# Physical Layer

The physical layer is level one in the seven level OSI model of computer networking as well as in the five layer TCP/IP reference model. It performs services requested by the data link layer.
The physical layer is the most basic network layer, providing only the means of transmitting raw bits rather than packets over a physical data link connecting network nodes. No packet headers nor trailers are consequently added to the data by the physical layer. The bit stream may be grouped into code words or symbols, and converted to a physical signal, which is transmitted over a physical transmission medium (e.g. coaxial cable, optical fiber ,or twisted pair). The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes of the electrical connectors, which frequencies to broadcast on, what modulation scheme to use and similar low-level parameters are specified here. An analogy of this layer in a physical mail network would be the roads along which the vans carrying the mail drive (i.e., the transmission medium is the roads while the signals are the vans).
The physical layer determines the bit rate in bits per second (bit/s or bps), also known as channel capacity, digital Bandwidth, maximum throughput or connection speed.
The major functions and services performed by the physical layer are:
* Bit-by-bit node-to-node delivery
* Providing a standardized interface to physical transmission media, including
** Mechanical specification of electrical connectors and cables, for example maximum cable length
** Electrical specification of transmission line signal level and electrical impedance
** Radio interface, including electromagnetic spectrum frequency allocation and specification of signal strength, analog bandwidth, etc.
** Specifications for Infrared(IR) radiation over optical fiber or a wireless IR communication link
Template:IPstack
* Modulation
* Line coding
* Bit synchronization in synchronous serial communication
* Start-stop signalling and Flow control in asynchronous serial communication
* Circuit mode multiplexing
* Carrier sense and collision detection utilized by some level 2 multiple access protocols
* Equalization filtering, training sequences, pulse shaping and other signal processing of physical signals
The physical layer is also concerned with
* Point-to-point, multipoint or point-to-multipoint line configuration
* Physical network topology, for example bus, ring, mesh or star network.
* Serial or parallel communication
* Simplex, half duplex or full duplex transmission mode

## Physical signaling sublayer

In a local area network (LAN) or a metropolitan area network (MAN) using open systems interconnection (OSI) architecture, the physical signaling sublayer is the portion of the physical layer that:
* interfaces with the medium access control sublayer| (MAC)which is a part of Data Link Layer
* performs character encoding, transmission, reception and decoding
* performs mandatory isolation functions.
Source: from w:Federal Standard 1037C

## Examples

* V.92 telephone network modems
* xDSL
* IRDA| physical layer
* USB physical layer
* Firewire
* EIA RS-232, EIA-422, EIA-423, RS-449, RS-485
* ITU Recommendations: see ITU-T
* DSL
* ISDN
* T1 and other T-carrier links, and E1 and other E-carrier links
* 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and other varieties of the Ethernet physical layer
* Varieties of 802.11
* SONET/SDH
* GSM radio interface
* Bluetooth physical layer
* IEEE 802.11x Wi-Fi physical layers

## Hardware equipment (network node) examples

* Network adapter|
* Repeater
* Ethernet hub
* Modem
Note: Physical layer Associated with transmission of unstructured bit streams over a physical link. Responsible for the mechanical, electrical and procedural characteristics that establish, maintain and deactivate the physical link.

## WAN Technologies
### DSL
### T1

Frame Relay
Frame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. Frame Relay originally was designed for use across Integrated Services Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well. This chapter focuses on Frame Relay's specifications and applications in the context of WAN services.
Frame Relay is an example of a packet-switched technology. Packet-switched networks enable end stations to dynamically share the network medium and the available bandwidth. The following two techniques are used in packet-switching technology:
•Variable-length packets
•Statistical multiplexing
Variable-length packets are used for more efficient and flexible data transfers. These packets are switched between the various segments in the network until the destination is reached.
Statistical multiplexing techniques control network access in a packet-switched network. The advantage of this technique is that it accommodates more flexibility and more efficient use of bandwidth. Most of today's popular LANs, such as Ethernet and Token Ring, are packet-switched networks.
Frame Relay often is described as a streamlined version of X.25, offering fewer of the robust capabilities, such as windowing and retransmission of last data that are offered in X.25. This is because Frame Relay typically operates over WAN facilities that offer more reliable connection services and a higher degree of reliability than the facilities available during the late 1970s and early 1980s that served as the common platforms for X.25 WANs. As mentioned earlier, Frame Relay is strictly a Layer 2 protocol suite, whereas X.25 provides services at Layer 3 (the network layer) as well. This enables Frame Relay to offer higher performance and greater transmission efficiency than X.25, and makes Frame Relay suitable for current WAN applications, such as LAN interconnection.

### ATM
## Analog modems
## PAP/CHAP

PPP=> PPP stands for Point-to-Point Protocol, and is an Open standard protocol works with same & different company routers.

## References

PAP =>PAP is two handshaking protocol means it sends User name & password in clear text while authentication which means it can be easily accessed which means it is unsecure.
CHAP => CHAP is Three way handshaking protocol means it sends User name in clear text whereas password in an encrypted format.so it is secure as compare to PAP.