What Is The Function Of Pad Field In Csma Cd

what is the function of pad field in csma cd

CCNA - ICND1 Book Chapter 1 - Building a Simple Network

Chapter 1   Cisco Press "Interconnecting Cisco Network Devices, Part 1 (ICND1): CCNA Exam 640-802 and ICND1 Exam 640-822" .

Summary of Exploring the Functions of Networking

The key purpose of this section was to get a basic understanding of the key components in a computer network and how the network is used by business. The main points are as follows:

·         A network is a connected collection of computing devices that communicate with each other to carry data in homes, small businesses, and enterprise environments.

·         You have four major categories of physical components in a computer network: the computer, interconnections, switches, and routers.

·         The major resources that are shared in a computer network include data and applications, physical resources, storage devices, and backup devices.

·         The most common network user applications include e-mail, web browsers, instant messaging, collaboration, and databases.

·         The terms that describe networks include characteristics around network performance and structure such as speed, cost, security, availability, scalability, reliability, and topology.

·         A physical topology describes the layout for wiring the physical devices, while a logical topology describes how information flows to devices within the networks.

·         In a physical bus topology, a single cable connects all the devices together.

·         In a physical star topology, each device in the network is connected to central device with its own cable.

·         When a star network is expanded to include additional networking devices that are connected to the main networking device, it is called an extended-star topology.

·    Ina ring topology, all the hosts are connected to one another in the form of a ring or circle. A dual-ring topology provides a second ring for redundancy.

·         A full-mesh topology connects all devices to each other for redundancy, while a partial-mesh topology provides multiple connections for only some devices.

Summary of Exploring the Functions of Networking

The key purpose of this section was to get a basic understanding of the key components in a computer network and how the network is used by business. The main points are as follows:

·         A network is a connected collection of computing devices that communicate with each other to carry data in homes, small businesses, and enterprise environments.

·         You have four major categories of physical components in a computer network: the computer, interconnections, switches, and routers.

·         The major resources that are shared in a computer network include data and applications, physical resources, storage devices, and backup devices.

·         The most common network user applications include e-mail, web browsers, instant messaging, collaboration, and databases.

·         The terms that describe networks include characteristics around network performance and structure such as speed, cost, security, availability, scalability, reliability, and topology.

·         A physical topology describes the layout for wiring the physical devices, while a logical topology describes how information flows to devices within the networks.

·         In a physical bus topology, a single cable connects all the devices together.

·         In a physical star topology, each device in the network is connected to central device with its own cable.

·         When a star network is expanded to include additional networking devices that are connected to the main networking device, it is called an extended-star topology.

·    Ina ring topology, all the hosts are connected to one another in the form of a ring or circle. A dual-ring topology provides a second ring for redundancy.

·         A full-mesh topology connects all devices to each other for redundancy, while a partial-mesh topology provides multiple connections for only some devices.

·         Application layer: The application layer handles high-level protocols, including issues of representation, encoding, and dialog control. The TCP/IP model combines all application-related issues into one layer and ensures that this data is properly packaged for the next layer.

·         Transport layer: The transport layer deals with QoS issues of reliability, flow control, and error correction. One of its protocols, TCP, provides for reliable network communications.

·         Interne! layer: The purpose of the Internet layer is to send source datagrams from any network on the internetwork and have them arrive at the destination, regardless of the path they took to get there.

·         Network access layer: The name of this layer is broad and somewhat confusing. It is also called the host-to-network layer. It includes the LAN and WAN protocols and all the details in the OSI physical and data link layers.

OSI Model Versus TCP/IP Stack

Both similarities and differences exist between the TCP/IP protocol stack and the OSI reference model. Figure 1-25 offers a side-by-side comparison of the two models.

Similarities between the TCP/IP protocol stack and the OSI reference model include the following:

·         Both have application layers, though they include different services.

·         Both have comparable transport and network layers.

·         Both assume packet-switched technology, not circuit-switched. (Analog telephone calls are an example of circuit-switched technology.)

OSI Model Versus TCP/IP

The differences that exist between the TCP/IP protocol stack and the OSI reference model include the following:

·         TCP/IP combines the presentation and session layers into its application layer.

·         TCP/IP combines the OSI data link and physical layers into the network access layer.

TCP/IP protocols are the standards around which the Internet developed, so the TCP/IP protocol stack gains credibility just because of its protocols. In contrast, networks are not typically built on the OSI reference model, even though the OSI reference model is used as a guide.

Summary of Securing the Network

Security is an important part of any computer network. When you are building a network, a strong security policy should be part of the foundation. The following items represent a summary of considerations for building a strong security policy:

·         Sophisticated attack tools and open networks continue to generate an increased need for network security policies and infrastructure to protect organizations from internally and externally based attacks.

·         Organizations must balance network security needs against e-business processes, legal issues, and government policies. Establishing a network security policy is the first step in changing a network over to a secure infrastructure.

·         The strategy of information assurance affects network architecture.

·         Providing physical installation security for network devices is very important.

·         Network devices should be protected against password attacks through controlled access methods and strong passwords.

Summary of Understanding the Host-to-Host Communications Model

This following summarizes ihe host-lo-host communications model key points:

·         The OS! reference model defines the network functions that occur at each layer.

·         The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

·         The data link layer defines how data is formatted for transmission.

·         The network layer provides connectivity and path selection between two host systems that might be located on geographically separated networks.

·         The transport layer segments data from the system of the sending host and reassembles the data into a data stream on the system of the receiving host.

·         The session layer establishes, manages, and terminates sessions between two communicating hosts.

·         The presentation layer ensures that the information sent at the application layer of one system is readable by the application layer of another system.

·         The application layer provides network services, such as e-mail, file transfer, and web services, to applications of the users.

·         The information sent on a network is referred to as data or data packets. If one computer wants to send data to another computer, the data must first be packaged by a process called encapsulation.

·         When the remote device receives a sequence of bits, the physical layer at the remote devices passes the bits of data up the protocol stack for manipulation. This process is referred to as de-encapsulation.

·         TCP/IP is now the most widely used protocol for a number of reasons, including its flexible addressing scheme, usability by most operating systems and platforms, its many tools and utilities, and the need to be connected to the Internet.

·         The components of the TCP/IP stack are the network access, Internet, transport, and application layers.

·         The OSI reference model and the TCP/IP stack are similar in structure and function, with correlation at the physical, data link, network, and transport layers. The OSI reference model divides the application layer of the TCP/IP stack into three separate layers.

Address Classification

		The first 3 bits are 110.
Range of network numbers:	Range of network numbers: 128.0.0.0 to 191.255.0.0.	Range of network numbers: 192.0.0.0 to 223.255.255.0.
Number of possible networks: 127(1 through 126 are usable: 127 is reserved).	Number of possible networks: 16,384.	Number of possible networks:
Number of possible values in the host portion: 16.777.216.*	Number of possible values in the host portion: 65.536, *	Number of possible values in the host portion: 256.*

	RFC 1918 Internal Address Range
	10.0.0.0 to 10.255.255.255
	172.16.0.0 to 172.31.255.255
	192.168.0.0 to 192.168.255.255

Summary of TCP/IP's Internet Layer

The following list summarizes key points about TCP/IP's Internet layer:

·         IP network addresses consist of two parts: the network ID and the host ID.

·         IPv4 addresses have 32 bits that arc divided into octets and are generally shown in dotted decimal form (for example. 192.168.54.18).

·         IPv4 addresses are divided into A. B. and classes to be assigned to user devices.

·         Classes D and E are used for multicast and research, respectively.

·         The first few bits of an address determine the class.

·         Certain IP addresses (network and broadcast) are reserved and cannot be assigned to individual network devices.

·         Internet hosts require a unique public IP address, but private hosts can have any valid private address that is unique within the private network.

·         DCHP assigns IP addresses and parameters to host devices automatically.

·         DNS is a TCP/IP application that resolves domain names like Cisco.com into IP addresses to be used by the application.

·         Hosts provide tools that can be used to verify the IP addresses of the device. Windows tools are Network Connections and IPCONFIG.

The characteristics of TCP

The services provided by TCP run in the host computers at either end of a connection, not in the network. Therefore, TCP is a protocol for managing end-to-end connections. Because end-to-end connections can exist across a series of point-to-point connections, these end-to-end connections are called virtual circuits. The characteristics of TCP are as follows:

·         Connection-oriented: Two computers set up a connection to exchange data. The end systems synchronize with one another to manage packet flows and adapt to congestion in the network.

·         Full-duplex operation: A TCP connection is a pair of virtual circuits, one in each direction. Only the two synchronized end systems can use the connection.

·         Error checking: A checksum technique verifies that packets are not corrupted.

·         Sequencing: Packets are numbered so that the destination can reorder packets and determine if a packet is missing.

·         Acknowledgments: Upon receipt of one or more packets, the receiver returns an acknowledgment to the sender indicating that it received the packets. If packets are not acknowledged, the sender can retransmit the packets or terminate the connection if the sender thinks the receiver is no longer on the connection.

·         Flow control: If the sender is overflowing the buffer of the receiver by transmitting too quickly, the receiver drops packets. Failed acknowledgments alert the sender to slow down or stop sending. The receiver can also lower the flow to slow the sender down.

·         Packet recovery services: The receiver can request retransmission of a packet. If packet receipt is not acknowledged, the sender resends the packets.

Names for Encapsulated Data by Layer

TCP Header Field Descriptions

	Number of the calling port
	Number of the called port
	Number used to ensure correct sequencing of the arriving data
	Number of 32-bit words in the header
	Control functions such as setup and termination of a session
	Number of octets that the device is willing to accept
	Calculated checksum of the header and data fields
	Indicates the end of the urgent data
	One currently defined: maximum TCP segment size
	Upper-layer protocol data

UDP Header Field Descriptions

	Number of the calling port
	Number of the called port
	Length of UDP header and UDP data
	Calculated checksum of the header and data fields
	Upper-layer protocol data

Summary of Understanding TCP/IP's Transport and Application Layers

The following are the key points that were discussed in this section:

·         UDP is a protocol that operates at the transport layer and provides applications withaccess to the network layer without the overhead and reliability mechanisms of TCP.UDP is a connectionless, best-effort delivery protocol.

·         TCP is a protocol that operates at the transport layer and provides applications with access to the network layer. TCP is connection-oriented, provides error checking, delivers data reliably, operates in full-duplex mode, and provides some data recovery functions.

·         TCP/IP supports a number of applications, including FTP, TFTP, and Telnet.

·          IP uses a protocol number in the datagram header to identify which protocol to use for a particular datagram.

·          Port numbers map Layer 4 to an application.

·          If you use TCP as the transport layer protocol, before applications can transfer data,both sending and receiving applications inform their respective operating systems that a connection will be initiated. After synchronization has occurred, the two end systems have established a connection and data transfer can begin.

·          Flow control avoids the problem of a transmitting host overflowing the buffers in the receiving host and slowing network performance.

·         TCP provides sequencing of segments with a forward reference acknowledgement. When a single segment is sent, receipt is acknowledged, and the next segment is then sent.

·         TCP window size decreases the transmission rate to a level at which congestion and data loss do not occur. The TCP window size allows for a specified number of unacknowledged segments to be sent.

·         A fixed window is a window with an unchanging size that can accommodate a specific flow of segments.

·         A TCP sliding window is a window that can change size dynamically to accommodate the flow of segments.

·         TCP provides the sequencing of segments by providing sequence numbers and acknowledgment numbers in the TCP headers.

Summary of Exploring the Packet Delivery Process

The following summarizes the key points that were discussed in this lesson.

·         Layer 1 devices provide the connection to the physical media and its encoding.

·         Layer 2 devices provide an interface between the Layer 2 device and the physical media.

·         Layer 2 addresses are MAC addresses.

·         The network layer provides connectivity and path selection between two host systems.

·         Layer 3 addresses provide identification of a network and a host, such as an IP address.

·         Before a host can send data to another host, it must know the MAC address of the other device.

·         ARP is a protocol that maps IP addresses to MAC addresses.

·         TCP uses a three-way handshake to establish a session before sending data.

·         Most operating systems offer tools to view the device ARP table as well as tools like ping and traceroute to test IP connectivity.

Ethernet Compared to the OSI Model

·         Preamble: This field consists of 7 bytes of alternating 1s and 0s, which synchronize the signals of the communicating computers.

·         Start-of-frame (SOF) delimiter: This field contains bits that signal the receiving computer that the transmission of the actual frame is about to start and that any data following is part of the packet.

·         Destination address: This field contains the address of the NIC on the local network to which the packet is being sent.

·         Source address: This field contains the address of the NIC of the sending computer.

·         Type/length: In Ethernet II, this field contains a code that identifies the network layer protocol. In 802.3, this field specifies the length of the data field. The protocoinformation is contained in 802.2 fields, which are at the LLC layer. The newer 802.3 specifications have allowed the use of Ethertype protocol identifiers when not using the 802.2 field.

·         Data and pad: This field contains the data that is received from the network layer on the transmitting computer. This data is then sent to the same protocol on the destination computer. If the data is too short, an adapter adds a string of extraneous bits to "pad"the field to its minimum length of 46 bytes.

·         Frame check sequence (FCS): This field includes a checking mechanism to ensure that the packet of data has been transmitted without corruption.

The three major types of network communications are as follows:

·         Unicast: Communication in which a frame is sent from one host and addressed to one specific destination. In unicast transmission, you have just one sender and one receiver. Unicast transmission is the predominant form of transmission on LANs and within the Internet.

·         Broadcast: Communication in which a frame is sent from one address to all other addresses. In this case, you have just one sender, but the information is sent to all connected receivers. Broadcast transmission is essential when sending the same message to all devices on the LAN.

·         Multicast: Communication in which information is sent to a specific group of devices or clients. Unlike broadcast transmission, in multicast transmission clients must be members of a multicast group to receive the information.

Ethernat Address

A 48-bit Ethernet MAC address has two components:

·         24-bit Organizational Unique Identifier (OUI): The letter "O" identifies the manufacturer of the NIC card. The IEEE regulates the assignment of OUI numbers. Within the OUI, the two following bits have meaning only when used in the destination address:

— Broadcast or multicast bit: This indicates to the receiving interface that the frame is destined for all or a group of end stations on the LAN segment.

— Locally administered address bit: Normally the combination of OUI and a 24-bit station address is universally unique; however, if the address is modified locally, this bit should be set.

·         24-bit vendor-assigned end station address: This uniquely identifies the Ethernet hardware.  Summary of Understanding Ethernet

The following summarizes the key points that were discussed in this lesson:

·         A LAN is a network that is located in a limited area, with the computers and other components that are part of this network located relatively close together.

·         Regardless of its size, several fundamental components are required for the operation of a LAN, including computers, interconnections, network devices, and protocols.

·         LANs provide both communication and resource-sharing functions for their users and can be configured in various sizes, including both SOHO and enterprise environments.

·         Ethernet was developed in the 1970s by DEC, Intel, and Xerox and was called DIX Ethernet. In the 1980s, an IEEE workgroup body defined a new Ethernet standard for public use, and it was called Ethernet 802.3 and Ethernet 802.2.

·         Ethernet LAN standards specify cabling and signaling at both the physical and data link layers of the OSI model.

·         Stations on a CSMA/CD LAN can access the network at any time before sending data. CSMA/CD stations listen to the network to determine whether it is already in use. If it is in use, they wait. If it is not in use, the stations transmit. A collision occurs when two stations listen for the network traffic, hear none, and transmit simultaneously.

·         An Ethernet frame consists of fields, including preamble, start-of-frame delimiter, destination address, source address, type/length, data and pad, and frame check sequence.

·         You find three major kinds of communications in networks: unicast, multicast, and broadcast.

·         The address used in an Ethernet LAN is the means by which data is directed to the proper receiving location.

·         The MAC sublayer handles physical addressing issues, and the physical address is a 48-bit number usually represented in hexadecimal format.

Summary of Connecting to an Ethernet LAN

This section summarizes the key points that were discussed in this lesson:

·         A NIC or LAN adapter plugs into a motherboard and provides an interface for connecting to the network.

·         The MAC address is burned onto each NIC by the manufacturer, providing a unique, physical network address that permits the device to participate in the network.

·         The cable and connector specifications used to support Ethernet implementations are derived from the EIA/TIA standards body.

·         The categories of cabling defined for the Ethernet are derived from the EIA/TIA-568 (SP2840) Commercial Building Telecommunications Wiring Standards.

·         Several connection media are used for Ethernet with RJ-45 and GBIC being the most common.

·         A GBIC is a hot-swappable I/O device that plugs into a Gigabit Ethernet port on a network device to provide a physical interface.

·         UTP cable is a four-pair wire. Each of the eight individual copper wires in UTP cable is covered by an insulating material, and the wires in each pair are twisted around each other.

·         A crossover cable connects between similar devices like router to router, PC to PC, or switch to switch.

·         A straight-through cable connects between dissimilar devices like switch to router or PC to switch.

A network is a connected collection of devices that can communicate with each other. Networks in homes, small businesses, or large enterprises allow users to share resources such as data and applications (e-mail, web access, messaging, collaboration, and

databases), peripherals, storage devices, and backup devices. Networks carry data (or data packets) following rules and standards called protocols, each with its own specialized function. Networks can be evaluated in terms of both performance and structure, using measures such as speed, cost, security, availability, scalability, reliability, and topology. Ethernet is the most common type of LAN used today. Standards unique to Ethernet specify Ethernet LAN cabling and signaling at both the physical and data link layers of the OSI reference model. Bits that are transmitted over an Ethernet LAN are organized into frames. Ethernet LANs manage the signals on a network using a process called CSMA/CD.  The OSI reference model facilitates an understanding of how information travels through a network, by defining the network functions that occur at each layer. Most networks operate under the rules defined by TCP/IP. TCP/IP defines a 32-bit address that is represented by 4 octets separated by a period. This host address can be manually configured or obtained from a DHCP server.

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