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www.en.wikipedia.org/wiki/Physical_layer

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Posted by crouse

try this

www.en.wikipedia.org/wiki/Physical_layer

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Posted by waqasahmad

The physical layer defines the mechanical, procedural, and electrical standards for accessing the media so that you can transmit your Layer 2 frames.

All signaling at Layer 1 is digital, which means that we are sending binary bits onto the wire. This can mean energizing a copper cable with electricity, where "electricity on" indicates a binary 1 and "electricity off" indicates a binary 0; or, it can mean blinking a laser down an optical fiber where on = 1 and off = 0. Wireless systems do much the same thing.

By defining standards for the physical layer, we can be assured that if we buy an RJ-45 patch cord, it will fit into and work properly in any interface designed to use it.

Sending Data Between Hosts:
The OSI model describes the framework for this flow. As we move down the layers from application to physical, the data is encapsulated, which means that headers and trailers are added by each layer. The process of creating a piece of data on one host and sending it to another host:

• At Layer 7, the user generates some data, perhaps an email message or a Word document. This data is passed down to Layer 6.
• At Layer 6, the data is formatted so that the same application on the other host can recognize and use it. The data is passed down to Layer 5.
• At Layer 5, the request to initiate a session for the transfer of the data is started. The data is passed down to Layer 4.
• At Layer 4, the data is encapsulated as either a TCP or UDP segment. The choice depends on what application generated the data. Source and destination port numbers are added, as are sequence and acknowledgment numbers and window size. The segment is passed down to Layer 3.
• At Layer 3, the segment is encapsulated with a Layer 3 header and becomes a packet. The packet header contains source and destination IP addresses and a label indicating what Layer 4 protocol it is carrying. The packet is passed down to Layer 2.
• At Layer 2, a header with source and destination MAC addresses is added. This encapsulation creates the frame. The trailer at this layer contains an error-checking calculation called the FCS (Frame Check Sequence). The frame header also contains a label indicating which Layer 3 protocol it is carrying (IP, IPX, and so on). The frame is sent to the interface for transmission onto the media (Layer 1).
• At Layer 1, the binary string that represents the frame is transmitted onto the media, whether electrically, optically, or by radio. Bits are transmitted across the media to the network interface of the other host.
• When received by the other host, the Layer 1 bits are sent up to Layer 2.
• At Layer 2, the destination MAC is examined to make sure that the frame was intended for this host. The FCS is calculated to check the frame for errors. If there are errors, the frame is discarded. If there are none, the frame is decapsulated and the packet is sent to the correct Layer 3 protocol based on the protocol ID in the header.
• At Layer 3, the destination IP address is checked to see if it is intended for this host. The packet header is checked to see which Layer 4 protocol to send it to. The packet is decapsulated, and the segment is sent up to Layer 4.
• At Layer 4, the destination port in the segment header is checked and the segment is decapsulated. The data is sent to the correct upper layer application. Depending on the application, it might go directly to Layer 7 or through 5 and 6.

This process of encapsulation, transmission, and decapsulation makes data flow in an organized and manageable fashion down the OSI stack on the sender, across the transmission media, and up the OSI stack on the receiving host. It is important to understand that layer 3 on the sender is communicating with layer 3 on the receiver as well by way of the information in the headers.

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Posted by campbell123

Please Visit:

www.en.wikipedia.org/wiki/Physical_layer

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Posted by sagitraz

Physical layer is the 1st layer of OSI model.
and on that layer data format is in bit/byte.
On this layer cable, HUB , Repeater etc devices are working. This layer is responsible for physical connectivity.

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Posted by saurabh2dixit