means that both the OSI applications layer and the user interact directly with the software application.

This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application-layer functions typically include identifying communication partners, determining resource availability, and synchronising communication.

When identifying communication partners, the application layer determines the identity and availability communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronising communication, all communication between applications requires co-operation that is managed by the application layer.

Two key types of application-layer implementations are TCP/IP applications and OSI applications. TCP/IP applications are protocols, such as Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP), that exist in the Internet Protocol suite. OSI applications are protocols, such as File Transfer Access Management (FTAM), Virtual Terminal Protocol (VTP), and Common Management Information Protocol (CMIP), that exist in the OSI suite. Ro

In general, networking protocols and the data traffic that they support can be characterised as being either connection-oriented or connectionless. In brief, connection-oriented data handling involves using a specific path that is established for the duration of a connection. Connectionless data handling involves passing data through a permanently established connection.

Connection-oriented service involves three phases: connection establishment, data transfer, and connection termination.

During the connection-establishment phase, single path between the source and the destination systems is determined. Network resources typically are reserved at the time to ensure a consistent grade of service, such as a guaranteed throughput rate.

In the data transfer phase, data is transmitted sequentially over the path that has been established. Data always arrives at the destination system in the order in which it was sent.

During the connection-termination phase, an established connection that is no longer needed is terminated. Further communication between the source and destination systems requires that a new connection be established.

Connection-oriented network service carries two significant disadvantages over connectionless, static-path selection and the static reservation of network resources. Static-path selection can create difficulty because all traffic must travel along the same static path. A failure anywhere along the path causes the connection to fail. Static reservation of network resources causes difficulty because it requires a guaranteed rate of throughput and, thus, a commitment of resources that other network users cannot share. Unless the connection uses full, uninterrupted throughput, bandwidth is not used efficiently.

Connection-oriented services, however are useful for transmitting data from applications that don’t tolerate delays and packet resequencing. Voice and video applications are typically based on connection-oriented services.

As another disadvantage, connectionless network service does not predetermine the path from the source to the destination system, nor are packet sequencing, data throughput, and other network resources guaranteed. Each packet must be completely addressed because different paths through the network may be selected for different packets, based on a variety of influences. Each packet is transmitted independently by the source system and is handled independently by intermediate network devices.

Connectionless service, however, offers two important advantages over connection-oriented service: dynamic-path selection and dynamic-bandwidth allocation. Dynamic-path selection enables traffic to be routed around network failures because paths are selected on a packet-by-packet basis. With dynamic-bandwidth allocation, bandwidth is used more efficiently because network resources are not allocated a bandwidth that they will not use.

Connectionless services are useful for transmitting data from applications that can tolerate some delay and resequencing. Data-based applications typically are based on connectionless service.

A data-link layer address uniquely identifies each physical network connection of a network device. Data-link addresses sometimes are referred as physical or hardware addresses. Data-link addresses usually exist within a flat address space and have a pre-established and typically fixed relationship to a specific device.

End systems generally have only one physical network connection, and thus have only one data-link address. Routers and other internetworking devices typically have multiple physical network connections and therefore also have multiple data-link addresses.

A network-layer address identifies an entity at the network layer of the OSI layers. Network addresses usually exist within a hierarchical address space and sometimes are called virtual or logical addresses.

The relationship between a network address and a device is logical and unfixed; it typically is based either on a physical network characteristics (the device is on a particular network segment) or on groupings that have no physical basis (the device is part of an Appletalk zone). End systems require one network-layer address for each network-layer protocol they support. (This assumes that the device has only one physical network connection.) Routers and other internetworking devices require one network-layer address per physical network connection for each network layer protocol supported. A router, for example, with three interfaces each running AppleTalk, TCP/IP, and OSI must have three network-layer addresses for each interface. The router therefore has nine network-layer interfaces.

Media Access Control (MAC) addresses consist of a subset of data-link layer addresses. MAC addresses identify network entities in LANs that implement the IEEE MAC addresses of the data-link layer. As with most data-link addresses, MAC addresses are unique for each LAN interface.

MAC addresses are 48 bits in length and are expressed as 12 hexadecimal digits. The first 6 hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus compromise the Organisational Unique Identifier (OUI). The last 6 hexadecimal digits comprise the interface serial number, or another value administered by the specific vendor. MAC addresses sometimes are called burned-in-addresses (BIAs) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the interface card initialises.

Different protocol suites use different methods for determining the MAC address of a device. The following three methods are used most often: Address Resolution Protocol (ARP) maps network addresses to MAC addresses. Hello protocol enables network devices to learn the MAC addresses of other network devices. MAC addresses are either embedded in the network-layer address or are generated by an algorithm

Address resolution is the process of mapping network addresses to the Media Access Control (MAC) addresses. This process is accomplished by using the Address Resolution Protocol (ARP), which is implemented by many protocol suites. When a network address is successfully associated with a MAC address, the network device stores the information in the ARP cache. The ARP cache enables devices to send traffic to a destination without creating ARP traffic because the MAC address of the destination is already known.

The process of address resolution differs slightly, depending on the network environment. Address resolution on a single LAN begins when End system A broadcasts an ARP request onto the LAN in an attempt to learn the MAC address of End system B. The broadcast is received and processed by all devices on the LAN, although only End system B replies to the ARP request by sending an ARP reply containing its MAC address to End System A. End System A receives the reply and saves the MAC address of End System B in its ARP cache. (The ARP cache is where network addresses are associated with MAC addresses.) Whenever End System A must communicate with End System B, it checks the ARP cache, finds the MAC address of System B, and sends the frame directly without first having to use an ARP request.

Address resolution works differently, however, when source and destination devices are attached to different LANs that are interconnected by a router. End System Y broadcasts an ARP request onto the LAN in an attempt to learn the MAC address to End System Z. The broadcast is received and processed by all devices on the LAN, including Router X, which acts as a proxy for End System Z by checking its routing table to determine that End System Z is located on a different LAN. Router X then replies to the ARP request from End system Y, sending an ARP reply containing its own MAC address as if it belonged to End System Z. End System Y receives the ARP reply and saves the MAC address of Router X in its ARP cache in the entry for End System Z. When End System Y must communicate with End System Z, it checks the ARP cache, finds the MAC address of Router X, and sends the frame directly without using ARP requests. Router X receives the traffic from End System Y and forwards it to End System Z on the other LAN.

The Hello protocol is a network-layer protocol that enables network devices to identify one another and indicate that they are still functional. When a new end system powers up, for example, it broadcasts Hello messages onto the network, Devices on the network then return Hello replies, and Hello messages are also sent at specific intervals to indicate that they are still functional. Network devices can learn the MAC addresses of other devices by examining Hello-protocol packets.

Three protocols use predictable MAC addresses. In these protocol suites, MAC addresses are predictable because the network layer either embeds the MAC address in the network-layer address or uses an algorithm to determine the MAC address. The three protocols are Xerox Network Systems (XNS), Novell Internetwork Packet Exchange (IPX), and DECnet Phase IV.

Flow control is function that prevents network congestion by ensuring that transmitting devices do not overwhelm receiving devices with data. Countless possible causes of network congestion exist. A high-speed computer, for example, may generate traffic faster than the network can transfer it, or faster than the destination device can receive and process it. The three commonly used methods for handling network congestion are buffering, transmitting source-quench messages, and windowing.

Buffering is used by network devices to temporarily store bursts of excess data in memory until they can be processed. Occasional data bursts are easily handled by buffering. Excess data bursts can exhaust memory, however, forcing the device to discard any additional datagrams that arrive.

Source-quench messages are used by receiving devices to help prevent their buffers from overflowing. The receiving devices sends source-quench messages to request that the source reduce its current rate of data transmission. First, the receiving device begins discarding received data due to overflowing buffers. Second, the receiving device begins sending source-quench messages to the transmitting device at the rate of one message for each packet dropped. The source device receives the source-quench messages and lowers the data rate until it stops receiving the messages. Finally, the source device then gradually increases the data rate as long as no further source-quench requests are received.

Windowing is a flow-control scheme in which the source device requires an acknowledgement from the destination after a certain number of packets have been transmitted. With a window size of three, the source requires an acknowledgement after sending three packets, as follows. First, the source device sends three packets to the destination device. Then, after receiving the three packets, the destination device sends an acknowledgement to the source. The source receives the acknowledgement and sends three more packets. If the destination does not receive one or more of the packets for some reason, such as overflowing buffers, it does not receive enough packets to send an acknowledgement. The source then retransmits the packets at a reduced transmission rate.

Pacing is used by protocols to insure that data is transmitted at a rate which the remote device can manage.

X.25 is an International Telecommunication Union Telecommunication Standardisation Sector (ITU-T) protocol standard for WAN communications. The X.25 standard defines how connections between user devices and network devices are established and maintained.

X.25 is designed to operate effectively regardless of the type of systems connected to the network. It is typically used in the packet switched networks (PSNs) of common carriers (the telephone companies). Subscribers are charged based on their use of the network.

The development of the X.25 standard was initiated by the common carriers in the 1970s. At that time, there was a need for WAN protocols capable of providing connectivity across public data networks (PDNs). X.25 is now administered as an international standard by the ITU-T.

X.25 network devices fall into three general categories:

Data terminal equipment (DTE) -- DTE devices are end systems that communicate across the X.25 network. They are usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers.

Data circuit-terminating equipment (DCE) -- DCE devices are special communications devices such as modems and packet switches. They provide the interface between DTE devices and a packet switching exchange (PSE), and are generally located in the carrier's facilities.

Packet switching exchange (PSE) -- PSEs are switches that compose the bulk of the carrier's network. They transfer data from one DTE device to another through the X.25 packet switched network (PSN).

The following figure shows the relationship between the three types of X.25 network devices:

The packet assembler/disassembler (PAD) is a device commonly found in X.25 networks. PADs are used when a DTE device (such as a character-mode terminal) is too simple to implement the full X.25 functionality.

The PAD is located between a DTE device and a DCE device. It performs three primary functions:

Buffering -- The PAD buffers data sent to or from the DTE device.

Packet assembly -- The PAD assembles outgoing data into packets and forwards them to the DCE device. (This includes adding an X.25 header.)

Packet disassembly -- The PAD disassembles incoming packets before forwarding the data to the DTE. (This includes removing the X.25 header.)

The following figure shows the basic operation of the PAD when receiving packets from the X.25 WAN:

The X.25 protocol suite maps to the lowest three layers of the OSI reference model. The following protocols are typically used in X.25 implementations:

• Packet Layer Protocol (PLP)

• Link Access Procedure, Balanced (LAPB)

• X.21bis and other physical layer serial interfaces (such as

EIA/TIA-232, EIA/TIA-449, EIA-530, G.703, and so forth)

The following figure maps the key X.25 protocols to the layers of the OSI reference model:

Frame Relay is a high-performance wide-area network (WAN) protocol that operates at the physical and data link layers of the Open System Interconnection (OSI) reference model.

Frame Relay was originally designed for use across Integrated Services Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well.

Frames Relay provides a data communications interface between user devices and network devices. This interface forms the basis for communication between user devices across a WAN.

Typical communication speeds for Frame Relay are between 56 Kbps and 2 Mbps (although lower and higher speeds are supported).

Frame Relay is considerably more efficient than X.25, the protocol for which it is often considered a replacement. Because it supports technological advances such as fibre-optic cabling and digital transmission, Frame Relay can eliminate time-consuming processes (such

as error correction and flow control) that are necessary when using older, less reliable WAN media and protocols.

Internationally, Frame Relay was standardised by the International Telecommunications Union - Telecommunications Sector (ITU-T). In the United States, Frame Relay is an American National Standards Institute (ANSI) standard.

The Local Management Interface (LMI) specification, developed in 1990, further extends the functionality of Frame Relay.

Devices attached to a Frame Relay WAN fall into two general categories:

Data terminal equipment (DTE) -- DTE are customer-owned end node and internetworking devices. Examples of DTE devices are terminals, personal computers, routers, and bridges.

Data circuit-terminating equipment (DCE) -- DCE are carrier-owned internetworking devices. In most cases, these are packet switches (although routers or other devices can be configured as DCE as well).

DTE and DCE devices are logical entities. That is, DTE devices initiate a communications exchange, and DCE devices respond.

The following figure shows the relationship between the two categories of devices:

Frame Relay provides connection-oriented data link layer communication. This service is implemented using virtual circuits.

A Frame Relay virtual circuit is a logical connection created between two data terminal equipment (DTE) devices across a Frame Relay packet-switched network (PSN).

Virtual circuits provide a bi-directional communications path from one DTE device to another. They are uniquely identified by a data link connection identifier (DLCI).

A virtual circuit can pass through any number of intermediate data circuit-terminating equipment (DCE) devices (switches) located within the Frame Relay PSN. A number of virtual circuits can be multiplexed into a single physical circuit for transmission across the network.

Frame Relay virtual circuits fall into two categories:

• Switched virtual circuit (SVC)

• Permanent virtual circuit (PVC)

A switched virtual circuit (SVC) is one of the two types of virtual circuits used in Frame Relay implementations. SVCs are temporary connections that are used when there is only sporadic data transfer between DTE devices across the Frame Relay network.

A communication session across an SVC consists of four operational states:

Call setup -- In this state, the virtual circuit between two Frame Relay DTE devices are established.

Data transfer -- In this state, data is being transmitted between the DTE devices over the virtual circuit.

Idle -- In this state, the connection between DTE devices is still active, but no data is being transferred.

Call termination -- In this state, the virtual circuit between DTE devices is terminated.

After the virtual circuit is terminated, the DTE devices must establish a new SVC if there is additional data to be exchanged.

A permanent virtual circuit (PVC) is one of two types of virtual circuits used in Frame Relay implementations. PVCs are permanently established connections that are used when there is frequent and consistent data transfer between DTE devices across the Frame Relay network.

Communication across a PVC does not require the call setup and termination states that are used with SVCs. PVCs are always in one of the following two operational states:

Data transfer -- In this state, data is being transmitted between the DTE devices over the virtual circuit.

Idle -- In this state, the connection between DTE devices is active, but no data is being transferred.

DTE devices can begin transferring data whenever they are ready because the circuit is permanently established.

Frame Relay virtual circuits are identified by data link connection identifiers (DLCIs). DLCI values are typically assigned by the Frame Relay service provider (for example, the Telephone Company).

Frame Relay DLCIs have local significance. That is, the values themselves are not unique in the Frame Relay WAN. Two DTE devices connected by a virtual circuit might use a different DLCI value to refer to the same connection.

The following figure shows how a single virtual circuit might be assigned a different DLCI value on each end of the connection:

Integrated Services Digital Network (ISDN) refers to a set of communication protocols proposed by telephone companies to permit telephone networks to carry data, voice, and other source material.

In general, ISDN provides a set of digital services that concurrently deliver voice, data, text, graphics, music, video, and information to end users. ISDN was developed to permit access over existing telephone systems.

ISDN services are offered by many carriers under tariff. ISDN is generally viewed as an alternative to Frame Relay and T1 wide-area telephone services (WATS).

In practical terms, ISDN has evolved into one of the leading technologies for facilitating telecommuting arrangements and internetworking small, remote offices into corporate campuses.

The following figure illustrates a conceptual ISDN environment:

ISDN is addressed by a suite of ITU-T standards, spanning the physical, data link, and network layers of the seven-layer OSI networking model:

Physical layer -- The ISDN Basic Rate Interface (BRI) physical layer specification is defined in International Telecommunication Union Telecommunication Standardisation Sector (ITU-T) I.430. The ISDN Primary Rate Interface (PRI) physical layer specification is defined in ITU-T I.431.

Data link layer -- The ISDN data link layer specification is based on Link Access Procedure on the D channel (LAPD) and is formally specified in ITU-T Q.920 and ITU-T Q.921.

Network layer -- The ISDN network layer is defined in ITU-T I.450 (also known as ITU-T Q.930) and ITU-T I.451 (also known as ITU-T Q.931). Together these two standards specify user-to-user, circuit-switched, and packet-switched connections.

ISDN applications require bandwidth. Typical ISDN applications and implementations include high-speed image applications (such as Group IV facsimile), high-speed file transfer, video conferencing, and multiple links into homes of telecommuters.

ISDN network components fall into three principal categories:

• ISDN terminal equipment

• ISDN termination devices

• ISDN reference points

ISDN specifies two basic terminal equipment types:

Terminal Equipment Type 1 (TE1) -- A TE1 is a specialised ISDN terminal, including computer equipment or telephones. It is used to connect to ISDN through a four-wire, twisted-pair digital link.

Terminal Equipment Type 2 (TE2) -- A TE2 is a non-ISDN terminal such as data terminal equipment (DTE) that predates the ISDN standards. A TE2 connects to ISDN through a terminal adapter (TA). An ISDN TA can be either a standalone device or a board inside the TE2.

ISDN specifies a type of intermediate equipment called a network termination (NT) device. NTs connect the four-wire subscriber wiring to two-wire local loops. There are three supported NT types:

NT Type 1 (NT1) device -- An NT1 device is treated as customer premises equipment (CPE) in North America, but is provided by carriers elsewhere.

NT Type 2 (NT2) device -- An NT2 device is typically found in digital private branch exchanges (PBXs). An NT2 performs Layer 2 and 3 protocol functions and concentration services.

NT Type 1/2 (NT1/2) device -- An NT1/2 device provides combined functions of separate NT1 and NT2 devices. An NT1/2 is compatible with NT1 and NT2 devices, and is used to replace separate NT1 and NT2 devices.

ISDN reference points define logical interfaces. Four reference points are defined in ISDN:

R reference point -- The R reference point defines the reference point between non-ISDN equipment and a TA.

S reference point -- The S reference point defines the reference point between user terminals and an NT2.

T reference point -- The T reference point defines the reference point between NT1 and NT2 devices.

U reference point -- The U reference point defines the reference point between NT1 devices and line-termination equipment in a carrier network. (This is only in North America, where the NT1 function is not provided by the carrier network.)

The following figure illustrates the various devices and reference points found in ISDN implementations, as well as their relationship to the ISDN networks they support:

The High-Level Data Link Control (HDLC) protocol is a popular ISO-standard, bit-oriented, link layer protocol. HDLC was derived from SDLC and specifies an encapsulation method of data on synchronous serial data links.

In general, HDLC is very similar to SDLC. However, there are several differences. HDLC supports a 32-bit checksum, does not support the loop or hub go-ahead configurations supported by SDLC, and supports three transfer modes. SDLC supports only one transfer mode.

The transfer modes supported by HDLC follow:

Normal response mode -- The normal response mode (NRM) is the transfer mode implemented with SDLC. Under NRM, secondaries cannot communicate with a primary until the primary gives permission.

Asynchronous response mode -- The asynchronous response mode (ARM) allows secondaries to initiate communication with a primary without explicit permission.

Asynchronous balanced mode -- The asynchronous balanced mode (ABM) transfer mode introduced the concept of the combined node. A combined node can act as a primary or secondary station. All ABM communication is between multiple combined nodes. Any combined stations can initiate data transmission without permission.

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of V.42.

The Point-to-Point Protocol (PPP) is generally viewed as the successor to the Serial Line IP (SLIP) protocol. PPP provides router-to-router and host-to-network connections over both synchronous and asynchronous circuits.

PPP emerged in the late 1980s in response to a lack of encapsulation protocols for the Internet that was blocking growth of serial-line access. PPP was basically created to solve remote Internet connectivity problems. PPP supports a number of network layer protocols, including

Novell IPX and DECnet.

The following figure illustrates a generalised view of a PPP environment:

PPP is defined using a number of International Organisation for Standardisation (ISO) standards:

• ISO 3309-1979 specifies the HDLC frame structure for synchronous environments.

• ISO 4335-1979 and ISO 4335-1979/Addendum 1-1979 specify control procedures.

PPP physical connections permit operation across any DTE/DCE interface, but require a duplex circuit that can operate in either asynchronous or synchronous bit-serial mode. PPP physical connection requirements do not impose any restrictions regarding transmission rate.

Examples of supported physical interfaces include EIA/TIA-232-C, EIA/TIA-422, EIA/TIA-423, and V.35.

PPP datagram transmission employs three key components to provide effective data transmission:

Encapsulation -- PPP supports the High-Level Data Link Control (HDLC) protocol to provide encapsulation.

Link Control Protocol (LCP) -- An extensible LCP is used to establish, configure, and test the data link connection.

Network Control Protocols (NCPs) -- A family of NCPs are used to establish and configure different network layer protocols.

The Synchronous Data Link Control (SDLC) protocol is a bit-synchronous data-link layer protocol developed by IBM Corp.

SDLC was developed by IBM during the mid-1970s for use in Systems Network Architecture (SNA) environments. Subsequent to the implementation of SDLC by IBM, SDLC formed the basis for numerous similar protocols, including HDLC and LAPB.

In general, bit-synchronous protocols have been successful because they are more efficient, more flexible, and in some cases faster than other technologies. SDLC is the primary SNA link layer protocol for wide-area network (WAN) links.

The following figure illustrates the relative position of SDLC links within the context of an SNA WAN environment:

SDLC was modified by the International Organisation for Standardisation (ISO) to create the High-Level Data Link Control (HDLC) protocol.

HDLC was subsequently modified by the International Telecommunication Union Telecommunication Standardisation Sector (ITU-T) to create Link Access Procedure (LAP) and then Link Access Procedure, Balanced (LAPB).

HDLC was eventually modified by the Institute of Electrical and Electronic Engineers (IEEE) to create the IEEE 802.2 specification.

SDLC supports a range of link types and topologies, including the following:

• Point-to-point and multipoint links

• Bounded and unbounded media

• Half-duplex and full-duplex transmission facilities

• Circuit- and packet-switched networks

The Link Access Procedure, Balanced (LAPB) protocol emerged as the ITU-T X.25 version of a bit-oriented data link protocol. LAPB is integrated into the X.25 protocol stack and shares the same frame format, frame types, and field functions as SDLC and HDLC.

Link Access Procedure, Balanced (LAPB) is a data link layer protocol that manages communication and packet framing between DTE and DCE devices. LAPB is a bit-oriented protocol that ensures that frames are correctly ordered and error-free.

There are three types of LAPB frames:

Information frame (I-frame) -- These frames carry upper-layer information and some control information. Their functions include sequencing, flow control, and error detection and recovery. I-frames carry send and receive sequence numbers.

Supervisory frame (S-frame) -- These frames carry control information. Their functions include requesting and suspending transmissions, reporting on status, and acknowledging the receipt of I-frames. S-frames carry only receive sequence numbers.

Unnumbered frame (U-frame) -- These frames carry control information. Their functions include link setup and disconnection, and error reporting. U-frames carry no sequence numbers.

LAPB is restricted to the ABM transfer mode. With LAPB, circuits can be established by either the data terminal equipment (DTE) or the data circuit-terminating equipment (DCE). Stations initiating calls are determined to be primaries. Responding stations are secondaries.

You configure Cisco routers from the user interface that runs on the router console or terminal. You can also configure Cisco routers using remote access. Cisco Internetwork Operating System (Cisco IOS) software provides a command interpreter called EXEC. EXEC interprets the commands you type and carries out the corresponding operations. You must log in to the router before you can enter an EXEC command.

For security purposes, the EXEC has two levels of access to commands: user mode and privileged mode.

• User mode - Typical tasks include those that check the router status.
• Privileged mode - Typical tasks include those that changes the router configuration.

Router con0 is now available

Press RETURN to get started.

User Access Verification

Password:

Router> enable

Password:

Router# disable

Router>

Router> exit

Router# clok

Translating "CLOK"

% Unknown command or computer name, or unable to find computer address

Router# cl?

clear clock

Router# clock

% Incomplete command.

Router# clock ?

set Set the time and date

Router# clock set ?

% Incomplete command.

Router# clock set ?

Current Time (hh:mm:ss)

(^) Indicate error in syntax.

(?) To reveal additional arguments to commands.

<Crtl><P> or Up arrow - Last (previous) command line

<Crtl><N> or Down arrow - More recent command recall

Router> show history - Show command buffer

Router> terminal history size number-of-lines - Set command buffer size

Router> terminal no editing - Disable advanced editing features

Router> terminal editing - Reenable advanced editing

<Tab> - Entry completion

<Crtl><Z> - Backs you out of configuration mode

Automatic scrolling of long lines.

<Crtl><A> - Move to the beginning of the command line.

<Crtl><E> - Move to the end of the command line.

<Esc><B> - Move back one word.

<Crtl><F> - Move forward one character

<Crtl><B> - Move back one character.

<Esc><F> - Move forward one word.

Internal configuration components are as follows:

RAM/DRAM – Stores routing tables, ARP cache, fast-switching cache, packet buffering (shared RAM), and packet hold queues. RAM also provides temporary and/or running memory for the router’s configuration file while the router is powered on. RAM content is lost when you power down or restart.

NVRAM – Non-volatile RAM stores the router’s backup configuration file. NVRAM content is retained when you power down or restart.

Flash – Erasable, reprogrammable ROM. Flash memory holds the operating system image and microcode. Having Flash memory allows you to update software without removing and replacing chips on the processor. Flash content is retained when you power down or restart. Multiple copies of IOS can be stored in flash memory.

ROM – Contains power on diagnostics, a bootstrap program, and operating system software. To perform software upgrades, remove and replace pluggable chips on the CPU.

Interfaces – Network connections through which packets enter and exit the router. Interfaces

are on the motherboard or on separate interface modules.

Whether accessed from the console or by a Telnet session through an auxiliary port, the router can be replaced in several modes. Each mode provides different functions:

• User EXEC mode – A "look-only" mode in which the user can view some information about the router, but cannot change anything.

• Privileged EXEC mode – Supports the debugging and testing commands, detailed examination of the router, manipulation of configuration files, and access to configuration modes.

• Setup mode – Presents an interactive prompted dialogue at the console that helps the new create a first-time basic configuration.

• Global configuration mode – Implements powerful one-line commands that perform simple configuration tasks.

• Other configuration modes – Provide more complicated multiple-line configurations.

• RXBOOT mode – A maintenance mode that can be used, among other things, to recover lost passwords.

Internetwork Operating System - Router# show version

Programs - Router# show processes CPU/show protocols

Active Configuration File - Router# sho running-config/write term

Tables and Buffers - Router# sho mem/sho stacks/sho buffers

Backup Configuration File - Router# show startup-config/sho config.

Operating Systems - Router# show flash.

Interfaces - Router# sho interfaces.

Console privileged EXEC mode - Router# enable.

Auxiliary virtual terminal port - Telnet

show version - Displays the configuration of the system hardware, the software version , the names and sources of configuration files, and the boot images.

show processes - Display information about the active processes.

show protocols - Displays the configured protocols. This command shows the status of any configured Layer 3 (network) protocol.

show mem - Shows statistics about the router’s memory, including memory free pool statistics.

show stacks - Monitors the stack use of processes and interrupt routines and displays the reason for the last system reboot.

show buffers - Provides statistics for the buffer pools on the network server.

show flash - Shows information about the Flash memory device.

show running-config (write term on Cisco IOS Release 10.3 or earlier) - Displays the active configuration file.

show startup-config (show config on Cisco IOS Release 10.3 or earlier) - Displays the backup configuration file.

show interfaces - Displays statistics for all interfaces configured on the router.

Cisco Discovery Protocol (CDP) provides a single proprietary command that enables network administrators to access a summary of the multiple protocols and address configured on other directly connected routers.

CDP runs over a data link layer connecting lower physical media and upper-network-layer protocols. Because CDP operates at this level, two or more CDP devices that support different network-layer protocols can learn about each other.

Physical media supporting the Subnetwork Access Protocol (SNAP) connect CDP devices. These can include all LANs, Frame Relay and SMDS WANs, and ATM networks.

When a Cisco device running Cisco IOS Release 10.3 and later boots up, CDP starts up by default. CDP can then automatically discover neighbouring Cisco devices running CDP, regardless of which protocol suite or suites are running.

Discovered devices extend those having TCP/IP. CDP will discover directly connected Cisco devices regardless of which protocol suite they run.

Once CDP discovers a device, it can display any of the various upper layer-layer protocol address entries used on the discovered device’s port – IPX, AppleTalk Datagram Delivery Protocol (DDP), DECnet CLNS, and others.

Each router running CDP exchanges information about any protocol entries it knows with its neighbours. The administrator can display the results of this CDP information exchange on a console connected to a router configured to run CDP on its interfaces.

The network manager uses a show command to display information about the networks directly connected to the router. Frames formed by CDP provide information about each CDP neighbour device. Value include the following:

• Enable CDP on each interface

CDP begins automatically upon a device’s system startup. The CDP function normally starts by default when a Cisco product boots up with Cisco IOS Release 10.3 or later.

Although CDP runs by default, you must explicitly enable it on the device’s interface using the command cdp enable. For example, the graphic shows the cdp enable command that you use on the E0 and S0 interfaces on the router named router A.

This command begins CDP’s dynamic discovery function on the device’s interfaces. Advertisement and discovery using CDP involves data-link frame exchanges. Only directly connected neighbours exchange CDP frames.

A router caches any information it receives from its CDP neighbours. If a subsequent CDP frame indicates that any of the information about a neighbour has changed, the router discards the older information in favour of the newer information.

Use the command show cdp interface to display the values of the CDP timers, the interface status, and the encapsulation used by the CDP for its advertisement and discovery frame transmission.

Default values for timers set the frequency between CDP updates and for ageing CDP entries. These timers are set automatically at 60 seconds and 180 seconds, respectively. If the device receives a more recent update or if this holdtime value expires, the device must discard the CDP entry.

Notice that the output from this command includes all the Layer 3 addresses present in the neighbour router B – an administrator can see the IP, CLNS, and DECnet network addresses of the targeted CDP neighbour with the single command entry on router A.

The holdtime value indicates how long ago the CDP frame with this information. The command includes abbreviated version information about router B.

CDP was designed and implemented as a very simple, low-overhead protocol. A CDP frame can be as small as 80 octets, mostly made up of the ASCII strings that represent information like that shown.

Use the command show cdp neighbors to display the CDP updates received on the local router. Notice that for each local port, the display shows the following:

• Neighbour device ID

• Local port type and number

• Decremental holdtime in seconds

• Neighbour’s device capability code
• Hardware platform of the neighbour
• Neighbour’s remote port type and number

To display this information as well as information like that from show cdp entry, the administrator uses the optional show cdp neighbors detail.

Router configuration information can be generated by several means.

The privileged EXEC configure command can be used to configure from either a virtual (remote) terminal or the console terminal, allowing you to enter changes to an existing configuration at any time. The privileged EXEC configure command can also be used to load a configuration from a network TFTP server, allowing you to maintain and store configuration information at a central site.

Configuration command summary:

• configure terminal - Configure manually from the console terminal
• configure memory - Load configuration information from NVRAM.
• copy tftp running-config - Load configuration information from a network TFTP server.
• show running-config - Display the current configuration in RAM.
• copy running-config startup-config - Store the current configuration in RAM into NVRAM.
• copy running-config tftp - Store the current configuration in RAM on a network TFTP server.
• show startup-config - Display the saved configuration, which is the contents of NVRAM.
• erase startup-config - Erase the contents of NVRAM.

A current copy of the configuration can be stored on a TFTP server. Use the copy running-config tftp command to store the current configuration in RAM on a network TFTP server.

You can configure the router by retrieving the configuration file stored on one of your network servers. To do so, complete the following tasks:

Step 1 Enter configuration mode by entering the copy tftp running-config command.

Router# configure memory

Router#

Router# erase startup-config

Router#

Router# copy running-config startup

Using 5057 out of 32768 bytes

!

enable-password san-fran

!

interface Ethernet 0

ip address 131.108.100.5 255.255.255.0

!

-- More --

These commands manage the contents of NVRAM:

• configure memory - Load configuration information from NVRAM.
• erase startup-config - Erase the contents of NVRAM
• copy running-config startup-config - Store the current configuration in RAM (the running configuration) into NVRAM (as the startup configuration).
• show startup-config - Display the saved configuration, which is the contents of NVRAM.

Router(config)# line console 0

Router(config)# login

Router(config-line)# password cisco

Router(config)# line vty 0 4

Router(config-line)# login

Router(config-line)# password cisco

Router(config)# enable-password san-fran

Router(config)# service password-encryption

(set passwords here)

Router(config)# no service password-encryption

The line console 0 command establishes password on the console terminal.

The line vty 0 4 command establishes password protection on incoming Telnet sessions.

The enable-password command restricts access to the privileged EXEC mode.

The enable-secret password from the System Configuration Dialog to setup global parameters uses a Cisco-proprietary encryption process to alter the password character string.

Password can be further protected from display through the use of the service password-encryption command. The encryption algorithm does not match the Data Encryption Standard (DES).

Router(config)# hostname Tokyo

Tokyo#

Tokyo(config)# banner motd #

Welcome to router Tokyo

#

Tokyo(config)# interface e 0

Tokyo(config-if)# description Engineering LAN, Bldg. 18

The configuration of network devices determines the network’s behaviour. To manage device configurations, you need to list and compare configuration files on running devices, store configuration files on network servers for shared access, and perform software installations and upgrades.

One of the first basic tasks is to name your router. The name of the router is considered to be the host name and is the name displayed by the system prompt. If no name is configured, the system default router name is Router. You can name the router in global configuration mode. In the example shown, the router name is set to Tokyo.

You can configure a message-of-the-day banner to be displayed on all connected terminals. This banner is displayed at login and is useful for conveying messages that affect all network users, such as impending system shutdowns. To configure this message, use the banner motd command in global configuration mode.

The startup routines for Cisco IOS software have the goal of starting router operations. The router must deliver reliable performance connecting the user networks it was configured to serve. To do this, the startup routines must:

• Make sure that the router comes up with tested hardware.
• Find and load the Cisco IOS software that the router uses for its operating system.
• Find and apply the configuration statements about router-specific attributes, protocol functions, and interface addresses.

The router will make sure that it comes up with tested hardware. When a Cisco router powers up, it performs a power-on self-test. During this self-test, the router executes diagnostics from ROM on all modules. These diagnostics verify the basic operation of the CPU, memory, and interface circuitry.

After verifying the hardware functions, the router proceeds with software initialisation. Some startup routines act as fallback operations that are able to perform the router startup should other routines be unable to do so. This flexibility allows Cisco IOS software to start up in a variety of initial situations.

After the power-up self test on the router, the following events occur when the router initialises:

1. The generic bootstrap loader executes from ROM on the CPU card. A bootstrap is a simple, pre-set operation to load instructions that in turn cause other instructions to be loaded into memory, or cause entry into other configuration modes. The term comes from the data processing concept of the system "pulling itself up by its own bootstraps."
2. The operating system source is determined from the boot field of the configuration register. If the boot field indicates a Flash or network load, boot system commands in the configuration file indicate exact location of the image.
3. The operating system image is loaded into low-addressed memory. Once loaded and operational, the operating system determines the hardware and software components and lists the results on the console terminal.
4. The saved configuration file in NVRAM is loaded into main memory and executed one line at a time. These configuration commands start routing processes, supply addresses for interfaces, set media characteristics, and so on.
1. If no valid configuration file exists in NVRAM, the operating system executes a question-driven initial configuration routine referred to as the system configuration dialogue. This special mode is also called the setup dialogue.

Setup is not intended as the mode for entering complex protocol features in the router. Use setup to bring up minimal configuration. Instead of setup, network administrators use various config-mode commands for most router configuration tasks.

Router# show start-up config (show config)*

Router# show running-config (write term)*

Router# erase startup-config (write erase)*

Router# reload

Router# setup

*Use these commands for routers running Release 10.3 or earlier

• The top two commands above display the backup and active configuration files.
• The erase startup-config command deletes the backup configuration file in NVRAM.
• The reload command reloads the router, causing it to run through the entire configuration process.
• The last command is used to enter setup mode from the privileged EXEC prompt.

One routine for initial configuration is the setup mode. The primary purpose of the setup mode is to rapidly bring up a minimal-feature configuration for any router that cannot find its configuration from some other source.

For many of the prompts in the system configuration dialog of the setup command facility, default answers appear in square brackets ([ ]) following the question. Pressing the Return key allows you to use defaults. If the system was previously configured, the defaults that appear are the currently configured values. If you are configuring the system for the first time, the factory defaults are provided. If there is no factory default, as in the case of passwords, nothing is displayed after the question mark (?).

At this point, you can choose not to continue with system configuration dialog and can exit by entering No at the prompt. To begin the initial configuration process, enter Yes. You can press Control-C to terminate the process and start over at any time. When you are using the command form of setup (Router# setup), Control-C returns you to the privileged EXEC prompt (Router#).

If a "—More—" prompt appears, press the space bar to continue.

You are prompted for global parameters at the console. You use the configuration values you have determined for your router to enter the global parameters at the prompts.

The first global parameter allows you to set the router host name. This host name will precede the Cisco IOS prompts for all configuration modes. At initial configuration, the router name default is shown between the square brackets as [Router].

Use the next global parameters shown to set various passwords used on the router.

You must enter an enable secret password. When you enter a string of password characters for the prompt to "Enter enable secret," the characters are processed by Cisco-proprietary encryption. This can enhance the security of the password string. Whenever anyone lists the contents of the router configuration file, this enable password appears as a meaningless of characters. Setup recommends, but does not require that the enable password be different form the enable secret.

You are prompted for global parameters at the console. You use the configuration values you have determined for your route to enter the global parameters at the prompts.

You are prompted for parameters for each installed interface. You use the configuration values you have determined for your interface to enter the interface parameters at the prompts.

When you complete the configuration process for all installed interfaces on your router, the setup command facility presents the configuration command script that was created.

The setup command facility asks you whether you want to use this configuration. If you answer Yes, the configuration is executed and saved to NVRAM. If you answer No, the configuration is not saved and the process begins again. There is no default for this prompt; you must answer either Yes or No.

Once you have answered Yes to this last question, your system is ready to use. If you want to modify the configuration you have just established, you must configure manually.

The script tells you to use configuration mode to modify the command after setup has been used. The script file generated by setup generated by setup is additive; you can turn on features with setup, but you cannot turn them off. Also setup does not support many of the advanced features of the router or those features that require a more complex configuration.

Router# configure terminal

Router(config)# boot system flash IOS_filename

Router(config)# boot system tftp IOS_filename tftp_address

Router(config)# boot system rom

[Ctrl-Z]

Router(config)# copy running-config startup-config

• Configure register bits 3, 2, 1, and 0 set boot option

commands (0x2 default if router has flash)

• Check configuration register setting with show version

Writing C4500-I !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Router#

• Back up a current files prior to updating Flash

• Note message that image already exists

You can compare distance-vector routing to link-state routing in several key areas:

Class A: N.H.H.H

Class B: N.N.H.H

Class C: N.N.N.H

Note Class D and E addresses are also defined. Class D addresses start at 224.0.0.0 and are used for multicast purposes. Class E addresses start at 240.0.0.0 and are used for experimental purposes.

Note 127.0.0.1 is reserved for loopback address.

An IP address is 32 bits in size, written as four octets. The subnet mask is 32 bits in size, written as four octets. The layout of the subnet mask field is as follows:

The router extracts the IP destination address from the packet and retrieves the internal subnet mask.

172 16 2 0

• Network number extended by eight bits

2 255.255.192.0 2 16,382

2 255.255.255.192 2 62

• Assigns an address and subnet mask
• Starts IP processing on an interface

• Sets format of network mask as seen in show commands

• Defines static host name to IP address mapping

• Hosts/interfaces selectable by name or IP address

Note: trace is supported by IP, CLNS, VINES, and AppleTalk.

The router should automatically enter setup mode. If the router# prompt appears, enter the setup command.

The RIP protocol was originally specified in RFC 1058

network Command Description

Note Multiple encapsulation can be specified on an interface, but only if multiple network numbers have also been assigned. Although several encapsulation types can share the same interface, clients and servers with different encapsulation types cannot communicate directly with each other.

• Specify encapsulations when you configure IPX network.

• Configures round-robin load sharing over multiple equal metric paths

Command Description