- The physical layer defines the standards used to send and receive bits between 2 devices across physical network media, eg: maximum length of each type of cable, the number of wires inside the cable, the shape of the connector on the end of the cable, and the purpose of each pin or wire.
- The Electronic Industries Association and the newer Telecommunications Industry Alliance (EIA/TIA) are the standard organizations that define the Ethernet physical layer specifications.
- Figure 3-3 shows the wiring of the Category 5 (CAT5) Unshielded Twisted-Pair (UTP) straight-through and crossover cables.
Figure 3-3: CAT5 UTP Cables and RJ-45 Connector
- Pins 1 and 2 are used for transmitting data; while pins 3 and 6 are used for receiving data.
- Sometime multiple specifications are used to define the details of the physical layer.
Ex: RJ-45 (connector shape, number of pins) + Ethernet (pins usage – pins 1, 2, 3, and 6).
- Straight-through cable Pin 1 connects to pin 1 of the other end, pin 2 connects to pin 2, etc.
A straight-through cable has 2 identical ends.
- Crossover cable Pin 1 connects to pin 3, pin 2 connects to pin 6, and vice versa for both ends.
With such pin arrangement, it connects the transmit circuit of an NIC to the receive circuit of another NIC, and vice versa, which allows both NICs to transmit and receive at the same time.
It allows the creation of mini-LAN with 2 PCs without a switch or hub (point-to-point topology).
- Rollover cable Pin 1 connects to pin 8 of the other end, pin 2 to pin 7, pin 3 to pin 6, etc.
Mainly used to connect to the RJ-45 console ports which are available on most Cisco devices.
Also known as console cable.
- Most LAN cabling uses twisted-pairs (a pair of wires that were twisted together) cables, as they can greatly reduce electromagnetic interference caused by electrical current.
Straight-Through Cable
Figure 3-4: Common Network Topologies
- Physical topology defines how devices are physically connected; while logical topology defines how devices communicate (the data path) across the physical topology. The physical and logical topologies could be same or different depends on the Ethernet specifications.
- A bus topology uses a single cable to connect all devices (linear topology). Both ends of the cable must be terminated with a terminator to absorb signals that reach the end of the cable in order to prevent them from bouncing back at the end of the wire and causing collisions or errors.
If the terminators were removed, an entire network would stop working.
- In a star topology, a central device has many point-to-point connections to other devices. In a 10BaseT or 100BaseTX network, multiple PCs connect to a hub or switch (the center of the star).
Star topologies are also known as hub-and-spoke topologies.
- All type of networks has the limitations on the total length of a cable. Repeaters were developed to exceed the distance limitation of the Ethernet standard. They were deployed inline to overcome the attenuation problem. Any signal (including collisions) received from a port is reamplified or regenerated and forwarded out all ports without any interpretation of the meaning of bits. However, repeaters do not simply amplify the signal, as this might amplify noise as well. They act as signal conditioners that clean up signals prior to transmission.
- Hubs are multiple ports repeaters. All devices connected into a hub reside in the same collision and broadcast domains. Note that while a hub is a repeater, a repeater is not necessary a hub.
A repeater may have only 2 connectors, while a hub can have many more.
- Attenuation is the loss of signal strength as electrical signal travels across a cable. It is measured in decibels (dB). A higher quality cable will have a higher rated category and lower attenuation – CAT5 cables are better than CAT3 cables because they have more wire twists per inch and less crosstalk (unwanted signal interference from adjacent pairs); and therefore can run at higher speeds and longer distances.
- When an electrical signal is transmitted across a cable, it will introduce magnetic field and radio frequency interferences, which emit radiation that can interfere with other signals in other wires. Crosstalk is referred to as the situation where a wire affects another wire(s) by changing of the electrical signal which would cause bit errors.
- Twisted-pair cables (a pair of wires which are twisted together) are used to reduce emissions.
By using an opposite current on each wire, each wire produces an identical magnetic field in opposite direction, which can cancel each other out (cancellation).
10Base5 and 10Base2 10BaseT 100BaseTX
Hub Switch
- Physical Bus - Logical Bus
- Physical Star - Logical Bus
- Physical Star - Logical Star
- Another way to reduce emissions is by shielding the wires – the use of some materials placed around them to block the electromagnetic interference. Unfortunately, this makes the cables more expensive (materials and manufacturing costs), and less flexible (cannot be bended easily, which makes it more difficult to install).
- STP max length and speed for a network segment is 100m (328 feet) and 100Mbps respectively.
Figure 3-5: Common Network Cables and Connectors
- The TIA has defined several standards for UTP cabling and different categories of UTP (Unshielded Twisted Pair) cables. Below lists the all the UTP categories and their characteristics:
UTP Category Max Speed Rating Usual Applications
CAT1 < 1Mbps Analog voice (telephones). ISDN BRI. Not for data.
CAT2 4Mbps Mainly used in IBM Token Ring networks.
CAT3 10Mbps Analog voice (telephones) and 10BaseT Ethernet (data)
CAT4 16Mbps Mainly used in IBM Fast Token Ring networks.
CAT5 100Mbps Mainly used in 100BaseTX Fast Ethernet networks.
CAT5e 1Gbps Similar to CAT5 cable, but contains a physical separator between the 4 pairs to further reduce electromagnetic interference (more expensive than CAT5). Lower emissions and better for Gigabit Ethernet cabling.
CAT6 1Gbps+ Intended as a replacement for CAT5e. Capable of supporting multigigabit speeds.
- Coaxial cabling was used for 10Base5 and 10Base2 Ethernet networks. 10Base5 was referred to as thicknet while 10Base2 was referred to as thinnet, as 10Base5 used thicker coaxial cables.
- Coaxial cables are shielded. They have a single copper wire in the center, with plastic insulation and copper shielding.
- Connecting a host to a 10Base5 segment requires a vampire tap and the cabling is inflexible.
No cable stripping and connectors were used. Vampire taps pierce through insulating layer and makes direct contact with the core of a cable. Attachment Unit Interface (AUI) cables (15-pin shielded and twisted-pair) were also used to connect between vampire taps (MAU) and NICs.
UTP Cable STP Cable
- The 10Base2 Ethernet, which was developed after 10Base5 uses thinner and more flexible coaxial cabling. The cables are terminated with BNC (British Naval Connector, Bayonet Neill Concelman, or Bayonet Nut Connector) connectors, which was a lot easier to use than the vampire taps. A BNC T-connector was being used to connect a host into a 10Base2 segment.
- In those networks, a single cable problem could take down the entire Ethernet segment.
- Transceiver is Transmitter + Receiver. Original Ethernet was designed to use an external device called transceiver instead of the NIC itself for encoding and decoding of signals and bits.
Figure 3-6: 10Base5 and 10Base2 Network Connections
- Below are the main differences between optical cabling (fiber cabling) and electrical cabling:
i) Supports longer distances.
ii) Higher cost.
iii) Does not emit electromagnetic radiation. Immune to electromagnetic interferences (EMI) and electronic eavesdropping; hence provides better security.
iv) Supports 10Gbps Ethernet.
- Optical cabling uses a pair of strands (or threads) for data transmission in both directions.
- The cladding has a different Index of Refraction (IOR) than the core (the fiber). When the light hits the outer wall of the core, which is the inner wall of the cladding, the light will be reflected back into the core, and the light eventually travels from one end to another end of the fiber cable.
- Below lists the 2 general categories of optical cabling:
Single-mode Fiber manufacturing process for the light generation hardware. SMF provides longer distances, higher data rates, and less dispersion than MMF.
Multimode Fiber (MMF)
Uses larger diameter optical fiber core. Uses light-emitting diodes (LEDs) to generate light. LEDs generate multiple modes or wavelengths of light where each takes slightly different path, thus named as MM.
MMF is mostly deployed in short transmission distance environments.
Note: Modes are the number of paths that a light ray can follow when propagating down a fiber.
Dispersion is the spreading of light pulses as they propagate down a fiber.
- Optical cabling can transmit up to 10Gbps (MMF) and 100Gbps (SMF).
NIC
Figure 3-6A: 10Base5 Network Connection Figure 3-6A: 10Base2 Network Connection
Vampire Tap
AUI – Attachment Unit Interface MAU – Medium Attachment Unit
Figure 3-7: Single-Mode and Multimode Fiber Optics - Below lists some types of fiber-optic connectors:
ST connector Each strand is terminated with a barrel connector (like a BNC connector).
Twisted when connected into an interface card to secure the connection.
SC connector 2 strands are attached together as a single connector.
MT-RJ connector
Newer type of connector (Small Form Factor Pluggable, SFP). Similar to RJ-45 connector that ease the connectors to switch ports installation.
- Below lists the original and expanded IEEE Ethernet 802.3 standards:
Original IEEE 802.3 Standards:
10Base5 Up to 500 meters long. Physical and logical bus. Uses vampire taps and AUI cables. Up to 2500 meters with repeaters and 1024 users for all segments.
Terminators were used.
10Base2 Developed after 10Base5. Up to 185 meters (since it is ~200 meters, thus 2 in the name). Supports up to 30 workstations on a single segment. Physical and logical bus. Uses BNCs and T-connectors. Terminators were used.
10BaseT Up to 100 meters. Uses Category 3 UTP 2-pair wiring. Each device must connect into a hub or switch, and only 1 host per segment or wire. Physical star topology and logical bus with RJ-45 connectors and hubs.
Expanded IEEE 802.3 Standards:
100BaseT Up to 100 meters. Uses UTP.
100BaseT4 Up to 100 meters. Uses 4 pairs of Category 3 to 5 UTP.
100BaseTX Up to 100 meters. Uses 2 pairs of Cat 5 to 7 UTP or STP. 1 user per segment.
Physical and logical star topology with RJ-45 connectors and switches.
100BaseFX Up to 400 meters. Uses 2 strands of 62.5 or 125 microns MMF optical cable.
Point-to-point topology. Uses ST or SC connector.
1000BaseCX Copper twisted pair called twinax that can only run up to 25 meters.
1000BaseT Up to 100 meters. Uses Category 5, 5e, 6 UTP 4-pair wiring.
1000BaseSX Short-wavelength laser. MMF using 50 or 62.5 microns core and 850 nanometer laser. Up to 275 meters (62.5-micron) and 550 meters (50-micron).
1000BaseLX Long-wavelength laser. SMF or MMF that uses a 9-micron core and 1310 nanometer laser. Up to 550 meters (MMF) and 10KM (SMF). Lasers used in SMF provide higher output than LEDs used in MMF.
1000BaseXD Extended distance up to 50KM.
1000BaseZX Extended distance up to 70KM with a 9-micro core SMF.
- The “Base” in IEEE 802.3 standards is referred to as baseband signaling, a technology where only one carrier frequency or digital signal is being used at a time on the wire – when a device transmits, it uses the entire bandwidth on the wire and does not share with other. As compared to
Single-Mode Multimode
- Attachment Unit Interface (AUI) is defined in all original 802.3 standards as the standard Ethernet interface that allows the data link layer to remain unchanged while supporting any existing and new physical layer technologies (eg: BNC, UTP). Medium Attachment Unit (MAU) transceivers (also known as media converters) were used to provide conversion between 15-pin AUI signals and twisted-pair Ethernet cables (eg: 10Base2, 10BaseT). Networks connected via external transceivers (eg: AUI, MAU) can operate only in 10Mbps half-duplex.
- Media Independent Interface (MII) is used in Fast Ethernet and Gigabit Ethernet to provide faster bit transfer rate of 4 or 8 bits at a time. As compared to AUI, which is only 1 bit at a time.
- Below lists some IEEE 802.3 Ethernet standards:
802.3u Fast Ethernet (100BaseTX).
802.3ab Gigabit Ethernet over twisted-pair cable – CAT5 or CAT5e. 802.3z Gigabit Ethernet over fiber-optic.
802.3ae 10-Gigabit Ethernet (fiber and copper).
Note: UTP Gigabit Ethernet may operate in half-duplex mode with the 10/100Mbps Ethernet CSMA/CD mechanism. Fiber optic Gigabit Ethernet can only operate in full-duplex mode.
- Below are some features of IEEE 802.3ae 10-Gigabit Ethernet:
i) Allows only point-to-point topology. It is targeted for connections between high speed switching devices.
ii) Allows only full-duplex communication.
iii) Supports only optical fiber cabling. Supports copper cabling in the future.
- Auto-negotiation is a feature of Fast Ethernet that allows NICs and switch ports to negotiate to discover which mode they should operate at (10Mbps or 100Mbps, half duplex or full duplex).
There are doubts of the reliability of auto-negotiation, hence the speed and duplex settings for the switch ports and devices that seldom move (eg: servers, routers) should be configured statically.
The use of auto-negotiation should be limited to access layer switches ports.
- Wireless communication uses some form of electromagnetic energy that propagates through the air at varying wavelengths. Electromagnetic energy can pass through matters, but the matters often reflect the energy to certain degree and absorb part of the energy. Some wavelengths require line-of-sight for communication as they are unable to pass through matters well.
- The IEEE 802.11 Wi-Fi (Wireless Fidelity) is the most common and widely deployed WLAN.
A WLAN is a shared LAN as only one station can transmit at a time. A typical WLAN consists of PCs with wireless adapters, and a wireless access point (AP). Access points bridge traffic between the wired and wireless LANs.
- The IEEE 802.11 standards still uses IEEE 802.2 LLC, but with a different MAC header other than 802.3. An access point swaps an 802.3 header with an 802.11 header when bridging traffic.
- Below lists some IEEE 802.11 standards:
Standard Transmits Using Maximum Speed 802.11a 5GHz frequency band 54Mbps
802.11b 2.4GHz frequency band 11Mbps 802.11g 2.4GHz frequency band 54Mbps Note: 802.11g is backward-compatible with 802.11b.
Figure 3-8: 802.11 Framing
- 802.11b transmits at 11Mbps but has a maximum throughput of 7Mbps due to the shared bus architecture – WLANs are half-duplex communication, all devices share the same bandwidth and only one device can transmit at a time.
- Half-duplex Ethernet uses CSMA/CD in its operation while IEEE 802.11 WLAN uses CSMA/CA (Carrier Sense Multiple Access with Collisions Avoidance) in its operation.
Congestion avoidance monitors network traffic load to predict and avoid congestion via packet dropping. A common congestion avoidance mechanism is Random Early Detection (RED).
802.11 802.2 Data 802.3
802.3 802.2 Data 802.3
Switch Wireless
AP