A network is a set of devices connected by physical or virtual media links.
A network is a connection of two or more nodes via a physical or virtual link but also two or more networks connected by one or more nodes.
Link is the virtual or physical medium used to connect two or more computers.
Taking as example the response from the question above, the computers are called Nodes.
A node that is situated between two or more networks is commonly called router or Gateway. It generally translates and forwards messages from one network to another.
If the links are limited to a pair of nodes, that can be a point-point link.
If the links are shared by more than two nodes, this is Multiple Access.
- Distributed database
- Faster Problem solving
- Security through redundancy
- Collaborative Processing
- Performance: can be measured in many ways, including transmission and response time.
- Reliability: measured by frequency of failure, the time a link takes to recover from a failure, and the network's robustness.
- Security: protecting data from unauthorized access and viruses, malware and other similar threats.
- Number of Users
- Type of transmission medium
- Frequency of failure
- Recovery time of a network after a failure
- Unauthorized Access
- Viruses, malware and other similar threats.
A network protocol defines rules and conventions for communication between network devices.
- Syntax: the structure or data format, that is the order in which they are shown.
- Semantics: the meaning of every section of bits.
- Timing: when data should be sent and how fast it can be sent.
- Cost-effective Resource Sharing
- Support for common Services
Network performance is measured in Bandwidth (throughput) and Latency (Delay).
- Bandwidth of a network is given by the number of bits that can be transmitted over the network in a certain period of time.
- Latency corresponds to how long it takes for a message to travel from one end off a network to the other. It is strictly measured in terms of time.
The process of determining systematically how to forward messages toward the destination nodes based on its address.
The processes on each machine that communicate at a given layer.
When a switch receives data faster than the shared link can accommodate in its memory, for an extended period of time, then the switch will eventually run out of buffer space and some packets will have to be dropped. This state is congested state.
Defining a useful channel involves both understanding the applications requirements and recognizing the limitations of the underlying technology. The gap between what applications expects and what the underlying technology can provide is called semantic gap.
The duration of time it takes to send a message from one end of a network to the other and back, is called Round Trip Time, in short RTT.
- Unicasting: the message is sent from a source to a single computer in a network.
- Multicasting: the message is sent to a grup of destination computers simultaneously.
- Broadcasting: the message is sent to all computers in the network simultaneously.
Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.
- Frequency Division Multiplexing (FDM)
- Time Division Multiplexing (TDM)
-- Synchronous TDM
-- ASynchronous TDM Or Statistical TDM.
- Wave Division Multiplexing (WDM)
FDM is an analog technique that can be applied when the bandwidth of a link is greater than the combined bandwidths of the signals to be transmitted.
WDM is conceptually the same as FDM, except that the multiplexing and demultiplexing involve light signals transmitted through fiber optics channel.
TDM is a digital process that can be applied when the data rate capacity of the transmission medium is greater than the data rate required by the sending and receiving devices.
In STDM, the multiplexer allocates exactly the same time slot to each device at all times, whether or not a device has anything to transmit.
- Physical Layer
- Data Link Layer
- Network Layer
- Transport Layer
- Session Layer
- Presentation Layer
- Application Layer
- Physical Layer
- Data link Layer
- Network Layers
- Session Layer
- Presentation Layer
- Application Layer
- Transport layer
Physical layer coordinates the functions required to transmit a bit stream over a physical medium.
- Physical characteristics of interfaces and media
- Representation of bits
- Data rate
- Synchronization of bits
- Line configuration
- Physical topology
- Transmission mode
The Data Link Layer transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for node-node delivery.
- Physical Addressing
- Flow Control
- Error Control
- Access Control
The Network Layer is responsible for the source-to-destination delivery of packet possibly across multiple networks (links).
- Logical Addressing
The Transport Layer is responsible for source-to-destination delivery of the entire message.
- Service-point Addressing
- Segmentation and reassembly
- Connection Control
- Flow Control
- Error Control
The Session layer is the network dialog Controller. It establishes, maintains and synchronizes the interaction between the communicating systems.
- Dialog control
The Presentation layer is concerned with the syntax and semantics of the information exchanged between two systems.
The Application Layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as e-mail, shared database management and other types of distributed information services.
- Network virtual Terminal
- File transfer, access and Management (FTAM)
- Mail services
- Directory Services
- Leased Lines
- Last-Mile Links
- Wireless Links
- Single-Bit error: only one bit in the data unit has changed
- Burst Error: two or more bits in the data have changed
Data can be corrupted during transmission. For reliable communication, errors must be deducted and Corrected. Error Detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination. The common Error Detection methods are
- Vertical Redundancy Check (VRC)
- Longitudinal Redundancy Check (VRC)
- Cyclic Redundancy Check (VRC)
The concept of including extra information in the transmission solely for the purpose of comparison.
It is the most common and least expensive mechanism for Error Detection. In VRC, a parity bit is added to every data unit so that the total number of 1s becomes even for even parity. It can detect all single-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd.
In LRC, a block of bits is divided into rows and a redundant row of bits is added to the whole block. It can detect burst errors. If two bits in one data unit are damaged and bits in exactly the same positions in another data unit are also damaged, the LRC checker will not detect an error. In LRC a redundant data unit follows n data units.
CRC, is the most powerful of the redundancy checking techniques, is based on binary division.
Checksum is used by the higher layer protocols (TCP/IP) for error detection.
- Divide the data into sections
- Add the sections together using 1's complement arithmetic
- Take the complement of the final sum, this is the checksum.
Data link protocols are sets of specifications used to implement the data link layer. The categories of Data Link protocols are:
- Asynchronous Protocols
- Synchronous Protocols
-- Character Oriented Protocols
-- Bit Oriented protocols
The correction of errors is more difficult than the detection. In error detection, any error occured is checked. In error correction, the exact number of bits that are corrupted and location in the message are known. The number of the errors and the size of the message are important factors.
Forward error correction is the process in which the receiver tries to guess the message by using redundant bits.
Retransmission is a technique in which the receiver detects the occurrence of an error and asks the sender to resend the message. Re-sending is repeated until a message arrives that the receiver believes is error-freed.
In block coding, we divide our message into blocks, each of k bits, called datawords. The block coding process is one-to-one. The same dataword is always encoded as the same codeword.
"r" redundant bits are added to each block to make the length n = k + r. The resulting n-bit blocks are called codewords. 2n - 2k codewords that are not used. These codewords are invalid or illegal.
A linear block code is a code in which the exclusive OR (addition modulo-2) of two valid codewords creates another valid codeword.
Cyclic codes are special linear block codes with one extra property. In a cyclic code, if a codeword is cyclically shifted (rotated), the result is another codeword.
A device or program that uses predefined algorithms to encode, or compress audio or video data for storage or transmission use. A circuit that is used to convert between digital video and analog video.
A device or program that translates encoded data into its original format (e.g. it decodes the data). The term is often used in reference to MPEG-2 video and sound data, which must be decoded before it is output.
Framing in the data link layer separates a message from one source to a destination, or from other messages to other destinations, by adding a sender address and a destination address. The destination address defines where the packet has to go and the sender address helps the recipient acknowledge the receipt.
In fixed-size framing, there is no need for defining the boundaries of the frames. The size itself can be used as a delimiter.
In byte stuffing (or character stuffing), a special byte is added to the data section of the frame when there is a character with the same pattern as the flag. The data section is stuffed with an extra byte. This byte is usually called the escape character (ESC), which has a predefined bit pattern. Whenever the receiver encounters the ESC character, it removes it from the data section and treats the next character as data, not a delimiting flag.
Bit stuffing is the process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data, so that the receiver does not mistake the pattern 0111110 for a flag.
Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment.
Error control is both error detection and error correction. It allows the receiver to inform the sender of any frames lost or damaged in transmission and coordinates the retransmission of those frames by the sender. In the data link layer, the term error control refers primarily to methods of error detection and retransmission.
Error control is both error detection and error correction. It allows the receiver to inform the sender of any frames lost or damaged in transmission and coordinates the retransmission of those frames by the sender. In the data link layer, the term error control refers primarily to methods of error detection and retransmission. Error control in the data link layer is often implemented simply: Any time an error is detected in an exchange, specified frames are retransmitted. This process is called automatic repeat request (ARQ).
In Stop and wait protocol, sender sends one frame, waits until it receives confirmation from the receiver (okay to go ahead), and then sends the next frame.
Error correction in Stop-and-Wait ARQ is done by keeping a copy of the sent frame and retransmitting of the frame when the timer expires.
The protocol specifies that frames need to be numbered. This is done by using sequence numbers. A field is added to the data frame to hold the sequence number of that frame. Since we want to minimize the frame size, the smallest range that provides unambiguous communication. The sequence numbers can wrap around.
In networking and in other areas, a task is often started before the previous task has ended. This is known as pipelining.
The sliding window is an abstract concept that defines the range of sequence numbers that is the concern of the sender and receiver. In other words, the sender and receiver need to deal with only part of the possible sequence numbers.
A technique called piggybacking is used to improve the efficiency of the bidirectional protocols. When a frame is carrying data from A to B, it can also carry control information about arrived (or lost) frames from B; when a frame is carrying data from B to A, it can also carry control information about the arrived (or lost) frames from A.
A generic term for a section of a large network usually separated by a bridge or router.
- Transmission: is a physical movement of information and concern issues like bit polarity, synchronisation, clock etc.
- Communication: means the meaning full exchange of information between two communication media.
Series of interface points that allow other computers to communicate with the other layers of network protocol stack.
- X3: The function of PAD (Packet Assembler Disassembler) is described in a document
- X.28: The standard protocol has been defined between the terminal and the PAD
- X.29: The standard protocol that exists between the PAD and the network
Together, these three recommendations are often called triple X.
Frame relay is a packet switching technology. It will operate in the data link layer.
Telnet is also called as terminal emulation. It belongs to application layer.
The process that allows a network to self-repair networks problems. The stations on the network notify the other stations on the ring when they are not receiving the transmissions. Beaconing is used in Token ring and FDDI networks.
Redirector is a software that intercepts file or prints I/O requests and translates them into network requests. This comes under presentation layer.
- NETBIOS: is a programming interface that allows I/O requests to be sent to and received from a remote computer and it hides the networking hardware from applications.
- NETBEUI: is NetBIOS extended user interface. A transport protocol designed by microsoft and IBM for the use on small subnets.
A method for providing fault tolerance by using multiple hard disk drives. It is mentioned in Networking section because it can be used over network too.
When the computers on the network simply listen and receive the signal, they are referred to as passive because they don't amplify the signal in any way. Example for passive topology is linear bus.
Hybrid devices that combine the features of both bridges and routers.
A layer of a glass surrounding the center fiber of glass inside a fiber-optic cable.
A communications protocol used to connect computers to remote networking services including Internet service providers.
A gateway operates at the upper levels of the OSI model and translates information between two completely different network architectures or data formats.
The degeneration of a signal over distance on a network cable is called attenuation.
The address for a device as it is identified at the Media Access Control (MAC) layer in the network architecture. MAC address is usually stored in ROM on the network adapter card and is unique (or should be). The MAC form is 00:00:00:00:00:00 and it is called also hardware address.
- Bit rate: the number of bits transmitted during one second.
- Baud rate: the number of signal units per second that are required to represent those bits.
baud rate = (bit rate / N) where N is no-of-bits represented by each signal shift.
Every line has an upper limit and a lower limit on the frequency of signals it can carry. This limited range is called the bandwidth.
Signals are usually transmitted over some transmission media that are broadly classified in to two categories.
- Guided Media: These are those that provide a conduit from one device to another that include twisted-pair, coaxial cable and fiber-optic cable. A signal traveling along any of these media is directed and is contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic that accept and transport signals in the form of electrical current. Optical fiber is a glass or plastic cable that accepts and transports signals in the form of light.
-- Twisted - Pair cable (Shielded TP or Unshielded TP)
-- Coaxial Cable
-- Fiber-optic cable
- Unguided Media: this is the wireless media that transport electromagnetic waves without using a physical conductor. Signals are broadcast either through air. This is done through radio communication, satellite communication and cellular telephony.
-- Terrestrial microwave
-- Satellite Communication
It is a project started by IEEE to set standards to enable intercommunication between equipment from a variety of manufacturers. It is a way for specifying functions of the physical layer, the data link layer and to some extent the network layer to allow for interconnectivity of major LAN protocols.
It consists of the following:
802.1 is an internetworking standard for compatibility of different LANs and MANs across protocols.
802.2 Logical link control (LLC) is the upper sublayer of the data link layer which is non-architecture-specific, that is remains the same for all IEEE-defined LANs.
Media access control (MAC) is the lower sub-layer of the data link layer that contains some distinct modules each carrying proprietary information specific to the LAN product being used. The modules are Ethernet LAN (802.3), Token ring LAN (802.4), Token bus LAN (802.5).
802.6 is distributed queue dual bus (DQDB) designed to be used in MANs.
The data unit in the LLC level is called the protocol data unit (PDU).
The PDU contains of four fields a destination service access point (DSAP), a source service access point (SSAP), a control field and an information field. DSAP, SSAP are addresses used by the LLC to identify the protocol stacks on the receiving and sending machines that are generating and using the data. The control field specifies whether the PDU frame is a information frame (I - frame) or a supervisory frame (S - frame) or a unnumbered frame (U - frame).
- Repeater: Also called a regenerator, it is an electronic device that operates only at physical layer. It receives the signal in the network before it becomes weak, regenerates the original bit pattern and puts the refreshed copy back in to the link.
- Bridges: These operate both in the physical and data link layers of LANs of same type. They divide a larger network in to smaller segments. They contain logic that allow them to keep the traffic for each segment separate and thus are repeaters that relay a frame only the side of the segment containing the intended recipent and control congestion.
- Routers: They relay packets among multiple interconnected networks (i.e. LANs of different type). They operate in the physical, data link and network layers. They contain software that enable them to determine which of the several possible paths is the best for a particular transmission.
- Gateways: They relay packets among networks that have different protocols (e.g. between a LAN and a WAN). They accept a packet formatted for one protocol and convert it to a packet formatted for another protocol before forwarding it. They operate in all seven layers of the OSI model.
ICMP is Internet Control Message Protocol, a network layer protocol of the TCP/IP suite used by hosts and gateways to send notification of datagram problems back to the sender. It uses the echo test / reply to test whether a destination is reachable and responding. It also handles both control and error messages. It is also called ping.
- message: data unit created at the application layer
- segment or user datagram: data unit created at the transport layer
- datagram: data unit created at the network layer
- frame: datagram encapsulated at the link layer
- signal: the data unit transmitted along the transmission media
- ARP: The address resolution protocol (ARP) is used to associate the 32 bit IP address with the 48 bit physical address, used by a host or a router to find the physical address of another host on its network by sending a ARP query packet that includes the IP address of the receiver.
- RARP: The reverse address resolution protocol (RARP) allows a host to discover its Internet address when it knows only its physical address.
The header should have a minimum length of 20 bytes and can have a maximum length of 60 bytes.
- Class A - 0.0.0.0 - 127.255.255.255
- Class B - 220.127.116.11 - 18.104.22.168
- Class C - 192.0.0.0 - 22.214.171.124
- Class D - 126.96.36.199 - 188.8.131.52
- Class E - 240.0.0.0 - 247.255.255.255
- TFTP: The Trivial File Transfer Protocol (TFTP) allows a local host to obtain files from a remote host but does not provide reliability or security. It uses the fundamental packet delivery services offered by UDP.
- FTP: The File Transfer Protocol (FTP) is the standard mechanism provided by TCP / IP for copying a file from one host to another. It uses the services offer by TCP and so is reliable and secure. It establishes two connections (virtual circuits) between the hosts, one for data transfer and another for control information.
PS: do not mix here security with encryption. The data via FTP is securely transmitted but can be sniffed as it is transmitted as is.
- Server-based network: provide centralized control of network resources and rely on server computers to provide security and network administration
- Peer-to-peer network: computers can act as both servers sharing resources and as clients using the resources.
- BUS topology: In this each computer is directly connected to primary network cable in a single line. Advantages: Inexpensive, easy to install, simple to understand, easy to extend.
- STAR topology: In this all computers are connected using a central hub.Advantages: Can be inexpensive, easy to install and reconfigure and easy to trouble shoot physical problems.
- RING topology: In this all computers are connected in loop. Advantages: All computers have equal access to network media, installation can be simple, and signal does not degrade as much as in other topologies because each computer regenerates it.
A network in which there are multiple network links between computers to provide multiple paths for data to travel.
- In a baseband transmission: the entire bandwidth of the cable is consumed by a single signal.
- In broadband transmission: signals are sent on multiple frequencies, allowing multiple signals to be sent simultaneously.
In a Ethernet network, between any two points on the network, there can be no more than five network segments or four repeaters, and of those five segments only three of segments can be populated.
In token Ring, the hub is called Multistation Access Unit (MAU).
- Routable protocols: can work with a router and can be used to build large networks
- Non-Routable protocols: are designed to work on small, local networks and cannot be used with a router
It provides a framework for discussing network operations and design.
One of two sublayers of the data link layer of OSI reference model, as defined by the IEEE 802 standard. This sublayer is responsible for maintaining the link between computers when they are sending data across the physical network connection.
Virtual channel is normally a connection from one source to one destination, although multicast connections are also permitted. The other name for virtual channel is virtual circuit.
Along any transmission path from a given source to a given destination, a group of virtual circuits can be grouped together into what is called path.
Packet filter is a standard router equipped with some extra functionality. The extra functionality allows every incoming or outgoing packet to be inspected. Packets meeting some criterion are forwarded normally. Those that fail the test are dropped.
One of the main causes of congestion is that traffic is often busy. If hosts could be made to transmit at a uniform rate, congestion would be less common. Another open loop method to help manage congestion is forcing the packet to be transmitted at a more predictable rate. This is called traffic shaping.
Sending a message to a group is called multicasting, and its routing algorithm is called multicast routing.
When hierarchical routing is used, the routers are divided into so called regions, with each router knowing all the details about how to route packets to destinations within its own region, but knowing nothing about the internal structure of other regions.
It is a problem that can ruin TCP performance. This problem occurs when data are passed to the sending TCP entity in large blocks, but an interactive application on the receiving side reads 1 byte at a time.
The most common two letter combinations are called digrams. e.g. th, in, er, re and an. The most common three letter combinations are called trigrams. e.g. the, ing, and, and ion.
IDEA stands for International Data Encryption Algorithm.
Wide-mouth frog is the simplest known key distribution center (KDC) authentication protocol.
It is a system that performs a protocol translation between different electronic mail delivery protocols.
It is any routing protocol used within an autonomous system.
It is the protocol the routers in neighboring autonomous systems use to identify the set of networks that can be reached within or via each autonomous system.
It is a collection of routers under the control of a single administrative authority and that uses a common Interior Gateway Protocol.
It is a protocol used to advertise the set of networks that can be reached with in an autonomous system. BGP enables this information to be shared with the autonomous system. This is newer than EGP (Exterior Gateway Protocol). It is the routing protocol of the Internet, used to route traffic across the Internet.
It is a protocol formerly used to exchange routing information between Internet core routers.
It is a set of rules defining a very simple virtual terminal interaction. The NVT is used in the start of a Telnet session.
It is a host that has a multiple network interfaces and that requires multiple IP addresses is called as a Multi-homed Host.
It is an authentication service developed at the Massachusetts Institute of Technology. Kerberos uses encryption to prevent intruders from discovering passwords and gaining unauthorized access to files.
It is an Internet routing protocol that scales well, can route traffic along multiple paths, and uses knowledge of an Internet's topology to make accurate routing decisions.
It is using a router to answer ARP requests. This will be done when the originating host believes that a destination is local, when in fact is lies beyond router.
It is a very simple protocol used for transmission of IP datagrams across a serial line.
It is a simple protocol used to exchange information between the routers.
It is a sequence of IP addresses identifying the route a datagram must follow. A source route may optionally be included in an IP datagram header.