Physical Ethernet Standards

Physical Ethernet Standards

Objective:

• Select the components required to meet a network specification

Have I said that ethernet is the most popular LAN protocol? Ethernet started in the 1970s when Xerox needed a networking system to connect personal computers. Xerox joined forces with Digital Equipment Corp. (DEC) and Intel to develop the protocol, which is why the very first ethernet standards were referred to as DIX Ethernet. This section covers the progression
of ethernet standards from the earlier 10Mbps connections to the more recent 10 gigabit ethernet connections.

Each standard has a maximum connection length and speed. Individual ethernet standards also specify which cables and connectors can be used for network connectivity. You will be introduced to each group of standards starting with the 10Mbps ethernet connections, then the 100Mbps Fast Ethernet connections, 1Gbps ethernet, and 10Gbps ethernet connections.

Ethernet

The IEEE 802.3 ethernet standards are covered in the following sections. The following list contains all the ethernet standards that are covered in this chapter, in order.

• 10BASE2
• 10BASE5
• 10BASE-T
• 10BASE-FL
• 100BASE-T4
• 100BASE-TX
• 100BASE-FX
• 1000BASE-T
• 1000BASE-TX
• 1000BASE-CX
• 1000BASE-SX
• 1000BASE-LX
• 10GbE
  

10BASE2
10BASE2 networks are connected with RG-58 coaxial cables that use Bayonet Neill Concelman (BNC) connectors. There are no other hardware devices such as hubs or switches to connect devices, just the coaxial cables. This creates a physical bus topology. An electrical signal is sent by each device that wants to transmit data on that network. If more than one device sends a signal at the same time, this causes a collision and the signal is lost. To prevent loss of data transmissions, an algorithm called Carrier Sense Multiple Access Collision Detection (CSMA/CD) was defined. This algorithm sends a jam signal to notify the devices that there has been a collision. The devices then halt transmission for a random back-off time. CSMA/CD must be activated for 10Base ethernet LANs that are connected with a hub.

EXAM ALERT
For the exam, know the definition of CSMA/CD and its capability to act as an arbitrator for devices in an ethernet LAN.

The name 10BASE2 breaks down as follows:
10—10Mbps data transmission speed
BASE—Represents baseband, the signaling mode where the media can only send one signal per wire at a time
2—Actually refers to 185m or the maximum segment length (where 185 is rounded up to 200 and 2 is a multiple of 100m)

10BASE5
10BASE5 has the same characteristics as 10BASE2, but with a maximum segment length of 500m. The 5 is also a multiple of 100m.

10BASE-T
10BASE-T has a maximum segment length of 100m and has a 10Mbps data transmission speed. 10BASE-T can use Category 3, 4, or 5 unshielded twisted-pair (UTP) or shielded twisted-pair (STP) cables for connectivity. If you recall, UTP is the more common and cost-effective solution. STP has an additional shield that provides additional reduction of interference and attenuation, but it is also the more expensive solution. The following cables can be used with a 10BASE-T connection:

• Category 3—Data cable that can handle speeds up to 10Mbps.
  Although it is faster than the Cat2 cable, this was quite popular until network speeds surpassed the 10Mbps threshold.
  
• Category 4—Data cable that can handle speeds up to 16Mbps and is meant to be used with token ring LANs.
  
• Category 5—Data cable that can handle speeds up to 100Mbps and is currently the most popular cable selection.

10BASE-FL
10BASE-FL also has a 10Mbps data transmission speed, but it runs over fiber-optic cables. This option allows for a maximum segment length up to 2km.
Table 3.4 compares the 802.3 ethernet characteristics, listing the key characteristics of each specification.

As you can see, the early standards are all limited to 10Mbps. More recent ethernet specifications allow for faster data transmission speeds and are more popular for today’s networks.

Fast Ethernet

Fast Ethernet was derived for networks that needed speeds in excess of 10Mbps. The IEEE 802.3u defines standards for 100BASE-T4, 100BASE-TX, and 100BASE-FX. You may also hear them collectively referred to as 100BASE-X. Based on what you learned from the 10Base naming scheme, you would be correct to infer that the 100 represents 100Mbps. Also, all three standards are baseband like the 10Mbps family of protocols.

100BASE-T4
100BASE-T4 has the same characteristics as 100BASE-TX except that it can use Category 3, 4, or 5 UTP or STP cables.
100BASE-TX
100BASE-TX, like 10BASE-T, uses either UTP or STP. Category 5 UTP cable is used with this implementation. 10BASE-T has a maximum segment length of 100m.
100BASE-FX
100BASE-FX uses either single-mode or multimode fiber-optic cables to connect. Multimode (MM) fiber set for half-duplex can reach a distance of 412m. Single-mode (SM) fiber set for full-duplex can reach a distance of 10,000m. SC or ST connectors can be used. The drawback  as mentioned before with fiber implementations, is the high overhead.

• Multimode (MM) fiber—This is generally used for shorter distances and is ideal for a campus-sized network. MM also has a larger diameter of optical fiber than SM fiber.
  
• Single-mode (SM) fiber—This mode is used to span longer distances. SM also allows for a higher data rate than MM and faster data transmission speeds.

EXAM ALERT
Fiber-optic cable is not susceptible to EMI, Near-end Crosstalk (NEXT), or Far-end Crosstalk (FEXT).

Gigabit Ethernet

Gigabit Ethernet standards all have a data transmission speed of 1000Mbps (1Gbps) and use a baseband signaling mode. Gigabit Ethernet can be broken down into two IEEE standards, 802.3ab or 1000BASE-T and 802.3z or 1000BASE-X.
1000BASE-T 802.3ab
1000BASE-T or 1000BASE-TX is defined by the 802.3ab standard and can reach a maximum total distance per segment of 75m. This standard uses a minimum of Category 5 UTP cable with an RJ-45 connector.

• Category 5e—Data cable that can handle speeds up to 1Gbps; a popular choice for Gigabit Ethernet networks.
  
• Category 6—Cable that was created to exceed speeds of 1Gbps.
  Table 3.6 summarizes the primary points of interest that are relevant for the 1000BASE-T standard.

1000BASE-X 802.3z
1000BASE-X is the collective name for 802.3z standards 1000BASE-CX, 1000BASE-SX, and 1000BASE-LX that have the following characteristics respectively:

• 1000BASE-CX—1000BASE-CX is the unique standard in this family because it uses shielded copper wire cable with a 9-pin shielded connector instead of fiber-optic cable for connectivity. The maximum total distance per segment is a mere 25m.
  
• 1000BASE-SX—1000BASE-SX transmits short-wavelength laser over fiber-optic cable. Either 50-micron or 62.5-micron (diameter) MM fiber can be used with this option. Lengths may vary depending on the type of MM fiber and duplex chosen for each connection as follows:
  • Half-duplex 62.5-micron MM fiber connections can reach a maximum segment length of 275m.
  • Half-duplex 50-micron MM fiber connections can reach a maximum segment length of 316m.
  • Full-duplex 62.5-micron MM fiber connections can reach a maximum segment length of 275m.
  • Full-duplex 50-micron MM fiber connections can reach a maximum segment length of 550m.
    As you can see, the 50-micron MM fiber can offer longer segment distances. The 62.5- micron MM fiber reaches the same maximum segment length of 275m regardless of the duplex.
    
• 1000BASE-LX—1000BASE-LX transmits long-wavelength laser over fiber-optic cable. Either 50-micron or 62.5-micron (diameter) MM fiber can be used with this option. SM fiber can also be used with 1000BASE-LX, which differentiates this standard from 1000BASE-SX. The same MM fiber length restrictions apply based on the implementation of half- or full duplex. The following lengths apply when SM fiber is used:
  • Half-duplex SM fiber connections can reach a maximum segment length of 316m.
  • Full-duplex SM fiber connections can reach a maximum segment length of 5000m.
    Using full-duplex SM fiber allows for a huge increase in distance. As you can imagine, this is also the more expensive option.

10-Gigabit Ethernet (10GbE)

You guessed it: 1Gbps just wasn’t a fast enough option. Actually, it is just the nature of technology to constantly strive for faster speeds. Yet another new standard was defined by IEEE and labeled 802.3ae. Earlier in this chapter you saw 10BASE-2, which has data transmission speeds of 10Mbps. 10-Gigabit Ethernet transmits data at 10,000Mbps. That is quite an upgrade! IEEE 802.3ae uses 62.5-micron MM, 50-micron MM, or SM fiber-optic cabling for connectivity and a baseband signaling mode.

Long Reach Ethernet

Cisco Long Reach Ethernet (LRE) was developed to provide broadband service over existing telephone-grade or Category 1, 2, or 3 wiring. Speeds vary between 5–15Mbps and can reach a maximum segment length of up to 5000m. Cisco LRE may be a viable networking solution for a LAN or MAN that already has Category 1/2/3 cabling installed. A hotel could benefit from Cisco LRE to provide high-speed Internet or video conferencing solutions to their clientele.

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