OSI 7 Layers Reference Model

These 7 layers further divide the tasks of moving the data across the network into subtask and hence complete one communication cycle between two computers or two network devices. Each layer is assigned a task and the task is completed independently. The OSI layers have the clear and independent characteristics and tasks.

The 7 layers of the OSI models can be divided into upper and lower layers. I have defined the characteristics, tasks and features of each layer separately.

Layer 7: Application Layer

The application layer defines the interfaces for communication and data transfer. This layer also provides and support services such as job transfer, handles network access, e-mail, supports user applications and error recovery.

Protocols: FTP, DNS, SNMP (simple network management protocol), SMTP (simple message transfer protocol), FINGER (which gives user profile information), TELNET (terminal access protocol), TFTP (trivial ftp), BOOTP and SMB protocol are operated on the application layer.

Network Devices: Gateway network device is operated on the application layer.

Layer 6:Presentation Layer

The presentation layer presents the data into a uniform format and masks the difference of data format between two dissimilar systems. It also translates the data from application to the network format. Presentation layer is also responsible for the protocol conversion, encryption, decryption and data compression. Presentation layer is a best layer for cryptography.

Network Devices: Gateway Redirector is operates on the presentation layer.

Layer 5: Session Layer

Session layer establish and manages the session between the two users at different ends in a network. Session layer also manages who can transfer the data in a certain amount of time and for how long. The examples of session layers and the interactive logins and file transfer sessions. Session layer reconnect the session if it disconnects. It also reports and logs and upper layer errors.

Protocols: The protocols that work on the session layer are NetBIOS, Mail Slots, Names Pipes, RPC

Network Devices: Gateway

Layer 4: Transport Layer

Transport layer manages end to end message delivery in a network and also provides the error checking and hence guarantees that no duplication or errors are occurring in the data transfers across the network. Transport layer also provides the acknowledgement of the successful data transmission and retransmits the data if no error free data was transferred.

It also provides and error handling and connectionless oriented data deliver in the network.

Protocols: These protocols work on the transport layer TCP, SPX, NETBIOS, ATP and NWLINK.

Network Devices: The Brouter, Gateway and Cable tester work on the transport layer.

Layer 3: Network Layer

The network layer determines that how data transmits between the network devices. It also translates the logical address into the physical address e.g computer name into MAC address. It is also responsible for defining the route, managing the network problems and addressing. Router works on the network layer and if a sending device does not break the data into the similar packets as the receiving device then network layer split the data into the smaller units and at the receiving end the network layer reassemble the data.

Network layer routes the packets according to the unique network addresses. Router works as the post office and network layer stamps the letters (data) for the specific destinations.

Protocols: These protocols work on the network layer IP, ICMP, ARP, RIP, OSI, IPX and OSPF.

Network Devices: Network devices including Router, Brouter, Frame Relay device and ATM switch devices work on the network layer.

Layer 2:Data Link Layer

Defines procedures for operating the communication links

Frames packets

Detects and corrects packets transmit errors

Protocols: Logical Link Control

• error correction and flow control

• manages link control and defines SAPs

802.1 OSI Model

802.2 Logical Link Control

Media Access Control

• communicates with the adapter card

• controls the type of media being used:

802.3 CSMA/CD (Ethernet)

802.4 Token Bus (ARCnet)

802.5 Token Ring

802.12 Demand Priority

Network Devices: Bridge

Switch

ISDN Router

Intelligent Hub

NIC

Advanced Cable Tester

Layer 1: Physical Layer

Physical layer defines and cables, network cards and physical aspects. It defines raw bit stream on the physical media. It also provides the interface between network and network communication devices. It is also responsible for how many volts for 0 and how many for 1. Physical layer also checks the number of bits transmitted per second and two ways or one way transmission. Physical layer also dealing with the optical, mechanical and electrical features.

Protocols: Protocols that work on the physical layer are ISDN, IEEE 802 and IEEE 802.2

Network Devices: Hubs, Repeaters, Oscilloscope and Amplifier works on the network devices.

Layer 1+2 protocols

• Ethernet
• GFP ITU-T G.7041 Generic Framing Procedure
• OTN ITU-T G.709 Optical Transport Network also called Optical Channel Wrapper or Digital Wrapper Technology

Layer 2 protocols (Data Link Layer)

• ARCnetre Attached Resource Computer NETwork
• ARP Address Resolution Protocol
• RARP Reverse Address Resolution Protocol
• CDP Cisco Discovery Protocol
• DCAP Data Link Switching Client Access Protocol
• Dynamic Trunking Protocol
• Econetr
• FDDI Fiber Distributed Data Interface
• Frame Relay
• ITU-T G.hn Data Link Layer
• HDLC High-Level Data Link Control
• IEEE 802.11 WiFi
• IEEE 802.16 WiMAX
• LocalTalk
• L2F Layer 2 Forwarding Protocol
• L2TP Layer 2 Tunneling Protocol
• LAPD Link Access Procedures on the D channel
• LLDP Link Layer Discovery Protocol
• LLDP-MED Link Layer Discovery Protocol - Media Endpoint Discovery
• PPP Point-to-Point Protocol
• PPTP Point-to-Point Tunneling Protocol
• Q.710 Simplified Message Transfer Part
• NDP Neighbor Discovery Protocol
• RPR IEEE 802.17 Resilient Packet Ring
• SLIP Serial Line Internet Protocol (obsolete)
• StarLANr
• STP Spanning Tree Protocol
• Token ring is not a protocol but is a topology
• VTP VLAN Trunking Protocol

Layer 2+3 protocols

• ATM Asynchronous Transfer Mode
• Frame relay, a simplified version of X.25 welcome
• MPLS Multi-protocol label switching
• X.25

Layer 1+2+3 protocols

• MTP Message Transfer Part
• NSP Network Service Part

Layer 3 protocols (Network Layer)

• CLNP Connectionless Networking Protocol
• EGP Exterior Gateway Protocol
• EIGRP Enhanced Interior Gateway Routing Protocol
• ICMP Internet Control Message Protocol
• IGMP Internet Group Management Protocol
• IGRP Interior Gateway Routing Protocol
• IPv4 Internet Protocol version 4
• IPv6 Internet Protocol version 6
• IPSec Internet Protocol Security
• IPX Internetwork Packet Exchange
• SCCP Signalling Connection Control Part
• AppleTalk DDP

Layer 3 protocols (Network Layer management)

• IS-IS Intermediate System-to-Intermediate System
• OSPF Open Shortest Path First
• BGP Border Gateway Protocol
• RIP Routing Information Protocol
• ICMP Router Discovery Protocol: Implementation of RFC 1256
• Gateway Discovery Protocol (GDP) is a Cisco protocol similar to IRDP

Layer 3.5 protocols

• HIP Host Identity Protocol

Layer 3+4 protocol suites

• AppleTalk
• DECnet
• IPX/SPX
• Internet Protocol Suite
• Xerox Network Systems

Layer 4 protocols (Transport Layer)

• AH Authentication Header over IP or IPSec
• ESP Encapsulating Security Payload over IP or IPSec
• GRE Generic Routing Encapsulation for tunneling
• IL Originally developed as transport layer for 9P
• SCTP Stream Control Transmission Protocol
• Sinec H1 for telecontrol
• SPX Sequenced Packet Exchange
• TCP Transmission Control Protocol
• UDP User Datagram Protocol

Layer 5 protocols (Session Layer)

• 9P Distributed file system protocol developed originally as part of Plan 9
• NCP NetWare Core Protocol
• NFS Network File System
• SMB Server Message Block
• SOCKS "SOCKetS"

Other protocols

• Controller Area Network (CAN)

Layer 7 protocols (Application Layer)

• ADC, A peer-to-peer file sharing protocol
• AFP, Apple Filing Protocol
• BACnet, Building Automation and Control Network protocol
• BitTorrent, A peer-to-peer file sharing protocol
• BOOTP, Bootstrap Protocol
• CAMEL, an SS7 protocol tool for the home operator
• Diameter, an authentication, authorization and accounting protocol
• DICOM includes a network protocol definition
• DICT, Dictionary protocol
• DNS, Domain Name System
• DHCP, Dynamic Host Configuration Protocol
• ED2K, A peer-to-peer file sharing protocol
• FTP, File Transfer Protocol
• Finger, which gives user profile information
• Gnutella, a peer-to-peer file-swapping protocol
• Gopher, a hierarchical hyperlinkable protocol
• HTTP, Hypertext Transfer Protocol
• IMAP, Internet Message Access Protocol
• Internet Relay Chat (IRC)
• ISUP, ISDN User Part
• XMPP, an instant-messaging protocol
• LDAP Lightweight Directory Access Protocol
• MIME, Multipurpose Internet Mail Extensions
• MSNP, Microsoft Notification Protocol (used by Windows Live Messenger)
• MAP, Mobile Application Part
• NetBIOS, File Sharing and Name Resolution protocol - the basis of file sharing with Windows.
• NNTP, News Network Transfer Protocol
• NTP, Network Time Protocol
• NTCIP, National Transportation Communications for Intelligent Transportation System Protocol
• POP3 Post Office Protocol Version 3
• RADIUS, an authentication, authorization and accounting protocol
• Rlogin, a UNIX remote login protocol
• rsync, a file transfer protocol for backups, copying and mirroring
• RTP, Real-time Transport Protocol
• RTSP, Real-time Transport Streaming Protocol
• SSH, Secure Shell
• SISNAPI, Siebel Internet Session Network API
• SIP, Session Initiation Protocol, a signaling protocol
• SMTP, Simple Mail Transfer Protocol
• SNMP, Simple Network Management Protocol
• SOAP, Simple Object Access Protocol
• STUN, Session Traversal Utilities for NAT
• TUP, Telephone User Part
• Telnet, a remote terminal access protocol
• TCAP, Transaction Capabilities Application Part
• TFTP, Trivial File Transfer Protocol, a simple file transfer protocol
• WebDAV, Web Distributed Authoring and Versioning
• DSM-CC Digital Storage Media Command and Control