Network Models
A network is a combination of hardware and software that enable transmission of data from one location in the network to another
H/W : - transmission medium (cable, fiber optic, wireless), network cards, hubs/switches/router
S/W : - protocols and standards, applications
Layered Levels
- Basically for all communication systems there are multiple layers.
- Each layer is responsible for its own task, but to complete that task needs help from the layer below
- The lowest layer is always the transmission medium (there cannot be anything lower than that)
- The layers should be present in both sides of the communication (i.e. in the source and destination)
- The data moves through the reverse order in the destination when compared to the source
For the example above:
- Highest level is the user (which includes the sender and the receiver)
- The middle level is the post box
- The lower level is the post office
- The lowest level is the delivery van, or delivery motorcycle etc.
So in the sender side we have:
1) Highest layer: The sender writes the letter, and seals the letter in an envelop. The envelop is dropped in the mailbox
Protocol: the sender must put a destination address, source address, and postage stamp in the envelop. Without this, the mail wont be delivered
2) Middle layer: The postman carries the envelop from the mailbox to the post office
Protocol: the postman may verify that the letter has the appropriate postage stamp
3) Lower level: The post office receives lots of letter from the area it serves. The letters are sorted according to destination, and transferred to appropriate carriers
Protocol: Sort the letter
- domestic vs international,
- same area in the city vs different area in the city
- same city vs different city,
- same region vs different region
- check if the postage stamp in the envelope is sufficient for the letter to be delivered
- put an approval seal in the envelope on top of the postage stamp
Carrier phase - Lowest layer connecting the sender side to the receiver side:
- Carriers can be vans, motorcycle, ships, airplanes...
Receiver side:
l) Lower level: The destination post office receives the letter, and sorted for various areas served by the post office
2) Middle level: The postman will deliver the mail to the appropriate mailbox, which will be in he
3) Higher level: The receiver will pick up the mail from the mailbox, opens it and read
As mention before:
Hierarchy
- All tasks must be done in the order specified in the hierarchy
- The order is followed in reverse for the receiver side
- Each layer uses the services of the layer immediately below it
Two network models
- OSI : Open Systems Interconnection model.
- Developed by ISO (International Standards Organization)
- Dominant before the 1990s
-However the OSI model was never fully implemented, and did not gain prominence.
- TCP/IP: Transport Control Protocol / Inter-networking Protocol
- First introduced as a landmark paper on TCP by Vint Cerf and Bob Kahn as part of the "Inter-netting Project" in 1973.
- Outlined the protocols to achieve the end to end delivery of packets through a network
- Soon split by authorities into TCP/IP, where IP would handle the datagram routing, while TCP handled the higher level functions such as segmentation, reassembly, and error detection.
- During the 1990s, the TCP/IP model became gained prominence over the OSI model
- It was used and extensively tested in the Internet.
Open System Interconnection (OSI) network model
- First introduced by ISO during the late 1970s
- An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture
- Purpose: to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software.
- OSI is not a protocol, it is merely a model for understanding and designing a robust, flexible, and inter-operable network
- OSI is a layered framework for the design of network systems
- Consists of 7 separate but related layers
- Each layer defines a part of the process of moving information across a network
- Application Layer: The topmost layer, and the layer which a user is familiar with
- Presentation Layer
- Session Layer
- Transport Layer
- Network Layer
- Data-link Layer
- Physical Layer: The lowest layer, and consist of the actual transmission medium
- The lowest layer is designated as the 1st layer, and the highest layer is designated as the 7th layer
Layered Architecture
OSI is an example of Layered Architecture
For a message to travel from User A to User B, it must first travel down from Layer 7 of User A to the Layer 1 of User A, go the from User A to User B via Layer 1, and then travel up from Layer 1 of User B to Layer 7 of User B.
- E.g. User A sending email to User B
- In the process of going from User A to User B, the message may go through many intermediate nodes
- These intermediate nodes usually involves only the first 3 layers of the OSI model (Physical, Data-link, and Network Layers)
- Layer 1 is always present from User A till User B
- Restating Again: Each layer calls upon the services of the layer below it to function
- Restating Again:Each layer in turn provides services to the layer above it
- By defining and localizing the functionality as separate layers, the architecture of the OSI model is:
- Comprehensive and flexible.
- Allow complete interoperability between incompatible systems
- Interfaces between Layers:
- The passing of data and network information between adjacent layers is made possible by interfaces
- Interface defines the information and services a layer must provide to the layer above it
- Organization of the layers:
- The layers can be grouped together into three subgroups
- Layer 1, 2, and 3 are network support layers
- Network support layers are a combination of hardware and firmware
- Layer 1 is completely physical hardware
- Network support layer deal with physical aspects of moving data from one device to another device
- electrical specifications
- physical connections
- physical addressing
- transport timing
- reliability
- Layer 5, 6, 7 are user support layers
- User support layer provide interoperability between unrelated software systems
- User support layer is implemented mostly in software
- Layer 4 links the two subgroups and ensures that what the lower layers have transmitted is in a form the upper layers can use
Encapsulation
Data from each layer is "encapsulated" in the layer below it. It means that the layer "encapsulating" the data packet from the layer above does not know which portion of the encapsulated packet is actual data, and which portion is the header. Instead the layer treats the whole packet as data.
Each layer adds a header to the data, except for Layer 2, which adds both a header and a trailer
For the layer below, the data, the header, and possibly the trailer from layer above is treated as one data packet
In layer 1, the packet is converted into electrical signals (0s and 1s) that are then transmitted
When the message reach the receiver, the reverse process occurs, and headers and trailers are removed from the data packet by each layer, until the original data is recovered