Layer 1: physical layer
The major functions & services:
- Establishment & termination of a connectiont o a communications medium.
- Participation in the process whereby the communication resources are effectively: contention resolution & flow control.
- Modulation/conversion between the representation of digital data in user equipment & the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (copper & optical fiber)/over a radio link.
Parallel SCSI buses operate in this layer, remembered: logical SCSI protocol is a transport layer protocol that runs over this bus. Various physical-layer Ethernet standards are also in this layer; Ethernet & other LAN: token ring, FDDI, ITU-TG.hn,IEEE 802.11, personal area networks: Bluetooth,IEEE 802.15.4 incorporates both this layer & the data link layer.
Layer 2: data link layer
The data link layer provides the functional & procedural means to transfer data between network entities, detect & possibly correct errors in the physical layer.
Originally, this layer was intended for point-to-point & point-to-multipoint media, characteristic of wide area media in the telephone system. LAN architecture, which included broadcast-capable multi-access media, was developed independently of the ISO work in IEEE Project 802. IEEE work assumed sublayer-ing & management functions not required for WAN use.
In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as PPP, LAN,
the IEEE 802.2 LLC layer is not used for most protocols on the Ethernet, & on other LANs, its flow control & acknowledgment mechanisms are rarely used.
The ITU-TG.hn standard, which provides high-speed LANing over existing wires (power lines, phone lines & coaxial cables), includes a complete data link layer which provides both error correction & flow control by means of a selective repeat Sliding Window Protocol.
Both WAN & LAN service arrange bits from the physical layer into logical sequences: frames.
Not all physical layer bits necessarily go into frames, as some of these bits are purely intended for physical layer functions (every 8th bit of the FDDI bit stream is not used by the layer.
- Physical Addressing
- Flow Control
- Error Control
- Access Control
- Media Access Control(MAC)
WAN protocol architecture
Connection-oriented WAN data link protocols, in addition to framing, detect & may correct errors. They are also capable of controlling the rate of transmission. A WAN data link layer might implement a sliding window flow control & acknowledgment mechanism to provide reliable delivery of frames; that is the case for Synchronous Data Link Control (SDLC) & HDLC, & derivatives of HDLC such as LAPB & LAPD.
IEEE 802 LAN architecture
Practical, connectionless LANs began with the pre-IEEE Ethernet specification, which is the ancestor of IEEE 802.3. This layer manages the interaction of devices with a shared medium, which is the function of a media access control (MAC) sublayer. Above this MAC sublayer is the media-independent IEEE 802.2 Logical Link Control (LLC) sublayer, which deals with addressing & multiplexing on multi-access media.
Layer 3: network layer
The network layer provides the functional & procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network (in contrast to the data link layer which connects hosts within the same network), while maintaining the QoS requested by the transport layer. The network layer performs network routing functions, & might also perform fragmentation & reassembly, report delivery errors. Routers operate at this layer, sending data throughout the extended network & making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is not hierarchical.
The network layer may be divided into three sublayers:
- Subnetwork access – that considers protocols that deal with the interface to networks, such as X.25;
- Subnetwork-dependent convergence – when it is necessary to bring the level of a transit network up to the level of networks on either side
- Subnetwork-independent convergence – handles transfer across multiple networks.
An example of this latter case is CLNP, or IPv6 ISO 8473. It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, & from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of erroneous packets so they may be discarded. In this scheme, IPv4 & IPv6 would have to be classed with X.25 as subnet access protocols because they carry interface addresses rather than node addresses.
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 & error, & 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.
Layer 4: transport layer
The transport layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, & error control. Some protocols are state- & connection-oriented. This means that the transport layer can keep track of the segments & retransmit those that fail. The transport layer also provides the acknowledgement of the successful data transmission & sends the next data if no errors occurred.
OSI defines five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 & provides the least features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, & 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 & preservation of record boundaries. Detailed characteristics of TP0-4 classes are shown in the following table:
|Connection oriented network||Yes||Yes||Yes||Yes||Yes|
|Concatenation & separation||No||Yes||Yes||Yes||Yes|
|Segmentation & reassembly||Yes||Yes||Yes||Yes||Yes|
|Reinitiate connection (if an excessive number of PDUs are unacknowledged)||No||Yes||No||Yes||No|
|Multiplexing & demultiplexing over a 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|
An easy way to visualize the transport layer is to compare it with a Post Office, which deals with the dispatch & classification of mail & 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 & not strictly conforming to the OSI definition of the transport layer, the Transmission Control Protocol (TCP) & the User Datagram Protocol (UDP) of the Internet Protocol Suite are commonly categorized as layer-4 protocols within OSI.
Layer 5: session layer
The session layer controls the dialogues (connections) between computers. It establishes, manages & terminates the connections between the local & remote application. It provides for full-duplex, half-duplex, or simplex operation, & establishes checkpointing, adjournment, termination, & restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, & also for session checkpointing & 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. On this level, Inter-Process communication happen (SIGHUP, SIGKILL, End Process, etc.).
Layer 6: presentation layer
The presentation layer establishes context between application-layer entities, in which the higher-layer entities may use different syntax & semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, & passed down the stack.
This layer provides independence from data representation (e.g., encryption) by translating between application & network formats. The presentation layer transforms data into the form that the application accepts. This layer formats & encrypts data to be sent across a network. It is sometimes called the syntax layer.
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 fileto an ASCII-coded file, or serialization of objects & other data structures from & to XML.
Layer 7: application layer
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer & 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, & synchronizing communication. When identifying communication partners, the application layer determines the identity & 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 exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application-layer implementations also include:
- On OSI stack:
- On TCP/IP stack:
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