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OSI Networking Model Layer 1

 

1 - PHYSICAL LAYER - Media, signal and binary transmission (raw bit stream).

 

Function: Translates the bits generated by all the other layers into signals to send through the network, and translates them back into bits at the other end.

 

Data unit: Bit and Packets.

 

Specifications (examples): V.24, V.35, EIA/TIA-232, EIA/TIA-449, FDDI, 802.3, 802.5, Ethernet, RJ45, NRZ, NRZI.

 

Protocols (examples):IEEE 802, IEEE 802.2, ISO 2110, ISDN

 

Devices: Concentrators, Hubs (Passive, Active), Repeaters, Connectors, Multiplexer, TDR, Oscilloscope, Amplifier, Modem, Codec.

 

Topics & Methods Explained


Connection types

Point-to-Point:
A direct link between two devices. For example attaching a PC directly to a printer is a point-to-point link. A  Point-to-Point connection guarantees each station a specific transmission capacity or bandwidth.


Multipoint:
(Also called multidrop) is a link between three or more devices (e.g. star topology). Share the same bandwidth so the overall capacity is devided among every device connected to the media.

Physical topology

Bus:
One long cable called a backbone & short cables, called drop cables, can be attached to the backbone using taps. The backbone is terminated at both ends to remove the signal from the wire after it has passed all devices.

 

Ring:
A circular topology or closed loop of point-to-point links. Electric or EM signals are typically passed from device to device in only one direction. Signals are repeated or regenerated at each device so degradation is minimal.

 

Star:
A central device with drop cables extending in all directions. Each network device is connected via a point-to-point link to the central device called a hub, multiport repeater, or a concentrator.

 

Mesh:
A Point-to-Point connection between every device in the network. Extremely fault tolerant, and each link provides guaranteed capacity. Typically used in a hybrid with just the largest or most important sites interconnected.

 

Cellular:
A combined wireless Point-to-Point and multipoint strategy that divides a geographic area into cells. Devices within the cell communicate with a central station or hub which are interconnected to route data across the network.

Digital signaling

Current state:
Measure the presence or absence of a state or signal characteristic. For example, fiber optic networks represent data by turning a light source on or off. Encoding schemes: Unipolar, Polar, Return-to-Zero RZ, and Biphase.

 

State transition:
The transition between states are used to encode data on a digital signal. A transition from one state to another would indicate a binary 0 or 1. Encoding schemes: Manchester, Differential Manchester, & Non-Return-to-Zero NRZ.


Analog signaling

Current state:
Amplitude Shift Key ASK encodes binary data by varying the signal's amplitude between two or more levels. Frequency Shift Key FSK encodes binary data by varying the signal's frequency between two or more values.

 

State transition:
Phase shift key PSK encodes binary data by varying the phase of the signal to indicate a value. For example, a 180 degrees phase shift could represent 1, and no shift could indicate 0.

Bit synchronization


Asynchronous:
When data is being transmitted, the devices use an internal hardware clock to know how frequently to measure the signal.   stop bit - data bits - start bit (or state change)->   (no data=no signal transmission).

 

Synchronous:
Guaranteed State Change:a clocking sig. is imbedded within the data sig. Separate Clock Signals:1 channel transports the actual data, while the other provides a clock. Oversampling:the receiver samples the sig. faster than data rate.

Bandwidth use:

Broadband: (thick ethernet)
Use the transmission medium's capacity to provide multiple channels which are created by dividing up the medium's bandwidth using a technology called FDM. Using analog signals multiple simultaneous conversations are supported.

 

Baseband: (thin ethernet)
Use the transmission medium's entire capacity for a single channel. Either analog or digital signal can be used, but digital is more common. Multiple conversations can be combined on that single signal using a technology called TDM.

Multiplexing

Frequency-Division: (FDM)
Uses separate frequencies to establish multiple channels within the medium. The mux creates special carrier signals that operate on different freq. Data segments are added to the carrier signals and are removed, at the other end.

 

Time-Division: (TDM)
Divides a sig. channel into short time slots. Bits, blocks of bits, bytes, or frames can be placed into each time slot as long as the time interval is not exceeded. Used on baseband, also on an individual channel of a broadband system.

 

Stat.Time-Division: (StatTDM)
Time slots are allocated dynamically to active devices on a first-come, first-served or priority basis. A control field identifies the owners of each slot's data so that the receiving mux can appropriately split out all the individual signals.

 

 

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