OSI Flashcards
What are the Seven layers in the OSI model?
Application Presentation Session Transport Network Data Link Physical "Please Do Not Toss Sausage Pizza Away"
How many layers are in the OSI model?
Seven
What does the First Layer of the OSI model do?
Physical - Manages signal to and from physical network connections.
The physical layer has the following major functions:
It defines the electrical and physical specifications of the data connection. It defines the relationship between a device and a physical transmission medium (e.g., a copper or fiber optical cable). This includes the layout of pins, voltages, line impedance, cable specifications, signal timing, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.
It defines the protocol to establish and terminate a connection between two directly connected nodes over a communications medium.
It may define the protocol for flow control.
It defines transmission mode i.e. simplex, half duplex, full duplex.
It defines the network topology as bus, mesh, or ring being some of the most common.
The physical layer of Parallel SCSI operates in this layer, as do the physical layers of Ethernet and other local-area networks, such as Token Ring, FDDI, ITU-T G.hn, and IEEE 802.11 (Wi-Fi), as well as personal area networks such as Bluetooth and IEEE 802.15.4.
What does the Second Layer of the OSI model do?
Data Link - Packages data in frames appropriate to network transmission method.
The data link layer provides node-to-node data transfer – a reliable link between two directly connected nodes, by detecting and possibly correcting errors that may occur in the physical layer. The data link layer is divided into two sublayers:
Media Access Control (MAC) layer - responsible for controlling how devices in a network gain access to data and permission to transmit it.
Logical Link Control (LLC) layer - controls error checking and packet synchronization.
The Point-to-Point Protocol (PPP) is an example of a data link layer in the TCP/IP protocol stack.
The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete data link layer that provides both error correction and flow control by means of a selective-repeat sliding-window protocol.
What does the Third Layer of the OSI model do?
Network - Establishes network connections; translates network addresses into their physical counterparts and determines routing.
The network layer provides the functional and procedural means of transferring variable length data sequences (called datagrams) from one node to another connected to the same network. It translates logical network address into physical machine address. A network is a medium to which many nodes can be connected, on which every node has an address and which permits nodes connected to it to transfer messages to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting the network find the way to deliver (“route”) the message to the destination node. In addition to message routing, the network may (or may not) implement message delivery by splitting the message into several fragments, delivering each fragment by a separate route and reassembling the fragments, report delivery errors, etc.
Datagram delivery at the network layer is not guaranteed to be reliable.
A number of layer-management protocols, a function defined in the management annex, ISO 7498/4, belong to the network layer. These include routing protocols, multicast group management, network-layer information and error, and network-layer address assignment. It is the function of the payload that makes these belong to the network layer, not the protocol that carries them.[5]
What does the Fourth Layer of the OSI model do?
Transport - Ensures accurate delivery of data though flow control, segmentation and reassembly, error correction, and acknowledgement.
The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host via one or more networks, while maintaining the quality of service functions.
An example of a transport-layer protocol in the standard Internet stack is Transmission Control Protocol (TCP), usually built on top of the Internet Protocol (IP).
The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state- and connection-oriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred. The transport layer creates packets out of the message received from the application layer. Packetizing is a process of dividing the long message into smaller messages.
OSI defines five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the fewest features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0-4 classes are shown in the following table:[6]
Feature name TP0 TP1 TP2 TP3 TP4
Connection-oriented network Yes Yes Yes Yes Yes
Connectionless network No No No No Yes
Concatenation and separation No Yes Yes Yes Yes
Segmentation and reassembly Yes Yes Yes Yes Yes
Error recovery No Yes Yes Yes Yes
Reinitiate connectiona No Yes No Yes No
Multiplexing / demultiplexing over single virtual circuit No No Yes Yes Yes
Explicit flow control No No Yes Yes Yes
Retransmission on timeout No No No No Yes
Reliable transport service No Yes No Yes Yes
a If an excessive number of PDUs are unacknowledged.
An easy way to visualize the transport layer is to compare it with a post office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM’s SNA or Novell’s IPX over an IP network, or end-to-end encryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a network-layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames inside transport packet.
Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the transport layer, the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite are commonly categorized as layer-4 protocols within OSI.
What does the Fifth Layer of the OSI model do?
Session - Establishes, maintains and terminates user connections.
The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls.
What does the Sixth Layer of the OSI model do?
Presentation - Allows host and applications to use a common language, performs data formatting, encryption, and compression.
The presentation layer establishes context between application-layer entities, in which the application-layer entities may use different syntax and semantics if the presentation service provides a big mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the protocol stack.
This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.[7]
The original presentation structure used the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serialization of objects and other data structures from and to XML.
What does the Seventh Layer of the OSI model do?
Application - Provides interface between software applications and network for interpreting applications requests and requirements.
The application layer is the OSI layer closest to the end user, which means both the OSI application 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 synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network or the requested communication exists. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.
What are some Protocols & Standards for OSI Layer Seven?
Application - BOOTP, DHCP, DNS, FTP, HTTP, HTTPS, IMAP4, PING, POP3, NSLOOKUP, NTP, SFTP, SMTP, SNMP, Telnet, TFTP.
What are some Protocols & Standards for OSI Layer Six?
Presentation - MIME, SSL, TLS,
What are some Protocols & Standards for OSI Layer Five?
Session - RTP, SIP.
What are some Protocols & Standards for OSI Layer Four?
Transport - TCP, UDP.
What are some Protocols & Standards for OSI Layer Three?
Network - ARP, ICMP, IGMP, IP, IPsec, RARP, RIP.
What are some Protocols & Standards for OSI Layer Two?
Data Link - L2TP, PPP, PPTP, SLIP.
