1. visualization
- Figure 3.2, p. 36
- logically, 7 layers of functionality at sender station and at receiver station
- layers depicted vertically, with layer 7 at top and 1 at bottom (protocol stack)
- sender depicted to left of receiver, with gap in between
2. interface and protocol
- on sender side, info flows from layer n to layer n-1
- on receiver side, info flows from layer n-1 to layer n
- interface
is services that layer n-1 provides layer n
- virtual peer-peer communication between sender layer n and receiver layer n
- protocol
is rules and conventions for virtual peer-peer communication
3. headers
- Figure 3.3, p. 38
- protocol for each layer requires additional information added to data as header
- header added at each layer on sender side
- header stripped at each layer on receiver side
- examples of header info: checksum, sender/receiver address, service type
- layer n software considers layer n+x (x=1,2,..) headers as data (payload)
Layer 1: Physical layer (Fig 3.4, p 39)
Purpose: convert between bits and signals, transmit over physical communication medium
Major issues:
- what type of signals?
- how are bits represented by signals?
- what physical medium to use?
- what is network topology (interconnection pattern)?
- what devices does physical network connection require?
Layer 2: Data Link Layer (Fig 3.5, p 40)
Purpose: error-free delivery of frames (data+header+trailer) from one station to next
Major issues:
- what info goes into header/trailer?
- how are stations addressed?
- how are transmission errors detected?
- how are transmission errors corrected?
- how is frame transmission synchronized? (e.g. bit pattern)
- how is transmission flow between stations regulated?
- how is access to shared transmission channel regulated?
For LANs, this layer has been subdivided into logical link control (LLC) and media access control (MAC) sublayers
Layer 3: Network layer (Fig 3.7, p 42)
Purpose: control subnet operation -- source to destination delivery of packet (header+frame)
Major Issues:
- what info goes into header? (e.g. end-end (logical) addresses: source and destination)
- how is packet switched when it arrives at intermediate node (IMP*)?
- how is packet routed from source to destination?
- how is transmission flow between network nodes regulated?
- how is internetworking handled (passing packet between different network technologies)?
Note: network layer addresses are logical, not physical, because they must be uniform across different underlying (data link) technologies.
Layer 4: Transport layer (Fig 3.9, p 44)
Purpose: source to destination delivery of messages (collection of packets). Note: this is the first end-end layer. Considers source to be directly connected to destination.
Major Issues:
- what info goes into header?
- how is message segmented into packets?
- how are packets reassembled into messages?
- how are packets delivered to correct application (port address)?
- is connection between source and destination maintained? and if so, how?
Layer 5: Session layer (Fig 3.11, p 46)
Purpose: provide session-oriented services between source and destination
Major Issues:
- what info goes into header?
- how is connection established?
- how is transmission synchronized?
- how is connection terminated?
- how is connection resumed if service interrupted?
Layer 6: Presentation layer (Fig 3.12, p 47)
Purpose: provide common data representation services
Major Issues:
- what info goes into header?
- how will data be translated into format understood by destination?
- will data be encrypted/decrypted? and if so, how?
- will data be compressed/uncompressed? and if so, how?
- what other security features are necessary?
Layer 7: Application layer (Figure 3.13, p 48)
Purpose: provide specific network application software to users
Major Issues:
- what network services does user require?
- what is the most effective user interface?
- etc, etc.