target="_blank" rel="nofollow" href="#ulink_0a892328-a604-5242-a09a-869bb9ff0dcf">Table 1.2 Networking functions provided by each layer
| Function | 1 Physical | 2 Data Link | 3 Network | 4 Transport |
| Transmission of bitstreams over physical media | X | |||
| Enabling/disabling physical network interface | X | |||
| Node-to-node data transfer over a shared medium | X | |||
| Forwarding/routing | X | X | ||
| Error control | X | X | X | |
| Flow control | X | X | X | |
| Multiplexing/splitting | X | X | X | |
| Ordering | X | X | X | |
| Fragmentation/reassembly | X | X |
The OSI replicates some functions in most layers, blurring the distinction among them. It becomes apparent that what distinguishes the layers isn't what they do but what they don't do. Higher layers lack functionality provided by lower layers, something you'd expect given the hierarchical structure of layers. One layer whose functions differ starkly from the others is the Physical layer.
Layer 1: The Physical Layer
The main function of the Physical layer is to convert bits to electromagnetic energy such as light, electrical current, or radio waves, and transmit them over some medium such as fiber-optic or copper cables or the airwaves. Whereas the functions of the other layers are performed in software, this particular function is performed by a node's physical network interface.
A challenge of using electromagnetic energy to send bits is that the physical media can carry only one bitstream at a time. In the early days of networking, two nodes would be connected via a pair of wires. If both simultaneously sent a signal, their signals would interfere with each other and create a collision. Hence, both nodes were in the same collision domain. To avoid this, the nodes had to use half-duplex communication wherein only one node could transmit at a time. Half-duplex wired communication may seem an irrelevant relic from the past, but as you'll learn in a moment, during its heyday half-duplex communication had an unfortunate impact on the Ethernet standards that still haunts us to this day. Broadcast storms and the infamous Spanning Tree Protocols (STPs) can be traced back to the early use of half-duplex communication.
Today, full-duplex communication is the norm in wired networks and something we take for granted. All that's needed for full-duplex communication is for the physical interface to separate the transmit and receive functions. Twisted-pair copper cabling, for example, does this by using two pairs of wires: one pair for transmitting and another pair for receiving. Likewise, fiber-optic cables have separate strands for transmitting and receiving. Wavelength-division multiplexing achieves full-duplex communication on a single fiber strand by using one light frequency for transmitting and another for receiving.
Layer 2: The Data Link Layer
The primary function of the Data Link layer is to facilitate data transfer between two (and only two) nodes that are connected to a shared medium. Some physical media can support only two nodes, as is the case with a crossover cable or point-to-point serial link. Other media, such as wireless, can support more than two nodes.
When only two nodes share the same media, data transfer is easy. As long as both nodes are aware of the point-to-point nature of the link, one node can send the data, and the other node receives it, knowing that it's the intended recipient. The Point-to-Point Protocol (PPP) and High-level Data Link Control (HDLC) are two common layer 2 protocols used on T1 serial links.
But when multiple nodes can share a medium, as they did with early Ethernet, things get tricky. At this point you're rightly thinking that with the exception of wireless, nobody connects nodes to a shared medium anymore. Hubs went out of fashion long ago. Now we connect devices to switches (the marketing term for bridges). However, switches actually simulate the behavior of a shared medium. Time for a quick history lesson.
A Brief History of Ethernet
The original Ethernet standards from the 1970s were designed for nodes all connected to a shared electrical bus that often took the form of a thick yellow cable (you may have heard the term Thicknet). Whenever one node would transmit a signal, all other nodes connected to the cable would receive it. Communication was half-duplex, and all nodes were in the same collision domain. As a way of detecting errors introduced by collisions, the original Ethernet II (DIX) specification got a frame check sequence (FCS, sometimes called a cyclic redundancy check, or CRC) to detect errors. Even today as back then, nodes discard frames that fail the FCS check.
The multi-access nature of Ethernet made it necessary to assign each node's network interface a unique, 48-bit Media Access Control (MAC) address. The sending node would construct a frame that included the destination node's MAC address and the data to send. All nodes would receive the frame, but only the destination node would process it.
Now let's fast-forward to today. We still use MAC addresses, even though the original rationale for using them is long gone. To maintain backward compatibility over the years, we never got rid of them. Figure 1.3 shows the original DIX frame format that we still use today. We're still using a technology designed specifically for devices that were all sharing a thick yellow cable. Today, however, instead of nodes sharing this thick yellow cable, they're connected to a switch.
Figure 1.3 Layer 2 frame and layer 1 packet, structurally identical to the revised (1997) IEEE 802.3 format that we use today
