Physical Layer | Layer 1 | The OSI-Model

OSI Layer 1 – Physical Layer

In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer. The implementation of this layer is often termed PHY.

The physical layer consists of the basic networking hardware transmission technologies of a network. It is a fundamental layer underlying the logical data structures of the higher level functions in a network. Due to the plethora of available hardware technologies with widely varying characteristics, this is perhaps the most complex layer in the OSI architecture.

The physical layer defines the means of transmitting raw bits rather than logical data packets over a physical link connecting network nodes. The bit stream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a hardware transmission medium. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to broadcast on, the modulation scheme to use and similar low-level parameters, are specified here.

Within the semantics of the OSI network architecture, the physical layer translates logical communications requests from the data link layer into hardware-specific operations to affect transmission or reception of electronic signals.Typical hardware on this layer: repeaters, hubs, cables, plugs,

Wikipedia

Services

• Bit-by-bit or symbol-by-symbol delivery

• Modulation

• Line coding

• Bit synchronization

• Start-stop signalling

• Circuit switching

• Multiplexing

• Carrier sense and collision detection

• Physical network topology, like bus, ring, mesh or star network

• …

Popular Physical Layer Technologies

1-Wire

ARINC 818

Avionics Digital Video Bus

Bluetooth physical layer

CAN bus

(controller area network) physical layer

DSL

EIA RS-232

also: EIA-422, EIA-423, RS-449, RS-485

Etherloop

Ethernet physical layer

10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and others

GSM

Um air interface physical layer

G.hn/G.9960

physical layer

I²C, I²S

IEEE 1394 interface

ISDN

IRDA

physical layer

ITU

Mobile Industry Processor Interface physical layer

OTN

Optical Transport Network

SPI

SMB

Telephone network modems

USB

physical layer

Advertisements

Data Link Layer | Layer 2 | The OSI-Model

OSI Layer 2 – Data Link Layer

The data link layer or layer 2 is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between adjacent network nodes in a wide area network (WAN) or between nodes on the same local area network (LAN) segment.[1] The data link layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the physical layer.

The data link layer is concerned with local delivery of frames between devices on the same LAN. Data-link frames, as these protocol data units are called, do not cross the boundaries of a local network. Inter-network routing and global addressing are higher-layer functions, allowing data-link protocols to focus on local delivery, addressing, and media arbitration. This way, the data link layer is analogous to a neighborhood traffic cop; it endeavors to arbitrate between parties contending for access to a medium, without concern for their ultimate destination. When devices attempt to use a medium simultaneously, frame collisions occur. Data-link protocols specify how devices detect and recover from such collisions, and may provide mechanisms to reduce or prevent them.

Examples of data link protocols are Ethernet for local area networks (multi-node), the Point-to-Point Protocol (PPP), HDLC and ADCCP for point-to-point (dual-node) connections. In the Internet Protocol Suite (TCP/IP), the data link layer functionality is contained within the link layer, the lowest layer of the descriptive model.Common hardware on this layer: Bridge, Switch (Multiport bridge)

Wikipedia

Services

• Encapsulation

• Frame synchronization

• Logical link control (Error & Flow control)

• Media access control (MAC, LAN switching, Physical addressing, QaS, VLAN, …)

Popular Data Link Layer Protocols

ARP

Address Resolution Protocol

ARCnet

ATM

CDP

Cisco Discovery Protocol

CAN

Controller Area Network

Econet

Ethernet

EAPS

Ethernet Automatic Protection Switching

FDDI

Fiber Distributed Data Interface

Frame Relay

HDLC

High-Level Data Link Control

IEEE 802.2

provides LLC functions to IEEE 802 MAC layers

IEEE 802.11

wireless LAN

I²C

LattisNet

LAPD

Link Access Procedures, D channel

LLDP

Link Layer Discovery Protocol

LocalTalk

MIL-STD-1553

MPLS

Multiprotocol Label Switching

NDP

Nortel Discovery Protocol

SDN

OpenFlow

PPP

Point-to-Point Protocol

Profibus

SpaceWire

SLIP

Serial Line Internet Protocol (obsolete)

SMLT

Split multi-link trunking

IEEE 802.1aq

Shortest Path Bridging

Spanning Tree Protocol

StarLan

Token ring

UDLD

Unidirectional Link Detection

UNI/O

1-Wire

Network Layer | Layer 3 | The OSI-Model

OSI Layer 3 – Network Layer

In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers, since it knows the address of neighboring network nodes, and it also manages quality of service (QoS), and recognizes and forwards local host domain messages to the Transport layer (layer 4). The data link layer (layer 2) is responsible for media access control, flow control and error checking.

The network 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.

Wikipedia

Functions

• Connection model

• Host addressing

• Message forwarding

Popular Network Layer Protocols

DDP

Datagram Delivery Protocol

DVMRP

Distance Vector Multicast Routing Protocol

ICMP

Internet Control Message Protocol

IGMP

Internet Group Management Protocol

IPsec

Internet Protocol Security

IPv4/IPv6

Internet Protocol

IPX

Internetwork Packet Exchange

PIM-DM

Protocol Independent Multicast Dense Mode

PIM-SM

Protocol Independent Multicast Sparse Mode

RSMLT

Routing Information Protocol

Shortest Path Bridging

Transport Layer | Layer 4 | The OSI-Model

OSI Layer 4 – Transport Layer

In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet Protocol Suite and the Open Systems Interconnection (OSI). The protocols of the layer provide host-to-host communication services for applications.[1] It provides services such as connection-oriented data stream support, reliability, flow control, and multiplexing.

The details of implementation and semantics of the Transport Layer of the TCP/IP model (RFC 1122), which is the foundation of the Internet, and the Open Systems Interconnection (OSI) model of general networking, are different. In the OSI model the transport layer is most often referred to as Layer 4 or L4, while numbered layers are not used in TCP/IP.

The best-known transport protocol of TCP/IP is the Transmission Control Protocol (TCP), and lent its name to the title of the entire suite. It is used for connection-oriented transmissions, whereas the connectionless User Datagram Protocol (UDP) is used for simpler messaging transmissions. TCP is the more complex protocol, due to its stateful design incorporating reliable transmission and data stream services. Other prominent protocols in this group are the Datagram Congestion Control Protocol (DCCP) and the Stream Control Transmission Protocol (SCTP).

Wikipedia

Services

• Connection-oriented communication

• Same order delivery

• Reliability

• Flow control

• Congestion avoidance

• Port Multiplexing

Popular Transport Layer Protocols

ATP

AppleTalk Transaction Protocol

CUDP

Cyclic UDP

DCCP

Datagram Congestion Control Protocol

FCP

Datagram Congestion Control Protocol

IL

Fibre Channel Protocol

MPTCP

IL Protocol

RDP

Multipath TCP

RUDP

Reliable User Datagram Protocol

SCTP

Stream Control Transmission Protocol

SPX

Sequenced Packet Exchange

SST

Structured Stream Transport

TCP

Transmission Control Protocol

UDP

User Datagram Protocol

UDP-Lite

User Datagram Protocol

µTP

Micro Transport Protocol

Session Layer | Layer 5 | The OSI-Model

OSI Layer 5 – Session Layer

In the seven-layer OSI model of computer networking, the session layer is layer 5.

The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session-layer services are commonly used in application environments that make use of remote procedure calls (RPCs).

An example of a session-layer protocol is the OSI protocol suite session-layer protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the session-layer protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and provides synchronization points in the stream of exchanged messages.

Other examples of session layer implementations include Zone Information Protocol (ZIP) – the AppleTalk protocol that coordinates the name binding process, and Session Control Protocol (SCP) – the DECnet Phase IV session-layer protocol.

Within the service layering semantics of the OSI network architecture, the session layer responds to service requests from the presentation layer and issues service requests to the transport layer.

Wikipedia

Services

• Authentication

• Authorization

• Session restoration

Popular Session Layer Protocols

ADSP

AppleTalk Data Stream Protocol

ASP

AppleTalk Session Protocol

H.245

Call Control Protocol for Multimedia Communication

ISO-SP

OSI session-layer protocol (X.225, ISO 8327)

iSNS

Internet Storage Name Service

L2F

Layer 2 Forwarding Protocol

L2TP

Layer 2 Tunneling Protocol

NetBIOS

Network Basic Input Output System

PAP

Password Authentication Protocol

PPTP

Point-to-Point Tunneling Protocol

RPC

Remote Procedure Call Protocol

RTCP

Real-time Transport Control Protocol

SMPP

Short Message Peer-to-Peer

SCP

Session Control Protocol

SOCKS

the SOCKS internet protocol, see Internet socket

ZIP

Zone Information Protocol

SDP

Sockets Direct Protocol

Presentation Layer | Layer 6 | The OSI-Model

OSI Layer 6 – Presentation Layer

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an EBCDIC-coded text computer file to an ASCII-coded file.

The presentation layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in the form of datagrams or packets between hosts. This layer deals with issues of string representation – whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, e.g. “thisisastring\0”). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will deal with the rest.

Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can be thought of as the key functionality of the presentation layer.

Encryption is typically done at this level too, although it can be done on the application, session, transport, or network layers, each having its own advantages and disadvantages. Decryption is also handled at the presentation layer. For example, when logging on to bank account sites the presentation layer will decrypt the data as it is received.[1] Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are ‘objects’ in object-oriented programming, and the exact way that streaming video is transmitted.

In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HyperText Transfer Protocol (HTTP), generally regarded as an application-layer protocol, has presentation-layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the application layer.

Within the service layering semantics of the OSI network architecture, the presentation layer responds to service requests from the application layer and issues service requests to the session layer.

In the OSI model: the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format.

Wikipedia

Services

• Data conversion

• Character code translation

• Compression

• Encryption and Decryption

Sublayers

The Presentation OSI Layer is usually composed of 2 sublayers that are:

CASE common application service element

ACSE

Association Control Service Element

ROSE

Remote Operation Service Element

CCR

Commitment Concurrency and Recovery

RTSE

Reliable Transfer Service Element

SASE specific application service element

FTAM

File Transfer, Access and Manager

VT

Virtual Terminal

MOTIS

Message Oriented Text Interchange Standard

CMIP

Common Management Information Protocol

JTM

Job Transfer and Manipulation

MMS

Manufacturing Messaging Service

RDA

Remote Database Access

DTP

Distributed Transaction Processing

Application Layer | Layer 7 | The OSI-Model

OSI Layer 7 – Application Layer

An application layer is an abstraction layer that specifies the shared protocols and interface methods used by hosts in a communications network. The application layer abstraction is used in both of the standard models of computer networking; the Internet Protocol Suite (TCP/IP) and the Open Systems Interconnection model (OSI model).

Although both models use the same term for their respective highest level layer, the detailed definitions and purposes are different.

In TCP/IP, the application layer contains the communications protocols and interface methods used in process-to-process communications across an Internet Protocol (IP) computer network. The application layer only standardizes communication and depends upon the underlying transport layer protocols to establish host-to-host data transfer channels and manage the data exchange in a client-server or peer-to-peer networking model. Though the TCP/IP application layer does not describe specific rules or data formats that applications must consider when communicating, the original specification (in RFC 1123) does rely on and recommend the robustness principle for application design.

In the OSI model, the definition of the application layer is narrower in scope. The OSI model defines the application layer as the user interface responsible for displaying received information to the user. In contrast, the Internet Protocol model does not concern itself with such detail. OSI also explicitly distinguishes additional functionality below the application layer, but above the transport layer at two additional levels; the session layer and the presentation layer. OSI specifies a strict modular separation of functionality at these layers and provides protocol implementations for each layer.

Wikipedia

Popular Application Layer Protocols

9P

Plan 9 from Bell Labs distributed file system protocol

AFP

Apple Filing Protocol

APPC

Advanced Program-to-Program Communication

AMQP

Advanced Message Queuing Protocol

Atom

Publishing Protocol

BEEP

Block Extensible Exchange Protocol

Bitcoin

Digital currency

BitTorrent

peer-to-peer file sharing

CFDP

Coherent File Distribution Protocol

CoAP

Constrained Application Protocol

DDS

Data Distribution Service

DeviceNet

automation industry protocol

eDonkey

classic file sharing protocol

ENRP

Endpoint Handlespace Redundancy Protocol

FastTrack

filesharing, known from KaZaa and more

Finger

User Information Protocol

Freenet

censorship resistant p2p network

FTAM

File Transfer Access and Management

Gopher

Gopher protocol

HL7

Health Level Seven

HTTP

HyperText Transfer Protocol

H.323

Packet-Based Multimedia Communications System

IRCP

Internet Relay Chat Protocol

Kademlia

p2p hashtables

LDAP

Lightweight Directory Access Protocol

LPD

Line Printer Daemon Protocol

MIME (S-MIME)

Multipurpose Internet Mail Extensions and Secure MIME

Modbus

Serial communications protocol

MQTT Protocol

MQ Telemetry Transport

Netconf

Network Configuration Protocol

NFS

Network File System

NIS

Network Information Service

NNTP

Network News Transfer Protocol

NTCIP

National Transportation Communications for Intelligent Transportation System Protocol

NTP

Network Time Protocol

OSCAR

AOL Instant Messenger Protocol

PNRP

Peer Name Resolution Protocol

RDP

Remote Desktop Protocol

RELP

Reliable Event Logging Protocol

RIP

Routing Information Protocol

Rlogin

Remote Login in UNIX Systems

RPC

Remote Procedure Call

RTMP

Real Time Messaging Protocol

RTP

Real-time Transport Protocol

RTPS

Real Time Publish Subscribe

RTSP

Real Time Streaming Protocol

SAP

Session Announcement Protocol

SDP

Session Description Protocol

SIP

Session Initiation Protocol

SLP

Service Location Protocol

SMB

Server Message Block

SMTP

Simple Mail Transfer Protocol

SNTP

Simple Network Time Protocol

SSH

Secure Shell

SSMS

Secure SMS Messaging Protocol

TCAP

Transaction Capabilities Application Part

TDS

Tabular Data Stream

Tor

anonymity network

TSP

Time Stamp Protocol

VTP

Virtual Terminal Protocol

Whois (and RWhois)

Remote Directory Access Protocol

WebDAV

Web Distributed Authoring and Versioning

X.400

Message Handling Service Protocol

X.500

Directory Access Protocol (DAP)

XMPP

Extensible Messaging and Presence Protocol