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Physical Layer

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Providing transparent transmission of a bit stream over a circuit built from some physical communications medium. Converts data to signals for transmission over physical media such as copper wires and optical fibers.

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Signaling

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Converting data to signals for transmission over physical media.

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Signals

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Change that can be detected at the receiving end. A detectable transmitted energy that can be used to carry information.

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Physical Medium

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Is the transmission path over which a signal propagates. (Wires, coaxial cables, optical fibers). Relates to physical media/transmission media.

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Optical Fiber

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Thin strand of glass that guides light along its length. It is a physical medium.

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Factors that favor optical fiber:

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Data rates, distance, instalation, and costs.

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Data

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Numbers, letters, or other representation of information that can be processed by people or machines.

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Digital Signals

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Signals in which discrete steps are used to represent information. Are created when we turn a switch on or off depending upon whether the data to be sent is a 0 or a 1.

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Analog signals

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Created when we take a sine wave and modify some property of the sine wave. They have continuous nature rather than a pulsed or discrete nature

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Bit period

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A bit period is the amount of time required to transmit one bit of data.

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Bit

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Unit of information that designates one of two possible states of anything that conveys information.

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Coding

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the transformation of elements of one set to elements of another set. Simple Scheme =ASCII, or Unicode = complex

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Multiplexing

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The combination of two or more information channels over a common medium. EX: transmission of multiple TV channels

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UTP

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unshielded twisted pair. 8 wires are organized as four pairs. 2 pairs are used for information exchange. One pair of wires is to forward data path and another pair is usedd for the reverse data path. UTP cables are connected to computers and other communication devices through plugs called RJ 45 jacks.

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Cat5e

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3-4 twist per inch. all color coated.

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Data Link Layer

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Sends data between neighboring devices on the network (transferring data within a network). Two primary functions are addressing and error-detection.

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Addressing

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assures that signals from the physical layer reach the correct devices on the network.

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Error-detection

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Once signals are recieved from addressing, the error-detection function detects if any errors were introduced during the signal transmission.

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Ethernet

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Data-link layer technology. Low cost, high speed communication in small networks. Commonly called local area networks (LAN). No privacy in Ethernet.

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Broadcasting

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transmission of signals that may be simultaneously received by stations that usually make no acknowledgement.

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Packet

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Data link layer. A packet of data contains data and some overhead, such as addresses, necessary for the data transger to be successful.

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Collision

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A collision is the situation that occurs when two or more demands are made simultaneously on a system that can only handle one demand at any given instants.

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Medium Access Control (MAC)

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Method used to determine who gets to send data over a shared medium. MAC method used in Ethernet is called CSMA/CD. Carrier Sense Multiple Access with Collision Detection

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Multiple Access

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Scheme that gives more than one computer access to the network for the purporse of transmitting informations.

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Carrier Sense

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An ongoing activity of a data station in a multiple access networkd to detect whether another station is transmitting.

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Collision Detection

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The requirement that a transmitting computer that detects another signal while transmitting data, stops transmitting that data.

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Cyclic Redundency Check (CRC)

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CRC is an error checking algorithm that checks data integrity by computing a polynomial algorithm basked checksum.

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Ethernet Addresses

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48 bits in length. first 24 bits determine the organizationally unique identifier (OUI) or Manufactor ID.

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Network Layer

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Transfers data packets from a source computer to a destination computer via one or more networks.

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Routing

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The data link layer cannot exchange data between two computers, and the network layer becomes necessary to transfer data accross networks

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Routers

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Routing is performed in networks by devices called routers. Routers connect networks. Example: when a network grows to the point where it is too large for an Ethernet, the network can be divided into multiple Ethernets. A router can be used to connect the Ethernets, creating a network of networks.

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IP

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Internet Protocol, used in Network Layer

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Best-effort-delivery

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Network service in which the network does not provide any guarantee that data will be delivered.

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IP Header Functions

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Version, header length, type of service, total length, identification, flags, time to live, protocol, header checksum

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Version HF

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The version field tells routers the version of IP begin used.

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Header length

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Specifies the length of the IP header

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Type of service

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The designers of IP allowed the source to specify the desired service priority.

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Total length

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Specifies the size of the packet, including the header information and data.

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Identification

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IP allows packets to be fragmented if necessary.

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Flags

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Indicates whether a packet may be fragmented, and whether it has in fact been fragmented.

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Fragment offset

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If an IP packet is fragmented, this field specifies the position of the current packet with respect to all other fragments with the same identification.

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Time to live

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Specifies the remaining life of the packet on the network.

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Protocol

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field in IP identifies the transport-layer technology that is sending and receiving the packet.

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Header Checksum

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This field carries error-detection information for the packet header. It is only calculated over the header, not over the packet data.

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Header Checksum vs Ethernet CRC

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CRC is for the entire frame whereas the IP checksum is only competed over the header. CRC is very robust at error detection and IP header checksum is less reliable.

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Source and destination addresses

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Fields identify the originating source and ultimate destination of the packet.

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Options

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Field allows the source of the packet to specify various kinds of optional information for use in routing.

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Padding

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This field consist of string of 0s to ensure that the IP header is a multiple of 32bit in size. Padding is necessary if options are used.

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Address (IP Header)

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is a unique label that helps locate an entity on a network.

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Network Part

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The first set of numbers in the IP Address. EX 131.91.128.84.... Network part is 131.91

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Host Part

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The last set of numbers in the IP address. EX: 131.91.128.84...Host part is 128.84

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Classless Inter-Domain Routing (CIDR)

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An address allocation scheme that eliminates the concept of address classes and allows address blocks of arbitrary length. Address blocks sizes of any power of 2 are possible with CIDR. EX: If need 2000 IP addresses, we would use 11 bits because 2^11 = 2048 IP addresses.

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IPv6

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IPv6 would have 128 bits in length rather than 32. 2^128 = 340 * 10*36 . There is no checksum header to be computed by routers and it. Functions are Version, Traffic class, Flow label, Payload Length, Next Header, Hop Limit

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Version (IPv6)

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The number is 6 for the version IPv6

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Traffic Class (IPv6)

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This field is similar to the type of service field in IPv4. It allows sender to specify a class of service for the packet.

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Flow label (IPv6)

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This field is similar to the traffic-class field. It allows senders to designate a few packets for special handling,

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Payload length (IPv6)

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This field is similar to the total-length field in IPv4 and specifies the size of the packet data.

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Next header (IPv6)

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Similar to protocol and specifies the transport-layer protocol that is using IP to deliver this packet.

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Hop Limit (IPv6)

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Similar to Time-to-live. Each router decrements this field by one. When the hop limit reaches 0, the packet is discarded.

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Transport Layer

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Segmentation. Reliability. Multiplexing. The transport layer receives data from applications and performs all the bookkeeping functions described above to make the inherently unreliable computer networks appear reliable to applications.

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Transmission Control Protocol (TCP)

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Is a highly reliable host-to-host transport-layer protocol over packet-switched networks. EX applications: emails and the web

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User Datagram Protocol (UDP)

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simplet transport-layer protocol that eliminates almost all the computational processing associated with TCP.

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Segmentation (TCP) Function

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Allows to transfer application data of arbitrary size. Breaks application data into segments of a manageable size for IP (smaller than 64k). Sends segments into sequential numbers.

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Datagrams

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Small segments broken down by TCP.

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Reliability (TCP) Function

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Receiver to periodically acknowledge received datagrams.

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Multiplexing (TCP) Function

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Allows multiple applications to share the same network interface card and network link. TCP enables multiplexing by providing multiple port addresses within each host.

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Flow Control (TCP) Function

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The control of the rate at which data are transmitted from a terminal so that the data can be received by another terminal. Flow control only slows down the sender. It does not make it go faster.

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Connection Establishment (TCP) Function

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ISN (initial sequence number) is an advancing number generator.

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Source and destination port addresses

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TCP header: These are the port numbers on the two sides of the data transfer

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TCP Header

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16 bits in length. Meaning that 2^16 port numbers are possible on each computer.

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Sequence Number

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TCP header: The sequence number of the first data byte in the datagram

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Acknowledgement number

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TCP Header: The nest sequence number expected by the sender of the datagram.

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Data Offset

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TCP Header: Size of the TCP header. Identifies where the data begins in the datagram.

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Reserved

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TCP Header: These bits have no current use, but may be used to add new functionality in the future.

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Control Bits

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TCP Header: Six bits indicating special information. Control bits indicate changes in TCP state, such as whether a new connection is being opened, or an existing connection is being closed.

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Window

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TCP Header: This is the window size, and it indicates the number of data bytes the send of the datagram is willing to accept beyond the acknowledgment number field of the datagram.

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Checksum

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TCP Header: A checksum on the entire datagram and the IP header. This is used for error detection.

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Urgent Pointer

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TCP Header: Used to indicate that the data in the dat gram must be processed immediately.

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Options (TCP)

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TCP Header: Potential options that can be defined, such as maximum receivable datagram size.

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Padding (TCP)

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0 or more bits to ensure that the TCP Header length is a multiple of 32 bits, This field is useful if options are used.

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UDP Header

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Useful for simpler transport protocol. EX: voice or video, real-time communication, Temperature, time etc.

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Application Layer

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Helps end users applications to use the data communications service provided by TCP and UDP. It hides TCP and IP from the end user. HTTP, SMTP

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HTTP Protocol

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Application layer function: HTTP is a protocol that facilitates the transfer of files between local and remote systems on the WWW

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HTML

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HyperText Markup Language. Hypertext is text that includes navigable links to other hypertext.

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HTTP

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simplest application layer protocol. Client sends a request for a file to the web server. The web server responds to the request by sending the requested file over TCP. Server provides status code that indicates the extent to which the server was successful in fulfilling the client's request. Server provides meta-data to decide how to display content.

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URL

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Uniform Resource Locator is character string describing the location and access method of a resource on the Internet.

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Email

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An electronic means for communication in which information- including texts, graphics, and sounds- is sent, stored, processed, and received. Messages are held in storage until called for by the addressee.

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MUA (Email)

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Mail-user-agent. Email Client such as Outlook. Read & Delete emails.

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MTA (Email)

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Mail-transfer-agent. Microsoft Exchange.

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SMTP

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Simple Mail Transfer Protocol, is the protocol used to transfer email between mail servers.

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DHCP

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Technology that enables automatic assignment and collection of IP addresses.

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DHCP Lease-Time

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The duration for which an IP address is provided.

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Automatic Allocation

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DHCP allocates IP addresses to hosts on the network on a first-come, first-serve basis. Once allocated, this IP address remains assigned to the host forever.

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Manual Allocation

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The network administrator manually specifies the IP addresses to be allocated to individual devices on the network. This IP address is also, permanently assigned.

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Dynamic Allocation

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IP addresses are leased out for short durations by the DHCP server. This is the most popular allocation.

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ARP

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Address Resolution Protocol. A protocol that dynamically determines the network-layer IP address associated with a data-link-layer physical hardware address.

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DNS

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DNS is the Domain Name System. It is the set of databases that performs the correspondence between the domain name and its IP address. DNS translates domain names to IP addresses.

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