Two models:
OSI Model, TCP/IP model.
OSI Model:
(open system interconnection)
- On the sender side a message will start at the top layer, descend the layers to the transmission media, and then travel up through the layers at the receiver's end.
- Transmission medium can be physical wire, or wireless..
- OSI was introduced to create some kind of standardization for how systems work globally
- OSI Model consists of seven layers, each with its own responsibilities
→ Data Link and Physical layers are typically associated with hardwaer
→ The remaining 5 are software based.
Physical layer
- bottom layer. related with hardware
- where the physcial networking medium operates, e.g. cables, devices
- the following things are monitored:
→ data rate: how much data can be transferred per unit of time
→ transmission mode
⇒ simplex
• one sender, one receiver
• the direction of transmission is fixed
• you can always give commands to your printer from your computer, but not vice versa
⇒ half duplex
• sender and receiver can both transmit data, but not at the same time
• e.g. walkie-talkies
⇒ full duplex
• bandwidth is shared, both sender and receiver can transmit data at the same time
• e.g. instant messaging
⇒ physical layer dictates which mode is used
→ physical topology
⇒ bus topology
⇒ star topology
⇒ mesh topology
⇒ ring topology
→ physical characteristics of interfaces and medium
⇒ everything has its own constraints and conditions
⇒ what connectors... hardware... etc
Data Link Layer
- MAC (media access control) addressing is used at this layer
- each device has a chip, network interface card, which will have an address
- MAC addr is a 48 bit address specifc to the device
- Data link layer has frames
- source and dest addr are on every frame
- main tasks of data link layer:
→ framing
→ physical layer addressing
→ flow control
⇒ mechanism by which the speed of transmission is controlled e.g. if the sender's speed exceeds receivers capacity.
→ error control
→ access control
⇒ if your bandwidth is shared, it needs to be determined who will send at what times
Network Layer
- IPv4, length 32 bits
- IPv6, length 128 bits
- Network layer handles logical addressing, ip addressing
- data is in “packets” at this layer
- routing is handled at this layer
Transport Layer
- Transport layer ensures reliable transmission of data by providing sequencing and acknowledgement notification
- main tasks of this layer
→ service point addressing
⇒ ip address + port number
⇒ e.g. 192.168.1.1/80
⇒ port numbers less than 1024 are standard port numbers
⇒ general purpose are > 1024
→ segmentation and reassembly
⇒ data is in “segments” at this layer
⇒ data can arrive in the wrong order or with missing pieces, transport layer rearranges and notices missing ones.
⇒ segments contain sequencing information
→ connection control
⇒ connectionless
• sender just transmits the data and hopes the receiver gets it
• when speed is priority and data loss can be tolerated, connectionless is preferred
⇒ connection oriented
• both parties have to “shake hands” and synchronize to send data
→ access control
→ error control
Session Layer
- responsible for establishing and maintaining communication between nodes
- decides which node will communicate first and for how long
- main tasks:
→ dialog control
→ synchronization
Presentation Layer
- “bridge” layer
- serves as a translator between application and other layers
- main tasks
→ translation
→ encryption
→ compression
Application Layer
- provides interfaces to the software that enables programs to use network services
- Mail services, directory services, virtual terminal, file transfer
Questions:
- Compression is performed by the Presentation layer
- Task of the Session layer: dialog control
- Service point addresses consist of IP addresses and Port numbers
Index