Written by Layer 1 of the OSI model is all about the physical transport of data from one computer to the next. It describes the connectors, cables, how data is transmitted over the air, as well as ‘dumb’ devices that helps the data get from point a to point b. This layer is all about specifications, mainly from IEEE (Institute of Electrical and Electronics Engineers,) TIA (Telecommunications Industry Association) and EIA (Electronic Industries Alliance.)
Layer 1 describes the medium itself and how to transmit on the medium. An example of layer 1 would be category 5 network cable. A network card is not a layer 1 device, but a layer 2 device (layer 2 devices will be described in another post.) The difference between a layer 1 device and a device on any of the layers of the OSI model is that layer 1 devices only transmit the data. A layer 1 device does not read any of the data or information from the transmitted signal. Network cards are layer 2 devices, so it reads the layer 2 PDU from the transmitted data.
I think I mentioned a few times that network cable operates at layer 1 of the OSI model. A repeater is another layer 1 device. Repeaters are used for longer cable runs. A repeater simply amplifies the signal. That is it. They do not read the information of the data at all. A multi-port repeater is called a hub. Hubs provide the ability to connect multiple computers together at a central point. There are two kinds of hubs: active and passive. An active hub is a powered hub that amplifies the signal. Passive hubs do not. It is very hard to find a hub nowadays. Switches are now mostly used. They operate at layer 2 of the OSI model (explained in a later post.)
There are a lot of different network cables. Copper cables can be divided into four categories: STP (Shield Twisted Pair,) UTP (Unshielded Twisted Pair,) coaxial and serial (that’s right, RS232 can be used in networks.) We will ignore RS232 serial, since that is old school used back in the 1980s. We will look a the other three:
Coaxial
Coaxial consists of an outer jacket surrounding a braided copper shielding, which surrounds a copper conductor. The copper shielding protects the signal from interference. However, it needs to be properly grounded or it will act like a big antenna. Coaxial is used from your cable company. However, it is a different kind than what is used for local computer networks. The backbone coaxial cable for local computer networks are known as thicknet. Thicknet is known as 10base5. This means that it transmits at 10 megabits and will go a distance up to 500 meters before having to be repeated. Thinnet, also known as ‘cheapnet,’ uses a thinner coaxial cable than thicknet. It is 10base2. This means that it transmits at 10 megabits and transmits up to 185 meters before a repeater is needed (the 2 is the rounded 200 meters.)
STP versus UTP cable
From looking at what these abbreviations mean, the obvious difference between STP and UTP is that STP is shielded. Both STP and UTP have multiple cables that are twisted in pairs. These twists create a field that cancels out noise, providing a way to protect the data. UTP is the more popular of the two. STP needs to be grounded, just like coaxial. It is most popular in Europe.
Twisted Pair Categories
Have you ever heard of Cat 5 cable? This refers to twisted pair cable that follows category 5 guidelines. What are the differences? below is a table of the categories and what they do:
| category | Speed | description |
| 1 | none | Traditional phone line. Not used in data transmission |
| 2 | 4 Mbps | No longer in use |
| 3 | 10 Mbps | Used in 10baseT networks |
| 4 | 16 Mbps | Used in Token Ring Networks |
| 5 | 100 Mbps | Transmits up to 100 Mbps (100baseT) |
| 5e (enhanced) | 1000 Mbps | Most common. Also known as ‘Gigabit Ethernet’ 1000baseT |
| 6 | 1000 Mbps | Uses ’24 American Wire Gauge (AWG).’ A little better than 5e |
When using twisted pair cabling, it is important to remember the direction of the data flow. Older networks used half-duplex. This means that a computer or device could not send and receive data at the same time. Nowadays, we use full-duplex. A device can send and receive data at the same time. How does this work? Twisted pair has one pair dedicated transmitting data and another pair dedicated receiving data. Think of the cable as two cables in one. The pair that is used to transmit data in the source device has to be used for receiving at the destination. Vice versa. Why is this important? You need to know what cable configuration to use in what situation.
Straight-through cable: Cable used to go from computer to hub/switch/router. With straight-though, the pair layout at one end of the cable is the same pair layout at the other end of the cable. The data goes ‘straight-through.’ When going from computer to hub/switch/router, the device assumes straight-through and crosses the receiving and transmitting pairs between all devices connected to it.
Cross-Over cable: Cable used to go from computer to computer, hub to hub, or switch to switch (not using the ‘up-link’ port.) Remember that the transmitting pair at one end has to go to the receiving end of the other? This is what cross-over does. it crosses the transmitting and receiving ends.
Fiber-Optics: Fiber optics is cabling that uses glass or plastic instead of copper to transmit the data. Instead of electricity, it uses light. The benefit of using light is that it is unaffected by noise. There is also virtually no resistance, so cables can be run at much greater length (try 2000 meters instead of just 100.) Since most of us are not using fiber optics, we will not get into the specifications for them.
Layer 1 is very basic. I was able to describe it in one post. However, some of the higher levels I will have to describe in multiple posts.
Resources:
http://www.ciscopress.com/articles/article.asp?p=31276
Join the forum discussion on this post
