OSI Model – The Lower Layers
The bottom four layers of the OSI Model are often referred to as the lower layers:
• Layer-4 – Transport layer
• Layer-3 – Network layer
• Layer-2 – Data-Link layer
• Layer-1 – Physical layer
Protocols that operate at these layers control the end-to-end transport of data between devices, and are implemented in both software and hardware.
OSI Model – The Transport Layer
The Transport layer (Layer-4) does not actually send data, despite its name. Instead, this layer is responsible for the reliable transfer of data, by ensuring that data arrives at its destination error-free and in order.
Transport layer communication falls under two categories:
• Connection-oriented – requires that a connection with specific agreed-upon parameters be established before data is sent.
• Connectionless – requires no connection before data is sent.
Connection-oriented protocols provide several important services:
• Segmentation and sequencing – data is segmented into smaller pieces for transport. Each segment is assigned a sequence number, so that the receiving device can reassemble the data on arrival.
• Connection establishment – connections are established, maintained, and ultimately terminated between devices.
• Acknowledgments – receipt of data is confirmed through the use of acknowledgments. Otherwise, data is retransmitted, guaranteeing delivery.
• Flow control (or windowing) – data transfer rate is negotiated to prevent congestion.
The TCP/IP protocol suite incorporates two Transport layer protocols:
• Transmission Control Protocol (TCP) – connection-oriented
• User Datagram Protocol (UDP) – connectionless
OSI Model – The Network Layer
The Network layer (Layer-3) controls internetwork communication, and has two key responsibilities:
• Logical addressing – provides a unique address that identifies both the host, and the network that host exists on.
• Routing – determines the best path to a particular destination network, and then routes data accordingly.
Two of the most common Network layer protocols are:
• Internet Protocol (IP)
• Novell’s Internetwork Packet Exchange (IPX).
IPX is almost entirely deprecated. IP version 4 (IPv4) and IP version 6 (IPv6) are covered in nauseating detail in other guides of OSI Model.
OSI Model – The Data-Link Layer
While the Network layer is concerned with transporting data between networks, the Data-Link layer (Layer-2) is responsible for transporting data within a network.
The Data-Link layer consists of two sublayers:
• Logical Link Control (LLC) sublayer
• Media Access Control (MAC) sublayer The LLC sublayer serves as the intermediary between the physical link and all higher layer protocols. It ensures that protocols like IP can function regardless of what type of physical technology is being used.
Additionally, the LLC sublayer can perform flow-control and error checking, though such functions are often provided by Transport layer protocols, such as TCP. The MAC sublayer controls access to the physical medium, serving as mediator if multiple devices are competing for the same physical link. Datalink layer technologies have various methods of accomplishing this Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD), and Token Ring utilizes a token.
The Data-link layer packages the higher-layer data into frames, so that the data can be put onto the physical wire. This packaging process is referred to as framing or encapsulation. The encapsulation type will vary depending on the underlying technology in OSI Model.
Common Data-link layer technologies include following:
• Ethernet – the most common LAN data-link technology
• Token Ring – almost entirely deprecated
• FDDI (Fiber Distributed Data Interface)
• 802.11 Wireless
• ATM (Asynchronous Transfer Mode)
The data-link frame contains the source and destination hardware (or physical) address of OSI Model. Hardware addresses uniquely identify a host within a network, and are often hardcoded onto physical network interfaces. However, hardware addresses contain no mechanism for differentiating one network from another, and can only identify a host within a network. The most common hardware address is the Ethernet MAC address OSI Model.
OSI Model – The Physical Layer
The Physical layer (Layer-1) controls the signaling and transferring of raw bits onto the physical medium. The Physical layer is closely related to the Data-link layer, as many technologies (such as Ethernet) contain both data-link and physical functions in OSI Model.
The Physical layer provides specifications for a variety of hardware:
• Connectors and transceivers
• Network interface cards (NICs)
• Wireless radios
Physical-layer devices and topologies are covered extensively in other OSI Model guides.