October 5, 2017

Linux Networking Hardware for Beginners: LAN Hardware


Network hardware
This two-part tutorial looks at networking hardware, from traditional switches and routers to smartphones and wireless hotspots.

Software is always changing, but hardware not so much. This two-part tour introduces networking hardware, from traditional switches and routers to smartphones and wireless hotspots.

Local Area Network

The traditional local area network is connected with an Ethernet switch and Cat cables. The basic components of an Ethernetwork are network interface cards (NICs), cables, and switches. NICs and switches have little status lights that tell you if there is a connection, and the speed of the connection. Each computer needs an NIC, which connects to a switch via an Ethernet cable. Figure 1 shows a simple LAN: two computers connected via a switch, and a wireless access point routed into the wired LAN.


Figure 1: A simple LAN.

Installing cable is a bit of work, and you lose portability, but wired Ethernet has some advantages. It is immune to the types of interference that mess up wireless networks (microwave ovens, cordless phones, wireless speakers, physical barriers), and it is immune to wireless snooping. Even in this glorious year 2017 of the new millennium there are still Linux distributions, and devices like IP surveillance cameras and set-top boxes, that require a wired network connection for the initial setup, even if they also support wi-fi. Any device that has one of those little physical factory-reset switches that you poke with a paperclip has a hard-coded wired Ethernet address.

With Linux you can easily manage multiple NICs. My Internet is mobile broadband, so my machines are connected to the Internet through a wireless hotspot, and directly to each other on the separate wired Ethernetwork for fast local communications. My workstations have easy wi-fi thanks to USB wireless interfaces (figure 2).


USB interface
Figure 2: USB wireless interfaces.

Switches come in "dumb" and managed versions. Dumb switches are dead simple: just plug in, and you're done. Managed switches are configurable and offer features like power over Ethernet (PoE), controllable port speeds, virtual LANs (VLANs), disable/enable ports, quality of service, and security features.

Ethernet switches route traffic only where it needs to go, between the hosts that are communicating with each other. If you remember the olden days of Ethernet hubs, then you remember that hubs broadcast all traffic to all hosts, and each host had to sort out which packets were meant for it. That is why one definition of a LAN is a collision domain, because hubs generated so much uncontrolled traffic. This also enabled easy snooping on every host connected to the hub. A nice feature on a managed switch is a snooping port, which may be called a monitoring port, a promiscuous port, or a mirroring port, which allows you to monitor all traffic passing through the switch.

Quick Ethernet cheat sheet:

  • Ethernet hardware supports data transfer speeds of 10, 100, 1000, and 10,000 megabits per second.
  • These convert to 1.25, 12.5, 125, and 1,250 megabytes per second.
  • Real-world speeds are half to two-thirds of these values.
  • Network bandwidth is limited by the slowest link, such as a slow hard drive, slow network interface, feeble CPU, congested router, or boggy software.
  • Most computers have built-in Ethernet interfaces.
  • Gigabit (1000 Mb/s) USB Ethernet interfaces are dirt cheap, currently around $25, and require USB 3.0.
  • Ethernet is backwards-compatible, so gigabit devices also support slower speeds.

A single user may not see much benefit from 10 Gigabit Ethernet, but multiple users will. You could use a 10 GigE link as your LAN backbone, and use slower hardware to connect your subnets and individual hosts.

What is Bandwidth?

Bandwidth means several things: latency, throughput, error rate, and jitter. Analogies are tricky, but we can illustrate this with a water pipe. The diameter of the pipe limits the total bandwidth: the larger the pipe, the more water it can deliver. Latency is how long you have to wait for the water to start coming out. Jitter measures how smoothly the water is delivered, or how erratically.

I can't think of a water analogy for error rate; in computer networking that is how many of your data packets are corrupted. Data transfers require that all packets arrive undamaged because a single bad packet can break an entire data file transfer. The TCP protocol guarantees packet delivery and re-sends corrupted and missing packets, so a high error rate results in slower delivery.

Having large bandwidth doesn't guarantee that you will enjoy smooth network performance. Netflix, for one example, requires only a minimum of 1.5 Mb/s. High latency, jitter, and error rates are annoying for data transmissions, but they are deadly for streaming media. This is why you can have an Internet account rated at 20-30 Mb/s and still have poor-quality video conferencing, music, and movies.

Ethernet Cables

Ethernet cables are rated in Cats, short for category: Cat 5, 6, 7, and 8. Cat 5 was deprecated in 2001, and it's unlikely you'll see it for sale anymore. Cat 5e and 6 support 10/100/1000 Mb/s. Cat 6a and 7 are for 10 Gb/s. (You also have the option of optical fiber cabling for 10 Gb/s, though it is more expensive than copper Cat 6a/7 cables.) Cat cables contain 4 balanced-signal pairs of wires, and each individual wire is made of either several copper strands twisted together, or one solid copper wire. Twisted-pair cables are flexible. Solid-core wires are stiffer and have less transmission loss.

Plenum cables are designed for permanent installations inside the plenum spaces in buildings, dropped ceilings, inside of walls, and underneath floors. Plenum cables are wrapped in special plastics that meet fire safety standards. These cost more than non-plenum, but don't cheap out because duh, do I have to explain why? Plenum cables should be solid-core rather than twisted pairs.

Patch cables are twisted-pair. Traditionally "patch" meant a short cable, for connecting computers to wall outlets, switches to routers, and for patch panels, though they can be as long as you need, up to about 300 feet for Cat 5e, 6, and 6a. For longer runs you'll need repeaters.

Come back next week for part 2, where we will learn how to connect networks, and some cool hacks for mobile broadband.

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