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OSI (Open Systems Interconnect) Model – 4
The two most common transport-layer protocols in use today include TCP (Transmission Control Protocol) found in TCP/IP and SPX (Sequential Packet Exchange) found in IPX/SPX. The corresponding protocol-stack network-layer protocols are IP (Internet Protocol) and IPX (Internetwork Packet Exchange).
NetBEUI, used in older Microsoft networking, is also a network-layer protocol; however it is nonroutable. When discussing the bus and star topologies, we introduced that the concept of one station listening to the cable before sending its data to ensure no other traffic was present is a characteristic of those topologies.
Noting that topologies are often driven by media-access methods, this characteristic is actually dictated by the media-access method rather than the topology. Two common media-access methods include Carrier Sense Multiple Access with Collision Detection (CSMA/CD) and token passing.
In CSMA/CD each computer or network device will listen, send data, and resend data, if needed. This method is known as a Contention Method because all devices on the network contend for access to the network wire. In token passing each computer or network device will wait for the “free token,” send the data, and then wait for acknowledgement.
The original version of Ethernet was a CSMA/CD media-access method and an IEEE (Institute of Electrical and Electronics Engineers) standard. Ethernet is the most common media-access method used in networking today. Investigators should note that although most people refer to the various implementations of Ethernet in the same way, the standard has changed significantly over time.