Network+/Architecture/Media/Introduction

Ethernet Overview

Ethernet has been with us since 1980. Early Ethernet was carried over stiff coax and later more malleable thin coax. Coax Ethernet installations require that the cable be snaked past each network device and that the device tap into the cable as it goes past. Although this wiring scheme reflects the way Ethernet uses a shared media to allow network communications, the station to station coax has serious real-world reliability problems. 10BASE-T was introduced in 1990 to address these problems. 10BASE-T uses durable and inexpensive twisted pair cable. Two pairs are required for each station: one pair for incoming traffic and one for outgoing. Home runs from each station to a central concentrator carry data to and from each station. With 10BASE-T, the vulnerable shared media portion of the network is now safely hidden in the closet and is not strewn all across the office. In a 10BASE-T system, a wiring fault typically takes out network service only to a single station.

10BASE-T includes diagnostic indicators that allow wiring faults to be easily identified. Because 10BASE-T offers both reliability and cost advantages over the original coax, most new Ethernet installations are of the twisted pair variety.

The newer Fast Ethernet used by CobraNet is wired just like 10BASE-T except that it has ten times the bandwidth, requires a slightly higher grade cable and has some distance limitations not found in 10BASE-T.

For more information on Ethernet, check out these network and Ethernet

Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the physical layer, through means of network access at the Media Access Control (MAC)/Data Link Layer, and a common addressing format.

Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology. It has been in use from the 1990s to the present, largely replacing competing LAN standards such as token ring, FDDI, and ARCNET. In recent years, Wi-Fi, the wireless LAN standardized by IEEE 802.11, is prevalent in home and small office networks and augmenting Ethernet in larger installations.

Ethernet is used in most, but not all, computer networking situations. It is a standard which defines rules that all computers follow to allow successful and efficient communication.

Common Network Cabling

Cable is a medium which provides physical path for data transmission. Several types exist. Some networks use the same type, other use multiple types. A common early cable was the RS-232 D Serial cable. It transmitted one bit at a time, very slow connection. Parallel cables can talk at one bit per line at a time - an 8 pin cable can have 8 bits at once assuming one bit is one line and software handles all handshaking.

RJ-11 is the terminator typically used on plugin phone cables. This cable type has been used for many years by TELCOs all over the world - six pin cable schmatics to follow at a later date.

RJ-45 connectors now terminate most common cables used for hub based or star style TCP/IP networks. RJ-45 terminated cable ends are shown to right. Schematics to follow at a later date. Now that we understand what cables are used the next step is to learn signal processing.

Early TCP/IP networks functioned with looped coax cables or very thick D connector style cables running from one computer to the next rather than via centralized star style hub components.

Physical Network Communication Mediums

Twisted-Pair Cable

Twisted-pair cabling is a copper wire that comes in two forms, shielded and unshielded. It is the most common form of wiring used in a network. It uses 8 wires twisted into pairs to cancel the effect of crosstalk (Noise from the adjacent wires). It is a relatively inexpensive form of LAN cabling. It can accommodate different topologies, but is mostly implemented in a star topology.

Unshielded Twisted-Pair Cable

UTP relies on the cancellation effect of twisting the wires to reduce Electromagnetic Interference (EMI). It is required to have a certain amount of twists per meter and it is connected using a Registered Jack 45 Connecter (RJ-45). UTP can run for 100 meters before the signal needs to be refreshed. UTP has advantages that make it ideal in some networks.

Easy to Install
Small, it does not take up much space in wiring ducts.
Cheapest type of cable.

UTP has 6 Categories.

Category 1:
- Only reliable for transmitting telephone communications, not regular data transmissions.
Category 2:
- Previously used in token rings. Speeds only up to 4kbit
Category 3:
- Works in 10BASE-T networks.. Transfer rate of 10mbit
Category 4:
- Used on 16mbit token ring networks.
Category 5:
- Transfer rate of 100mbit. Unreliable for 1000BASE-T networks.
Category 5e:
- Transfer rate of 1000mbit. Used in Gigabit Ethernet networks.
Category 6:
- Same as Cat 5e but made to a higher standard

Shielded Twisted-Pair Cable

Shielded Twisted-Pair Cable uses twisted pairs along with a metallic foil shielding to reduce the crosstalk and EMI. It is usually connected using an STP connecter but can also be connected with an RJ-45. Although it reduces the interference better than UTP, STP has many drawbacks that keep it from having a mainstream use.

More Expensive
Must be grounded at both ends
Harder to install

Because of these drawbacks it is rarely implemented in Ethernet networks. It is more common in Europe.

Coaxial Cable

Coaxial Cable uses a copper wire for the conductor, on top of this is insulation for the wire. The third layer consists of a metallic foil or woven copper braid as a shielding, followed by a rubber jacket on the top. It is often referred to as Thicknet or Thinnet, depending on the specification. Coaxial cable was found in early Ethernet networks (~1980).

Benefits:

It costs less to buy than Fiber Optic
It has speeds of 10mb/s to 100mb/s.
It costs more to install Coaxial cable.

Fiber Optic

Installation of fiber optic cabling is not the rocket science it once was. Great strides have been made in the durability cost and ease of termination of this media. Fiber optic cable offers two main advantages over twisted pair cable. First, data may be carried much further over fiber. Second, fiber is immune to electromagnetic interference.

There are two basic types of fiber in use today: Multimode and Single Mode. Multimode fiber is used extensively in the data communications industry. Fast Ethernet carried over multimode fiber is known as 100BASE-FX. Ethernet may be carried up to 2 kilometers on this fiber. Single mode fiber is used extensively in the telecom industry. Single mode fiber allows much greater run lengths than multimode fiber. Although there is no official standard for carrying Ethernet over single mode fiber, numerous datacom products offer this capability.

Two strands are required for each Ethernet link; one for transmit and one for receive. Both multimode and single mode fiber cables are available with varying numbers of strands. 4-strand cable costs about $0.80/meter.

Transfer rates of up to 10GB/s and a distance of up to 1000 meters. This is an expensive type of medium and takes a special connector to terminate the signal. It has higher bandwidth possibilities and is best suited for backbone installations. It has 3 parts to the cable: Core, Cladding and Buffer.

Core:
- This is where the light is transmitted
Cladding:
- Just outside the core it traps the light inside the core and helps guide it around corners.
Buffer:
- The hard plastic coating on the outside of the cable that protects the core from moisture and physical damage.

Fiber optic uses include grounding and bonding for:

LIghting protection systems
Grounding electrode systems
electrical bonding and grounding
Power protection
telecom bonding and grounding
Telecom circuit protector

External links

Bibliography

Network Cabling fundamentals, by Beth Verity