Physical layer

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OSI Model
7	Application layer
6	Presentation layer
5	Session layer
4	Transport layer
3	Network layer
2	Data link layer LLC sublayer MAC sublayer
1	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. 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 things are specified here. An analogy of this layer in a physical mail network would be a specification for various kinds of paper and ink, for example.

The physical layer determines the bit rate in bit/s, 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 impedance
- Radio interface, including electromagnetic spectrum frequency allocation and specification of signal strength, analog bandwidth, etc.
- Specifications for IR over optical fiber or a wireless IR communikation link

Internet protocol suite

5. Application layer

DNS, TLS/SSL, TFTP, FTP, HTTP, IMAP4, IRC, MIME, POP3, SIP, SMTP, SNMP, SSH, TELNET, RTP, SDP, …

4. Transport layer

TCP, UDP, RSVP, DCCP, SCTP, …

3. Network layer

IP (IPv4, IPv6), ICMP, IGMP, ARP, RARP, …

2. Data link layer

Ethernet, Wi-Fi, PPP, FDDI, ATM, Frame Relay, GPRS, Bluetooth, …

1. Physical layer

Modems, ISDN, SONET/SDH, RS232, USB, Ethernet physical layer, Wi-Fi, GSM, Bluetooth, …

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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

1 Physical signaling sublayer
2 Examples
3 Hardware equipment (network node) examples
4 See also

[edit] 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
performs character encoding, transmission, reception and decoding
performs optional isolation functions.

Source: from Federal Standard 1037C

[edit] 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

[edit] Hardware equipment (network node) examples

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.