I’m sorry that I didn’t post for about 4 days but I was changing my blog’s theme so I coded a lot in these days. In today’s post I will talk about the last OSI layer, called the Physical Layer. I want to describe his role, functionality as well as the protocols and standards used at this layer. The Physical Layer is the actual medium in which messages are transmitted. Frames from the Data Link Layer arrive at the Physical Layer for further processing. Here the frames are encoded into streams of bits and then are sent across the network through different media types (copper wires, fiber optic, wireless connections):
Physical layer creates signals that are sent across the medium.At this stage data was passed trough all layers and each one added it’s information. As a short recap, Data was created at the Application layer then sent to the Transport layer where it was encapsulated into a Segment. Here also the source and destination port numbers were added. Then the segment was sent to the Network layer where it was encapsulated into a packet and the source and destination IP addresses was added. After this phase the packet arrived at the Data Link layer where it was encapsulated into a frame by adding source and destination MAC addresses. When information arrives at the Physical Layer it is ready to be sent onto the medium as electrical, microwave or optical signals. These signals are received at the destination one after the other and interpreted so that the communication process is successful.
When talking about the Physical Layer you’ll have to know and understand the elements used here:
When a frame arrives it is encoded. This means that the signal is converted into a coded stream of bits. The codes can be understood by both parts, the sender and receiver. An example of a code is the start and the end of a frame, this could be a particular pattern like 11001100.
After the message is encoded, the Physical Layer generates the signals (the process is called signaling) that are transmitted between the source and destination. These signals can be either 1 or 0 but by transmitting a certain stream of bits (a pattern), these can be interpreted.
The signals must be synchroniszd between source and destination by using a clock signal. The clock signals are used so that information is sent intact between the source and destination.
Media in which signals are carried can have different speeds depending on the type used. The rate with which signals are transmitted can be measured by using three elements:
Bandwidth – this is the amount of data than can be transmitted between two points in a certain time period. This is actually the maximum speed which signals can be transmitted on a certain type of physical media.
It is measured in kilobits per second (kbps) or megabits per second (Mbps).
Kilobits per second equals 1000 Bits per second (1kbps = 1000 bps= 10^3 bits)
1 Mbps = 10^6 bps
1 Gbps (Giga)= 10^9 bps
1 Tbps (Terra)= 10^12 bps
Goodput – Measures the transfer rate of data, it is calculated after removing protocols overhead. You can find out more here: http://en.wikipedia.org/wiki/Throughput
There is a good article on Wikipedia in which you can see some different types of physical media and their characteristics: http://en.wikipedia.org/wiki/Ethernet_physical_layer.
Types of media used:
– is the most common type of physical media. Data is transmitted as electrical signals. It is a cheap communication media but it is likely to suffer because of interference. This is why we use the shielding twisted pairs to minimize signal degradation. One type of copper media is the UTP cable (Unshielded Twisted Pair). Open this link from Wikipedia : http://en.wikipedia.org/wiki/Unshielded_twisted_pair#Unshielded_twisted_pair_.28UTP.29
You can see on a right a picture with UTP cable. This type of cable uses 8 wires that are twisted in groups of 2. The cables are twisted in order to avoid crosstalk ( interference between two or more signals).
Fiber optic media – more expensive and much faster than copper media. This type of physical media uses light signals to transmit information between two devices. The communication is made in full duplex by using two dedicated fiber channels ( one for transmitting and one for receiving). The light signals are created and received by LED devices (light emitting diodes).
There are two types of optical fiber:
“Optical fibers typically include a transparent core
surrounded by a transparent cladding
material with a lower index of refraction. Light is kept in the core by total internal reflection. This causes the fiber to act as a waveguide. Fibers that support many propagation paths or transverse modes are called multi-mode fibers (MMF), while those that only support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a wider core diameter, and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 1,050 meters (3,440 ft).”
Wireless media – this type of media uses electromagnetic signals with different frequencies. Transmission can be made by either microwave or radio communication (Radio is the wireless transmission of signals through free space by electromagnetic radiation of a frequency significantly below that of visible light, in the radio frequency range, from about 30 kHz to 300 GHz. These waves are called radio waves. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space.
There are two main types of Wireless connections: WLAN or Wireless Local Area Connections and WWAN or Wireless Wide Area Connections. WLAN are used usually in a single location and do no cover large geographical areas whereas WWAN are used to communicate between two distant places.
You can see some common wireless standards here: http://en.wikipedia.org/wiki/IEEE_802.11.
Guys this is all for today, I’ve only included the main ideas needed to know when talking about the Physical Layer. You can surface the Internet for more information or leave a comment if you want to find out about something particular. I hope you will enjoy it and stay tuned for more.