Communications Layers Seminar Title

Communications Layers

You may have heard the term "communications stack" or "that is a layer 3 protocol".  This section discusses the 7-layer OSI stack and will also explain how TCP/IP applies to this way of understanding the functions performed at each of the protocol layers.


RadCom did an excellent poster of showing the various protocols and the placement of these protocols with the OSI layers.  Click here to get their PDF poster (813 KB).


The OSI Reference Model

The seven-layer reference model for Open Systems Interconnection (OSI) was developed by members of the International Standards Organization (ISO). It provides a common basis for the coordination of standards for the purpose of systems interconnection.  Each layer has a unique, defined function and, theoretically, each layer is independent of the protocol layer above and below it. This independence of each layer allows a system to be constructed that can use a mix & match approach to communications that can be optimize for a given task.

Figure 1.  The OSI 7-Layer Model

Layer 1: Physical Layer

The physical layer is responsible for the actual transmission of a bit stream across a physical circuit. It allows signals, such as electrical signals, optical signals, or radio signals, to be exchanged among communicating machines.   This layer addresses the cables, connectors, modems, and other devices used to permit machines to physically communicate and controls the generation and detection of signals that are interpreted as 0 bits and 1 bits.

Layer 2: Data Link Layer

The data link layer is responsible for providing data transmission over a single connection from one system to another.  Control mechanisms in the data link layer handle the transmission of data units, often called frames, over a physical circuit. This layer is also concerned with how bits are grouped and the beginning and ending of a "frame" of data.  With some types of data links, the data link layer may also perform procedures for flow control (starting & stopping data), frame sequencing, and recovery from transmission errors.

Layer 3: Network Layer

The network layer is concerned with making routing decisions and relaying data from one device to another through the network. Within the network, intermediate systems perform routing and relaying functions.  The application programs running in two end systems that wish to communicate should not need to be concerned with the route packets take nor with how many data links they must cross.

Layer 4: Transport Layer

The transport layer builds on the services of the lower layers to ensure a reliable end-to-end data transport service.  This layer for example might the task of asking for retransmission of a missing packet or reorder packets that arrive out of sequence.  The transport layer hides from the higher layers all the details concerning the actual moving of packets and frames from one computer to another and shields network users from the complexities of network operation.

The transport layer may also control the rate at which messages flow through the network to prevent and control congestion.

Layer 5: Session Layer

The session layer is responsible for organizing the dialog between two application programs and for managing the data exchanges between them. The top three layers are more concerned with services that are oriented to the application programs themselves.  To do this, the session layer imposes a structure on the interaction between two communicating programs.

The session layer defines three types of dialogs: two-way simultaneous interaction, where both programs can send and receive concurrently; two-way alternate interaction, where the programs take turns sending and receiving; and one-way interaction, where one program sends and the other only receives.

Layer 6: Presentation Layer

The presentation layer is interested in the meaning of the bits and deals with preserving the information content of data transmitted over the network. It is concerned with three types of data syntax that can be used for describing and representing data including abstract syntax, transfer syntax, and local concrete syntax.

The presentation layers between systems negotiate a common transfer syntax to be used to transfer the messages defined by a particular abstract syntax. If the local concrete syntax in the two communicating systems are different, an implementation of the presentation layer is responsible for transforming from the local concrete syntax to the transfer syntax in the sending system and from the transfer syntax to the local concrete syntax in the receiving system.

Layer 7: Application layer:

The application layer is concerned with high-level functions that provide support to the application programs using the network for communication.  This layer provides a means for application programs to access the system interconnection facilities to exchange information.  As far as the application layer is concerned, a program running in one computer sends a message, and the program running in the other computer receives it. The application layer is not concerned with any of the details related to how the message gets from the source computer to the destination computer.


The TCP/IP Internet Layering Mode

TCP and IP are separate layers within the "communications stack" and, in reality, there are over 30 different protocols involved but generically people call the entire suite of these protocols "TCP/IP".


Broadly, the TCP/IP software is organized into four conceptual layers that build on a fifth layer of hardware. These layers are slightly different than the OSI layers but they can be mapped onto it.

  • Application Layer - Messages or streams

  • Transport Layer - Transport protocol packets

  • Internet Layer - Create IP datagrams

  • Network Interface Layer - Network-specific frames

Functions of the TCP/IP Application Layer:

TCP/IP defines a wide range of application layer protocols that provide services to network users, including remote login, file copying, file sharing, electronic mail, directory services, and network management facilities. Some application protocols are widely used, others are employed only for specialized purposes. The following are the most commonly used TCP/IP application layer protocols:

  • PING Connectivity Testing

  • Telnet Remote Login

  • Rlogin Remote Login

  • Rsh Remote Execution

  • FTP File Transfer

  • TFTP File Transfer

  • SMTP Electronic Mail

  • Kerberos Authentication

  • X Windows Presentation

  • DNS Name Resolution

  • NFS Remote File Service

  • SNMP Network Management

Functions of the TCP/IP Network Interface Layer:

The main function of the network interface layer is to handle hardware-dependent functions and to present a standardized interface to the Internet layer of TCP/IP. The TCP/IP suite of protocols does not specify details concerning the protocols to be used in the network interface layer and below.  The network interface layer of TCP/IP is responsible for accepting messages from the Internet layer and preparing them for transmission across any desired type of data link technology.


An individual TCP/IP network may be a local area network, using LAN data link protocols such as Ethernet, Token Ring, or FDDI. An individual TCP/IP network may also be implemented using a wide area network data link technology, such as a point-to-point leased or dial-up line, satellite link, or specialized digital circuit. One of the reasons TCP/IP has become widely used is that it can be used in conjunction with almost any type of underlying physical circuit and data link technology.


One important function of the network interface layer is to examine each frame that the network interface card receives and to determine, from the way in which control bits in the frame are set, for which of the internet layer protocols the frame is intended, called a demultiplexing function.

Functions of the TCP/IP Internet Layer:

The TCP/IP internet layer provides routing and relaying functions for carrying packets of data from a source system to a destination system through an internet.  This is the layer at which routing decisions are made that determine the path over which each packet travels. TCP/IP protocols that operate in the Internet layer include the Internet Protocol (IP) and the Internet Control Message Protocol (ICMP), the Address Resolution Protocol (ARP) and the Reverse Address Resolution Protocol (RARP).


IP is the core protocol of the TCP/IP protocol suite. It provides a connectionless, best-effort data delivery service that is used in moving packets from one system to another through the internet.  The ICMP employs the services of IP to allow systems to report on error conditions and to provide information about unexpected circumstances.


The ARP helps a source system deliver data directly to a destination system when the two systems are on the same physical network. It allows the source system to determine the destination system's physical hardware address given the destination system's internet address. The RARP allows a system that does not yet have its internet address to obtain it. RARP is typically used to support workstations and intelligent terminals that do not have their own disk storage.

Functions of the TCP/IP Transport Layer:

The transport layer provides an end-to-end data delivery service that application processes use to exchange messages over the internet.  Protocols operating in the transport layer use the services or IP to deliver messages. The two major TCP/IP transport layer protocols are User Datagram Protocol (UDP) and Transfer Control Protocol (TCP).


UDP is the simpler of the two transport protocols.  It is a best-effort, connectionless transport layer protocol that adds little to the underlying IP datagram delivery service. TCP is a connection-oriented transport layer protocol that provides for reliable, sequenced stream data delivery.


An application process can use either UDP or TCP to request data transfer services. The protocol that an application developer chooses to use depends on whether the application requires only a best-effort, datagram data delivery service or whether it requires the reliability controls provided by a connection-oriented data transfer service.


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