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Mark Pearl

 

After reading this you should be able to

  • Explain basic data transmission concepts, including full duplexing, attenuation, latency, and noise
  • Describe the physical characteristics of coaxial cable, STP, UTP, and fiber-optic media
  • Compare the benefits and limitations of different networking media
  • Explain the principles behind and uses for serial connector cables
  • Identify wiring standards and the best practices for cabling buildings and work areas

Transmission Basics

  • Transit – means to issue signals along a network medium such as a cable
  • Transmission – refers to either the process of transmitting or the progress of signals after they have been transmitted
  • Transceiver – devices that transmits and receives signals

Analog and Digital Signaling

There are two main signaling methods:

  1. Analog – voltage varies continuously and appears as a wavy line when graphed over time
  2. Digital – strength of a signal is directly proportionate to the voltage – constant voltage either on or off

Analog Signals are characterized by four main properties

Analog Signal

  1. amplitude – measure of its strength at any given point in time
  2. frequency – number of times that a wave’s amplitude cycles from its starting point, through to its highest amplitude and back down again. Expressed in cycles per second (hertz)
  3. wavelength – distance between corresponding points on a wave’s cycle. Expressed in meters or feet (higher the frequency means the shorter the wavelength)
  4. phase – refers to the progress of a wave over time in relationship to a fixed point.
  • Advantage - Because analog signals are more variable than digital signals, they can convey greater subtleties with less energy.
  • Disadvantage - Analog signals voltage is varied and imprecise. Analog signals are more susceptible to transmission flaws such as noise or any type of interference that may degrade a signal.

Characteristics of Digital Signals

Digital Signal

  • Digital signals of composed of pulses of precise, positive voltages and zero voltages
  • Digital signals are more reliable than analogue signals
  • Noise affects digital signals less than analogue signals
  • Digital signals require many more pulses than analogue signals to send the same amount of information
  • Digital transmission is more efficient than analog transmission because it results in fewer errors and therefore requires less overhead to detect and compensate for errors
  • A positive pulse represents a 1 and a zero pulse represents a 0 (binary information)
  • Every pulse in a digital signal is called a binary digit
  • A bit can only have one of two possible values (1 or 0)
  • Eight bits form a byte (which gives it a decimal max value of 255)
  • One byte carries one piece of information – for example 01111001 means 121 on a digital network

Overhead – is a term used by networking professionals to describe the nondata information that must accompany data for a signal to be properly routed and interpreted by the network.

 

Data Modulation

Sometimes one needs to adapt data from one format to another (i.e. an analogue format to a digital format and back) – this is called modulation and demodulation – for example digital data can be transported over an analogue phone line using a modem (modulator / demodulator)

Data modulation is a technology used to modify analogue signals to make them suitable for carrying data over a communication path.

Modulation can be used to make a signal conform to a specific pathway.

  • In FM radio modulation (frequency modulation), data must travel along a particular frequency.
  • In AM radio modulation (amplitude modulation), data must travel along a particular amplitude.

Some technologies make use of modulation like DSL.

Simplex, Half-Duplex , and Duplex (direction in which signals travel)

Data transmission is characterized by the direction that it travels over the media.

  • Simplex transmission – Signals can only travel in one direction (from sender to receiver) (think of a football coach giving orders)
  • Half-duplex transmission – Signals can travel in both directions over a medium but in only one direction at a time. (think of a intercom where you need to press the button to talk)
  • Full-duplex transmission – Signals can travel in both directions over a medium at the same time. (think of a telephone conversation)

Full duplex is typically used on data networks.Usually full duplex uses multiple channels on the same medium. A channel is a distinct communication path between nodes. Channels may be separated either physically or logically.

Multiplexing

Multiplexing is a form of transmission that allows multiple signals to travel simultaneously over one medium. To carry multiple signals, the medium’s channel is logically separated into multiple smaller channels, or sub-channels. Networks rely on multiplexing to increase the amount of data that can be transmitted in a given time span over a given bandwidth.

There are several types of multiplexing including…

  • TDM (time division multiplexing( – which divides a channel into multiple intervals of time, or time slots. It then assigns a separate time slot to every node on the network, and in that time slot carries data for that node.
  • Statistical multiplexing – similar to TDM, but more efficient because time slots are unlikely to remain empty. Each node is assigned a time slot, but if it doesn’t use it – the slot is given to another node.
  • FDM (frequency division multiplexing) – assigns a unique frequency band to each communications sub channel. Signals are modulated with different carrier frequencies, then multiplexed to simultaneously travel over a single channel. First use of this was when phone companies discovered that they could put multiple phones on one physical line.
  • WDM (wavelength division multiplexing) – enables one fiber-optic connection to carry multiple light signals simultaneously (as many as 20 million phone conversations simultaneously). This works with any fiber-optic medium.
Relationships Between Nodes

Concerned with the number of nodes and who receives the data…

  • Point-to-point – Data transmission has one transmitter and one receiver
  • Point-to-multipoint – Data transmission has one transmitter and multiple receivers (Broadcast and Non-broadcast – broadcast is not concerned with who receives the data and sends it to everyone)
Throughput and Bandwidth

The data transmission characteristic most frequently discussed and analyzed by networking professionals is throughput.

  • Throughput - the measure of how much data is transmitted during a given period of time. It may also be called capacity or bandwidth.
  • Bandwidth  - is a measure of the difference between the highest and lowest frequencies that a medium can transmit

Often bandwidth and throughput are used interchangeably. While they are similar concepts, there is a difference.

Baseband and Broadband

 

Baseband  - is a transmission from in which digital signals are sent through direct current (DC) pulses applied to the wire. the direct current requires exclusive use of the wire’s capacity. As a result baseband systems can only transmit one signal at a time.

Broadband – is a form of transmission in which signals are modulated as radiofrequency analog waves that use different frequency ranges.

  • Baseband transmission supports half-duplexing. Ethernet is an example of a baseband system.  In Ethernet, each device on a network can transmit over the wire – but only one device at a time.
  • In traditional broadband systems, signals travel in only one direction.
  • Broadband transmission is generally more expensive than baseband transmission because of the extra hardware involved.
Transmission Flaws
  • Noise – is any undesirable influence that may degrade or distort a signal. Many different types of noise may affect transmission. (Examples of noise include EMI, RFI, Cross talk)
  • Attenuation – loss of a signals strength as it travels away from its source
  • Latency – the speed at which data travels determined by the medium (i.e. the speed that electrons travel)

 

Common Media Characteristics

Throughput
  • Probably the most significant factor in choosing a transmission method
  • Noise and devices connected to the transmission medium can further limit throughput
  • A noisy circuit spends more time compensating for errors etc.
Cost
  • Cost of upgrading a medium is not only dependent on the medium, but also on the connectors and hardware needed to support that medium
  • Take into consideration the cost of Installation, new infrastructure vs. reusing existing infrastructure, cost of maintenance and support, cost of a lower transmission rate affecting productivity and cost of obsolescence.
Noise Immunity
  • Noise can distort data signals
  • Some types of media are more susceptible to noise than others (i.e. fiber optics vs. coax cable)
  • In case of wired media, you may need to use a metal conduit or pipeline to protect cabling from noise
Size and Scalability
  • Three specifications determine the size and scalability of networking media
  1. Maximum nodes per segment (depends on attenuation and latency)
  2. Maximum segment length (depends on attenuation and latency plus segment type)
  3. Maximum network length ((dependent on cabling types etc.)
Connectors and Media Convertors
  • Connectors are pieces of hardware that connect the wire to the network device
  • Every networking medium requires a certain type of connector
  • Connectors are specific to the type of media you use, but that doesn’t restrict you to only one media per network
  • Using a media convertor, your can adapt a media from one type to another on the same network

Coaxial Cable

  • Coax cable was the foundation for Ethernet networks in the 1970’s
  • Over time twisted pair and fiber optic have replaced coax cable
  • Because of shielding , most coax has a high resistance to noise
  • Can also carry signals farther than twisted pair cable
  • More expensive than twisted pair cable
  • Many different types of coax including RG-6, RG-8, RG-58, RG-59
  • BNC stands for Bayonet Neil Concelman
  • A BNC connector is crimped, compressed, or twisted onto a coaxial cable

Coax

 

Twister Pair Cable

  • Consists of color-coded pairs of insulated copper wires, each with a diameter of 0.4 to 0.8 mm
  • Every two wires are twisted around each other to form pairs
  • The more twists per foot in a pair of wires, the more resistant the pair will be to cross talk
  • The number of twists per meter or foot is known as the twist ratio
  • A high twist ration can result in greater attenuation
  • Modern networks typically use cables that contain four wire pairs, in which one pair is dedicated to sending data and another pair is dedicated to receiving data
  • Twister pair cable is relatively inexpensive, flexible, and easy to install
  • It can span a significant distance before requiring a repeater (though not as far as coax)

Twisted pair cable

Shielded Twisted Pair

  • consist of twisted wire pairs that are not only individually insulated but also surrounded by a shielding made of a metallic substance such as foil
  • The shielding acts as a barrier to external electromagnetic forces

Unshielded Twisted Pair

  • UTP cabling consists of one or more insulated wire pairs encased in a plastic sheath
  • UTP is both less expensive and less resistant to noise than STP

Different categories of Twisted Pair support different bandwidths

  • Cat 3 – 10 Mbps
  • Cat 4 – 16 Mbps
  • Cat 5 – 1000 Mbps
  • Cat 6 and higher support higher signal rates

Comparing STP and UTP

  • Throughput – STP & UTP can transmit data at 10Mbps, 100 Mbps, 1 Gbps & 10 Gbps depending on the grade of cabling and the transmission method used
  • Connector – STP & UTP use RJ-45 modular connectors and data jacks
  • Noise immunity – Because of its shielding, STP is more noise resistant than UTP
  • Size and scalability – The maximum segment length for both STP & UTP is 100m

Fiber-Optic Cable

SMF (Single Mode Fiber)

  • Uses a narrow core through which light generated by a laser travels over one path, reflecting very little
  • Because of low reflection, light does not disperse as the signal travels along the fiber
  • This allows SMF to accommodate the highest bandwidths and longest distances without requiring repeaters
  • It has high costs so not recommended for LAN’s and WAN’s

MMF (Multimode Fiber)

  • Contains a core with a larger diameter than SMF over which many pulses of light generated by a laser or LED travel at different angles
  • Typically used to connect to a router
  • It is reliable

Fiber optic cable benefits over copper cabling

  • Extremely high throughput
  • High resistance to noise
  • Excellent security
  • Can carry signals for much longer distances before requiring repeaters
  • Industry standard for high-speed networking

Major drawback vs. copper cabling is cost

DTE and DCE Connector Cables

DTE – data terminal equipment

  • Refers to end user devices such as workstations, terminals or a console

DCE – data circuit terminating equipment

  • Refers to a device such as a multiplexer or modem that processes signals
  • DCE also supplies clock signal to synchronize transmission between DTE and DCE

Serial refers to a style of data transmission in which the pulses that represent bits follow one another along a single transmission line. Several types of serial cables exist including RS-232

With rollover cables, usual wire positions are exactly reversed in one of the two RJ-45 terminations. Rollover cables are mainly used to connect a console to a connectivity device, such as a router. Do not confuse rollover cables with crossover cables, which reverse the transmit and receive pairs from one end of the cable to the other.

Structured Cabling

Structured cabling - A popular standard for wiring is TIA/EIA’s joint 568 Commercial building Wiring Standard.

The standard suggests how networking media can best be installed to maximize performance and minimize upkeep. Structured cabling applies no matter what type of media or transmission technology a network uses.

In my opinion, the structured cabling standard is probably outside the scope of this course – check past exams if you want to verify this.

Best Practices for Cable Installation and Management

Read page 116 of the textbook. Again, not sure if this would apply to the course

Posted on Friday, January 13, 2012 6:31 AM UNISA COS 2626 Networks | Back to top


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