CCNA Study Notes1(1)
添加时间: 2007-9-8 2:46:25 作者: Cisco认证考试 阅读次数:50 来源: http://www.d9soft.com
1. Identify and describe the functions of each of the seven layers of the OSI reference model.
Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and the physical connectors to be used.
Data Link Layer
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 the following:
Physical addressing -- Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer.
Network topology -- Data link layer specifications often define how devices are to be physically connected (such as in a bus or a ring topology).
Error notification -- Error notification involves alerting upper layer protocols that a transmission error has occurred.
Sequencing of frames -- Sequencing of data frames involves the reordering of frames that are transmitted out of sequence.
Flow control -- Flow control involves moderating 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).
Network Layer
The network layer provides routing and related functions that allow 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-oriented and connectionless service from higher-layer protocols.
Transport Layer
The transport layer implements reliable internetwork data transport services that are transparent to upper layers. Transport layer functions typically include the following:
Flow control -- Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process.
Multiplexing -- Multiplexing allows data from several applications to be transmitted onto a single physical link.
Virtual circuit management -- Virtual circuits are established, maintained, and terminated by the transport layer.
Error checking and recovery -- Error checking involves various mechanisms for detecting transmission errors. Error recovery involves taking an action (such as requesting that data be retransmitted) to resolve any errors that occur.
Some examples of transport layer implementations follow:
Transmission Control Protocol (TCP), Name Binding Protocol (NBP), OSI transport protocols
Session Layer
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 coordinated by protocols implemented at the session layer. Some examples of session layer implementations follow:
Apple ZIP, DEC SCP, NFS, SQL, RPC, X Windows, ASP
Presentation Layer
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 layer of one system will be readable by the application layer of another system. Some examples of presentation layer coding and conversion schemes follow:
Common data representation formats -- The use of standard image, sound, and video formats allow the interchange of application data between different types of computer systems.
Conversion of character representation formats -- Conversion schemes are used to exchange information with systems using different text and data representations (such as EBCDIC and ASCII).
Common data compression schemes -- The use of standard data compression schemes allows data that is compressed at the source device to be properly decompressed at the destination.
Common data encryption schemes -- The use of standard data encryption schemes allows data encrypted at the source device to be properly unencrypted at the destination.
Presentation layer implementations are not typically associated with a particular protocol stack. Some well known standards follow:
Data: ASCII, EBCDIC, Encryption
Visual Imaging: PICT, TIFF, GIF, JPEG
Video: MIDI, MPEG, QuickTime
Application Layer
The application layer interacts with software applications that implement a communicating component. Application layer functions typically include the following:
Identifying communication partners -- The application layer identifies and determines the availability of communication partners for an application with data to transmit.
Determining resource availability -- The application layer must determine whether sufficient network resources for the requested communication are available.
Synchronizing communication -- Communication between applications requires cooperation that is managed by the application layer.
The application layer is the OSI layer closest to the end user. That is, both the OSI application layer and the user interact directly with the software application. Some examples of application layer implementations follow:
TCP/IP applications -- TCP/IP applications are protocols in the Internet Protocol suite, such as Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
OSI applications -- OSI applications are protocols in the OSI suite such as File Transfer, Access, and Management (FTAM), Virtual Terminal Protocol (VTP), and Common Management Information Protocol (CMIP).
2. Describe connection-oriented network service and connectionless network service and identify the key differences between them.
Connection-Oriented Network Service
Connection-oriented service involves three phases:
Connection establishment -- During the connection establishment phase, a single path between the source and destination systems is determined. Network resources are typically reserved at this time to ensure a consistent grade of service (such as a guaranteed throughput rate).
Data transfer -- During 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.
Connection termination -- 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 service has two significant disadvantages as compared to connectionless network service:
Static path selection -- Because all traffic must travel along the same static path, a failure anywhere along that path causes the connection to fail.
Static reservation of network resources -- A guaranteed rate of throughput requires the commitment of resources that cannot be shared by other network users. Unless full, uninterrupted throughput is required for the communication, bandwidth is not used efficiently.
Connection-oriented services are useful for transmitting data from applications that are intolerant of delays and packet re-sequencing. Voice and video applications are typically based on connection-oriented services.
Connectionless Network Service
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 might 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 offers two important advantages over connection-oriented service:
Dynamic path selection -- Because paths are selected on a packet-by-packet basis, traffic can be routed around network failures.
Dynamic bandwidth allocation -- Bandwidth is used more efficiently because network resources are not allocated bandwidth that they are not going to use.
Connectionless services are useful for transmitting data from applications that can tolerate some delay and re-sequencing. Data-based applications are typically based on connectionless service.
3. Describe data link addresses and network addresses and identify the key differences between them.
Data Link Layer Addresses
A data link layer address uniquely identifies each physical network connection of a network device. Data link addresses are sometimes referred to 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 typically 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. They therefore have multiple data link addresses.
Network Layer Addresses
A network layer address identifies an entity at the network layer of the OSI reference model. Network addresses usually exist within a hierarchical address space. They are sometimes called virtual or logical addresses. The relationship of a network address with a device is logical and unfixed. It is typically based either on 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. For example, a router 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 addresses.
4. Define and describe the function of a MAC address.
Media Access Control (MAC) addresses are a subset of data link layer addresses. MAC addresses identify network entities in LANs implementing the IEEE MAC sublayer of the data link layer. Like 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 are the manufacturer identification (or vendor code), called the Organizational Unique Identifier (OUI). These 6 digits are administered by the IEEE. The last 6 hexadecimal digits are the interface serial number or another value administered by the specific vendor. MAC addresses are sometimes called burned-in addresses (BIAs) because they are burned into read-only memory (ROM) and copied into random-access memory (RAM) when the interface card initializes.
5. Define flow control and describe the three basic methods used in networking.
Flow control is a function that prevents network congestion by ensuring that transmitting devices do not overwhelm receiving devices with data. There are a number of possible causes of network congestion. For example, a high-speed computer might generate traffic faster than the network can transfer it, or faster than the destination device can receive and process it. There are three commonly used methods for handling network congestion:
Buffering - 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. However, excess data bursts can exhaust memory, forcing the device to discard any additional datagrams that arrive.
Source quench messages - Source quench messages are used by receiving devices to help prevent their buffers from overflowing. The receiving device sends source quench messages to request that the source reduce its current rate of data transmission, as follows:
1. The receiving device begins discarding received data due to overflowing buffers.
2. The receiving device begins sending source quench messages to the transmitting device, at the rate of one message for each packet dropped.
3. The source device receives the source quench messages and lowers the data rate until it stops receiving the messages.
4. The source device then gradually increases the data rate as long as no further source quench requests are received.
Windowing - 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 acknowledgment after sending three packets, as follows:
1. The source device sends three packets to the destination device.
2. After receiving the three packets, the destination device sends an acknowledgment to the source.
3. The source receives the acknowledgment and sends three more packets.
4. 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 acknowledgment. The source, not receiving an acknowledgment, retransmits the packets at a reduced transmission rate.
6. Differentiate between the following WAN services: X.25 / LAPB, Frame Relay, ISDN / LAPD, SDLC. HDLC, PPP and DDR.
X.25 - ITU-T standard that defines how connections between DTE and DCE are maintained for remote terminal access and computer communications in PDNs. X.25 specifies LAPB, a data link layer protocol, and PLP, a network layer protocol. Frame Relay has to some degree superseded X.25.
Frame Relay - Industry-standard, switched data link layer protocol that handles multiple virtual circuits using HDLC encapsulation between connected devices. Frame Relay is more efficient than X.25, the protocol for which it is generally considered a replacement.
ISDN - Integrated Services Digital Network. Communication protocol, offered by telephone companies, that permits telephone networks to carry data, voice, and other source traffic.
SDLC - Synchronous Data Link Control. SNA data link layer communications protocol. SDLC is a bit-oriented, full-duplex serial protocol that has spawned numerous similar protocols, including HDLC and LAPB.
HDLC - High-Level Data Link Control. Bit-oriented synchronous data link layer protocol developed by ISO. Derived from SDLC, HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums.
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