CHAPTER 8
OSI PHYSICAL LAYER
THE PHYSICAL LAYER – Communications signal
The Open Systems Interconnection model (OSI model) was a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a way of sub-dividing a communications system into smaller parts called layers. Similar communication functions are grouped into logical layers. A layer provides services to its upper layer while receiving services from the layer below. On each layer, an instance provides service to the instances at the layer above and requests service from the layer below.
For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of that path. Two instances at one layer are connected by a horizontal connection on that layer.
Description of OSI Layer
According to recommendation X.200, there are seven layers, each generically known as an N layer. An N+1 entity requests services from the N entity.
At each level, two entities (N-entity peers) interact by means of the N protocol by transmitting protocol data units (PDU).
A Service Data Unit (SDU) is a specific unit of data that has been passed down from an OSI layer to a lower layer, and which the lower layer has not yet encapsulated into a protocol data unit (PDU). An SDU is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user.
The PDU at any given layer, layer N, is the SDU of the layer below, layer N-1. In effect the SDU is the 'payload' of a given PDU. That is, the process of changing a SDU to a PDU, consists of an encapsulation process, performed by the lower layer. All the data contained in the SDU becomes encapsulated within the PDU. The layer N-1 adds headers or footers, or both, to the SDU, transforming it into a PDU of layer N-1. The added headers or footers are part of the process used to make it possible to get data from a source to a destination.
OSI Model | ||||
Data unit | Layer | Function | ||
Host layers | Data | Network process to application | ||
Data representation, encryption and decryption, convert machine dependent data to machine independent data | ||||
Interhost communication | ||||
Segments | ||||
Media layers | Packet/Datagram | Path determination andlogical addressing | ||
Frame | Physical addressing | |||
Bit | Media, signal and binary transmission | |||
National Standards Institute (ANSI)
· The International Telecommunication Union (ITU)
· The Electronics Industry Alliance/Telecommunications Industry Association (EIA/TIA)
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· The American NNational telecommunications authorities such as the Federal Communication Commission (FCC) in the USA.
There are three basic forms of network media on which data is represented:
- · Copper cable
- · Fiber
- · Wireless
The representation of the bits - that is, the type of signal - depends on the type of media. For copper cable media, the signals are patterns of electrical pulses. For fiber, the signals are patterns of light. For wireless media, the signals are patterns of radio transmissions.
The Physical layer consists of hardware, developed by engineers, in the form of electronic circuitry, media, and connectors. Therefore, it is appropriate that the standards governing this hardware are defined by the relevant electrical and communications engineering organizations.
Similar to technologies associated with the Data Link layer, the Physical layer technologies are defined by organizations such as:
· The International Organization for Standardization (ISO)
· The Institute of Electrical and Electronics Engineers (IEEE)
The three fundamental functions of the Physical layer are:
The physical components
are the electronic hardware devices, media and connectors that transmit and carry the signals to represent the bits.
Data encoding
is a method of converting a stream of data bits into a predefined code.
Signaling
the method of representing the bits.
PHYSICAL SIGNALING AND ENCODING: Representing Bits
NRZ Signaling
This simple method of signaling is only suited for slow speed data links. NRZ signaling uses bandwidth inefficiently and is susceptible to electromagnetic interference.
Manchester Encoding
Although Manchester Encoding is not efficient enough to be used at higher signaling speeds, it is the signaling method employed by 10BaseT Ethernet (Ethernet running at 10 Megabits per second).
DATA CARRYING CAPACITY
Data transfer can be measured in three ways:
· Bandwidth
a measure of the width of a range of frequencies, measured in hertz
· Throughput
is the average rate of successful message delivery over a communication channel. This data may be delivered over a physical or logical link, or pass through a certain network node. The throughput is usually measured in bits per second (bit/s or bps), and sometimes in data packets per second or data packets per time slot.
· Goodput
is the application level throughput, i.e. the number of useful information bits, delivered by the network to a certain destination, per unit of time
PHYSICAL MEDIA- Connecting Communications
Types of Physical Media
· Ethernet media
=> is a family of computer networking technologies for local area networks (LANs) commercially introduced in 1980, Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies.
· Wireless media
=>Transmission of waves takes place in the electromagnetic (EM) spectrum. The carrier frequency of the data is expressed in cycles per second called hertz(Hz). Low frequency signals can travel for long distances through many obstacles but can not carry a high bandwidth of date while high frequency signals can travel for shorter distances through few obstacles and carry a narrow bandwidth. Also the noise effect on the signal is inversely proportional to the power of the radio transmitter
COPPER MEDIA
· Shielded twisted-pair cable (STP) combines the techniques of shielding, cancellation, and twisting of wires. Each pair of wires is wrapped in metallic foil. The four pairs of wires are wrapped in an overall metallic braid or foil. It is usually 150-Ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise within the cable such as pair to pair coupling and crosstalk. STP also reduces electronic noise from outside the cable, for example electromagnetic interference (EMI) and radio frequency interference (RFI).
·
Unshielded twisted-pair cable (UTP) is a four-pair wire medium used in a variety of networks. Each of the 8 individual copper wires in the UTP cable is covered by insulating material. In addition, each pair of wires is twisted around each other. This type of cable relies solely on the cancellation effect produced by the twisted wire pairs, to limit signal degradation caused by EMI and RFI. To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies
FIBER MEDIA
Fiber optic cables can be broadly classified into two types:
· single-mode
optical fiber carries a single ray of light, usually emitted from a laser, this type of fiber can transmit optical pulses for very long distances.
· Multimode
fiber typically uses LED emitters that do not create a single coherent light wave. Instead, light from an LED enters the multimode fiber at different angles.
WIRELESS MEDIA
carry electromagnetic signals at radio and microwave frequencies that represent the binary digits of data communications. As a networking medium, wireless is not restricted to conductors or pathways, as are copper and fiber media.
Types of Wireless Networks
· The IEEE and telecommunications industry standards for wireless data communications cover both the Data Link and Physical layers. Four common data communications standards that apply to wireless media are:
· Standard IEEE 802.11 - Commonly referred to as Wi-Fi, is a Wireless LAN (WLAN) technology that uses a contention or non-deterministic system with a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) media access process.
· Standard IEEE 802.15 - Wireless Personal Area Network (WPAN) standard, commonly known as "Bluetooth", uses a device pairing process to communicate over distances from 1 to 100 meters.
· Standard IEEE 802.16 - Commonly known as WiMAX (Worldwide Interoperability for Microwave Access), uses a point-to-multipoint topology to provide wireless broadband access.
