ARASH HABIBI LASHKARI
PUBLICATIONS
  Qualifications and Certificates
  Technical Committee Member
  Professional Experience
  Research & Developme
  Awards and Honors
  In the News
  Pulications
  Teaching
  Supervising
  Workshops
  Auxiliary Activities
Introduction to Computer Networks - Network Layers:
Bus
Seven Layer Model
The OSI model divides the complex task of computer-to-computer communications, traditionally called internetworking, into a series of stages known as layers. Layers in the OSI model are ordered from lowest level to highest. Together, these layers comprise the OSI stack. The stack contains seven layers in two groups namely Upper layers and Lower layers that are shown in the Figure below as Application and Data Transport Layers.
Bus
Introduction
When we browse the Internet, a physical connection allows for us to connect to the internet, either through a modem or through an Ethernet card in the case of a dedicated connection. A TCP/IP stack allows us to pass traffic and resolve web sites to IP addresses. Finally, applications, such as Netscape and Eudora, allow us to see the web sites and receive our e-mail.
The modem or Ethernet function has 2 parts. The modem or Ethernet drivers provide the computer with a way to communicate with the hardware. The PPP connection, also known as Dial-up Networking, allows your computer to access the modem. These two components provide the basis of getting a connection to the Internet.
The TCP/IP stack allows the computer to pass traffic across the link to the Internet in a meaningful way. That is, the TCP/IP stack allows your computer to speak the same "language" as the equipment at the other end of your connection. The TCP/IP stack also allows you to resolve friendly host names, such as www.verio.net, into an IP (Internet Protocol) address. Without the TCP/IP stack, we would be forced to go to each web site by its IP address instead of a name!
Finally, the applications allow us to interact with friendly software to interpret HTML code into web pages for us, interact with mail servers to exchange e-mail, connect to news servers to retrieve and post news articles, and exchange data with FTP servers to allow us to download files. Without these programs, the Internet would be much more difficult to navigate through.
Why were Layers created?
The principles that were applied to arrive at the seven layers are as follows:
• A layer should be created where a different level of abstraction is needed.
• Each layer should perform a well defined function.
• The function of each layer should be chosen in accordance with developing internationally standardized protocols.
• The layer boundaries should be chosen to minimize the information flow across the interfaces.
• The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy.
Having a way of categorizing each factor in an internet connection makes it easier for us to do our jobs as trouble-shooters.
We all inherently understand that if the modem is not plugged in, you're not going to be able to get your e-mail. The OSI model allows us to follow that logic further: for example, if you can browse the web by IP but can't see websites by name, you know that the problem is not on the Network layer, but on the Transport layer.
How Encapsulation Works on Layers?
The seven OSI layers use various forms of control information to communicate with their peer layers in other computer systems. This control information consists of specific requests and instructions that are exchanged between peer OSI layers. Control information typically takes one of two forms:
Header- Headers are prepended to data passed down from upper layers.
Trailer- Trailers are appended to data passed down from upper layers.
Bus
Imagine that System A is requesting information from System B. System A makes an HTTP (Layer 7) request, which gets prepended with a header and appended with footer. Layer 6 specifies whether it's a request for a GIF or an HTML document, and treats the Layer 7 header, data, and footer as its own data, prepending that with a header and appending it with footer. The same treatment happens on Layer 5, and so on.
System B receives the request on Layer 1, and begins the decapsulation process, stripping the Layer 1 headers and footers off to reveal the Layer 2 information, and so forth, all the way up to the 7th layer.
How Encapsulation Works on Layers?
The seven OSI layers use various forms of control information to communicate with their peer layers in other computer systems. This control information consists of specific requests and instructions that are exchanged between peer OSI layers. Control information typically takes one of two forms:
Header- Headers are prepended to data passed down from upper layers.
Trailer- Trailers are appended to data passed down from upper layers.
Application Layer
This layer provides the interface between applications and the Network Operating System (NOS). The Application layer provides network services and applications such as HTTP, FTP, TELNET and SMP.
Although no real world network operating systems conform exactly to the OSI model, it would be difficult to describe the operation of network devices and protocols without using the model as a reference.
Presentation Layer
At this layer applications communicate on a format for exchanging data. The Session layer provides character set conversion and formats the data. It performs encryption and decryption, compression and decompression.
Session Layer
This layer authenticates security and establishes a connection ID. It establishes, synchronizes, maintains and ends sessions. Remote Procedure Call (RPC) is a protocol that runs at the Session layer.RPC allows communication between processes on different systems.
Transport Layer
The Transport layer (OSI layer 4) uses connection-oriented protocols to provide a reliable end-to-end connection between the source computer and the destination computer. Transmission Control Protocol (TCP) is a transport layer protocol that provides flow control, multiplexing, error detection and recovery. At the transmitting end, the message is broken into smaller segments and each segment is given a sequence number. At the receiving end the segments are checked for errors. If the segments are received error free, they are reassembled in the proper sequence and an acknowledgement is sent to the transmitting computer. If the transmitting computer does not receive an acknowledgement, it resends the segments.
Network Layer
The Network layer (OSI layer 3) uses routable protocols to deliver data packets to networks connected through routers. To route data packets, the Network layer uses logical addressing. Routing is the process of moving data packets from one network or network segment to another. Each LAN or network segment has a unique logical address. The Network layer protocol adds a source node address and a destination node address to each data packet. A packet that has a destination not on the local subnet is sent to a node called a default gateway that can communicate outside the local subnet. Inter-network Packet Exchange (IPX) is a NetWare protocol that performs the Network layer functions for a NetWare IPX/SPX network. The IPX protocol uses a 32-bit network address and a 48-bit node address. IPX/SPX was a popular protocol suite for several years, but it has been replaced by the TCP/IP suite on most networks.
Data Link Layer
The Data Link layer (OSI layer 2) contains two sub-layers; the Logical Link Control (LLC), and the Media Access Control (MAC). IEEE specification 802.2 defines the LLC, while the IEEE specifications 802.3 and 802.5 define the MAC for Ethernet and Token Ring. All hosts on a network, including network devices such as printers and routers, must have a unique identifier called the Media Access Control address. The Data Link layer uses MAC addresses is to pass data frames from the Physical layer to the Network layer and vice versa. The use of MAC addresses permits the direction of data within the same network, but not across routers.
Bus
Logical Link Control Sub-layer
The Logical Link Control (LCC) sublayer of the data link layer manages communications between devices over a single link of a network. LCC is defined in the IEEE 802.2 specification. IEEE 802.2 defines a number of fields in data link layer frames that allow multiple higher-layer protocols to share a single physical data link. LLC supports both connectionless and connection-oriented services used by higher-layer protocols.
Media Access Control Sub-layer
The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which allow multiple devices to uniquely identify one another at the data link layer.
Bus
Physical Layer
The Physical layer (OSI layer 1) deals with the mechanical and electrical specifications of the network hardware. Layer 1 specifications define connectors, pin-outs, signal voltages, and related software.
The most common Physical layer component is the Network Interface Card (NIC). To install a NIC you need to assign computer resources such as an IRQ and I/O address. If the operating system and NIC are Plug-and-Play (PnP), these resources will be assigned automatically.
A repeater is a Physical layer device that amplifies, reconstructs and retransmits the signal, allowing you to extend the length of a network cable.
Bus
--------------------------------------------------------------
Highlights
OSI Model
TCP/IP Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Logical Link Control (LLC)
Media Access Control (MAC)
Physical Layer
New Brunswick, Canada     A.Habibi.L@gmail.com    A_Habibi_L@hotmail.com