Distributed system

A collection of independent computers that appears to its

users as a single coherent system.

The internet meets this definition. Independent OS, hardware configuration, all of it. Wants it to run, but appear as a single coherent system. Independent and coherent.

OSI layers vs TCP/IP

(7vs 5)

OSI                                                TCP/IP

Application                                   Application

Presentation                                Transport (host to host)

Session                                          Internet

Transport                                     Network Access

Network                                       Physical

Data Link

Physical

OSI

reference was developed by ISO

What does OSI stand for?

What does ISO stand for?

Open Systems Interconnection

International Organization for Standardization

The OSI model

Wht does it do?

What is a layer?

What is an instance?

• The Open Systems Interconnection model (OSI model) is a product of the Open Systems InterconnectionInternational Organization for Standardization. effort at the
• It is a way of sub-dividing a communications system into smaller parts called layers.
  
• A layer is a collection of conceptually similar functions that provide services to the layer above it and receives services from the layer below it.
• On each layer an instance provides services to the instances at the layer above and requests service from the layer below.
  • Instance - the execution of any running process.

What is an overhead bit?

In data transmission and telecommunication, overhead bits are nondata bits necessary for transmission (usually as part of headers, checksums, and such)

SNA

Standard Network Architecture.

Created by IBM, existed before OSI model.

Physical layer

• 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.
• The Physical Layer consists of the basic 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.

Data Link Layer

Number?

Function?

• Within the semantics of the OSI network architecture, the Data Link Layer protocols respond to service requests from the Network Layer and they perform their function by issuing service requests to 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. In 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. 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.

Network Layer

Number?

Function?

• The Network Layer is responsible for routing packets delivery including routing through intermediate routers, whereas the Data Link Layer 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.
  • Connection model
  • Host addressing
  • Message forwarding

Transport Layer

Number?

Function?

The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail. The Transport layer also provides the acknowledgement of the successful data transmission and if no error free data was transferred then sends the next data. (layer 4)

Session Layer

Number?

Function?

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). (Layer 5)

**This one is largely not used anymore.

Presentation layer

Number?

Function?

• 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. Note: An example of a presentation service would be the conversion of an EBCDIC-coded text 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 form of datagrams or packets between hosts. This layer deals with issues of string representation. (layer 6)
• **This layer is no longer really used, since most applications now allow character encoding on the Application layer.
  

Application layer

Number?

Function?

The common application layer services provide semantic conversion between associated application processes. Note: Examples of common application services of general interest include the virtual file, virtual terminal, and job transfer and manipulation protocols. (layer 7)