Ring topology - Also known as a ring network, the ring topology is a type of computer network configuration where each network computer and device is connected to each other forming a large circle (or similar shape). Each packet is sent around the ring until it reaches its final destination. Today, the ring topology is seldom used. Below is a visual example of a simple computer setup on a network using a ring topology?

Bus topology - A type of network setup where each of the computers and network devices are connected to a single cable or backbone. Below is a visual example of a simple computer setup on a network using the bus topology?

Mesh topology - A type of network setup where each of the computers and network devices are interconnected with one another, allowing for most transmissions to be distributed, even if one of the connections go down. This type of topology is not commonly used for most computer networks as it is difficult and expensive to have redundant connection to every computer. However, this type of topology is commonly used for wireless networks. Below is a visual example of a simple computer setup on a network using a mesh to topology?

Star topology - Also known as a star network, a star topology is one of the most common network setups where each of the devices and computers on a network connect to a central hub. A major disadvantage of this type of network topology is that if the central hub fails, all computers connected to that hub would be disconnected. Below is a visual example of a simple computer setup on a network using the star topology?

Tree topology - Also known as a star bus topology, tree topology is one of the most common types of network setups that is similar to a bus topology and a star topology. A tree topology connects multiple star networks to other star networks. Below is a visual example of a simple computer setup on a network using the star topology?

OSI seven-layer model

Application
The application layer interacts with software applications (such as Netscape or Outlook Express) that implement a communicating component. Such application programs are outside of the scope of the OSI model, but they translate an end-user’s typing into a Layer 7 request. Application layer functions typically include the following:
• Identifying communication partners - The application layer identifies and determines the availability of communication partners for an application with data to transmit.
• determining resource availability - The application layer must determine whether sufficient network resources for the requested communication are available.
• Synchronizing communication - Communication between applications requires cooperation that is managed by the application layer.

Presentation
The presentation layer provides a variety of encoding and encryption functions that are applied to the application layer data. These functions ensure that information sent from the application layer of one system will be readable by the application layer of another system. Some examples of presentation layer encoding and encryption schemes follow:
• Conversion of character representation formats - Conversion schemes are used to exchange information with systems using different text and data representations (such as EBCDIC and ASCII).
• Common data representation formats -the use of standard image, sound, and video formats (like JPEG, MPEG, and RealAudio) allow the interchange of application data between different types of computer systems.
• Common data compression schemes - The use of standard data compression schemes (like WinZip or GZip) allows data that is compressed at the source device to be properly decompressed at the destination.
• Common data encryption schemes - The use of standard data encryption schemes allows data encrypted at the source device to be properly unencrypted at the destination.

Session
The session layer establishes, manages, and terminates communication sessions between presentation layer entities. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer.
For example, SQL is a Session layer application that manages multiple queries to the SQL database. It's what allows multiple people to log in to, say, the Intranet at the same time.

Transport
The transport layer implements reliable internet work data transport services that are transparent to upper layers. Transport layer functions typically include the following:
• Flow control - Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process.
• Sliding Window - This allows the receiving computer to dictate to the receiving end how many packets the receiver is capable of receiving at one time.
• Multiplexing - Multiplexing allows data from several applications to be transmitted onto a single physical link.
• Virtual circuit management - Virtual circuits are established, maintained, and terminated by the transport layer.
• Three-way handshake - The three-way handshake is a connection establishment protocol. First, host A sends a SYN segment to host B in order to check that host B gets ready for establishing a TCP connection. Second, when host B receives the SYN segment that host A sent and is ready to start the TCP session, it sends a SYN and ACK segment back to host A. This ACK advertises an arrival of the first SYN segment to host A. Finally, host A sends an ACK segment for the second SYN and ACK segment that host B sent.
• Error checking and recovery - Error checking mechanisms for detecting transmission errors. Error recovery involves taking an action (such as requesting that data be retransmitted) to resolve any errors that occur.
The two most common Transport layer protocols are TCP and UDP.
Common Transport Layer Ports
21 FTP

22 SSH

23 telnet

25 SMTP

53 DNS

80 HTTP

110 POP3

143 IMAP

443 HTTPS

Network
The network layer provides routing and related functions that allow multiple data links to be combined into an internet work. This is accomplished by the logical addressing (as opposed to the physical addressing) of devices. The network layer supports both connection-oriented and connectionless service from higher-layer protocols.
Common protocols on the Network layer are BGP and OSPF. RIP is another Network layer protocol, but is not used on larger networks because of its inefficiency.

Data Link
The data link layer is where the logical information (i.e., IP addresses) is translated into the actual electrical pulses that travel over the physical layer. Frame Relay, ATM, and DSL all work on the Data Link layer.
Different data link layer specifications define different network and protocol characteristics, including the following:
• Physical addressing - Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer.
• Network topology - Data link layer specifications often define how devices are to be physically connected (such as in a bus or a ring topology).
• Error notification - Error notification involves alerting upper layer protocols that a transmission error has occurred.
• Sequencing of frames - Sequencing of data frames involves the reordering of frames that are transmitted out of sequence.
• Flow control - Flow control involves moderating the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.

Physical
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and the physical connectors to be used.
Common examples of things that work on the Physical layer are Fiber Optic cables, CAT5 (Ethernet) cables, and Copper Twisted Pair.