What are the seven layers of the OSI model?
- Application Layer - user interface to network services such as file transfer/database access, terminal emulation, etc. (e.g., RPCs) ·
- Presentation Layer - translates data format; data compression; redirector operations; network security (e.g., SMBs & NCPs)
- Session Layer - establish/maintain/terminate a connection; synchronization between user tasks with checkpoints, two way communication; name recognition (e.g., NetBIOS & WinSock)
- Transport Layer - Flow control; ensures reliable delivery of packets in sequence with no losses or duplications (e.g., TCP & UDP)
- Network Layer - translation of logical and physical addresses; determines route from source to destination; traffic control (IP & IPX)
- Data Link Layer - converts frames into bits (send side) and bits into frames (receive side); performs error checking; resends if no acknowledgement (e.g, the NIC driver)
- Physical Layer - transportation of raw (i.e., binary) data; defines cable & signaling specifications (e.g., the NIC & cable)
In real life things are never quite as precise as they are in models. For practical purposes you are generally only concerned with three 'composite' layers: 1) the 'lower layers' (physical and data link layers) that define the network architecture (e.g., Ethernet); 2) the 'network' or 'internetwork' layer (network and transport layers) where most of the addressing and logical transport functions are performed (e.g., TCP/IP); and the upper or 'application' layer (session, presentation, and application) where high level networking functions take place (e.g., redirector and RPCs). The boundaries between these 'composite' layers are separated by well-defined interfaces which on a Microsoft network are called NDIS and TDI.
Exam Tip. For purposes of certification testing, the following neumonic can be helpful in memorizing the names and order of the OSI layers:
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