95% of networks are Ethernet

Early history:

  • developed at Xerox Palo Alto by Metcalfe and Boggs
  • standardized by Xerox, Intel, and Digital Equipment Corporation
  • dev’d Ethernet I and Ethernet II stds in early 1980s
    • stds listed as IEEE “802” because first meeting was in Feb 1980… 802 committee!
    • 802 committee works with ISO
    • 802.1 for general stds, including security
    • 802.11 for wireless LAN (a thru x, says Mark)
    • 802.16 for WiMax

802.3 Working Group is in charge of creating Ethernet stds

  • “802.3” and “Ethernet” interchangeable
  • Ethernet stds are Open System Interconnect (OSI) stds
    • Layet 1 and 2 stds are almost always OSI stds
  • vendors may start building compliant products before 802.3 Group ratifies

Ethernet exists in the Physical and Data Link layers (1 and 2), transmitting frames across single network

Ethernet Physical Layer Standards:

  • 10BASE-T: 10 Mbps (obsolete), 4-pair Cat-3+
  • 100BASE-TX: 100 Mbps, 4-pair Cat-5+
  • 1000BASE-T (Gigabit Ethernet): 1 Gbps, 4-pair Cat-5+
  • 1000BASE-SX: Fiber! 1 Gbps
  • S = 850 nm, L = 1310 nm, E = 1550 nm

Baseband: signal transmitted as straight signal, electrons right from source

Broadband: signal modulated

Switches regenerate signals, removing propagation effects.

Data Link layer is split into Media Access Control (MAC) layer and Logical Link Control (yes, LLC) layer.

  • MAC layer specific to tech
  • LLC handles general rules (corporate IT guys don’t think about it)

Hey! remember: switches read MAC addresses; routers read IP addresses. Switches are really fast because they know where things are going: MAC addresses don’t change, so there’s nothing to check. IP addresses can change, so routers have to figure out where to send the signal.

Little Cisco/Linksys switches for $80 will get you by for workgroups of fewer than 10. Otherwise, get out the checkbook.

We generally set up switches in hierarchy. DSU for instance: one master switch in science center, talks to at least one main switch in each building…. Trouble with hierarchy: single point of failure means doom! 802.1D standard allows alternate paths as backup links! 802.1w RSTP (Rapid Spanning Tree Protocol).

Switches can provide Virtual LANs (VLANs): instead of each switch isolating the items beneath it, folks on one branch under one switch can virtually network with folks on other branches.

Remember: Ethernet isn’t reliable! No standard for rebroadcasting data! So how do we handle momentary traffic peaks? Via overprovisioning (build in more capacity than you anticipate needing) and priority (marking some traffic that can’t tolerate latency as high-priority while other packets are held back; if low-priority packets exceed the buffer, they get lost!).

Hub versus Switch Operation

  • Hubs are old: used a lot, broadcast signals to whole network
  • Switches directed signals to MAC address: signal in one port, out the other, all the rest remain available
  • Hubs are cheaper, but switch tech has become affordable enough that hubs have fallen away (to the point where they are hard to find!)
  • Carrier Sense Multiple Access/ Collision Detection (CSMA/CD) protocol runs hubs
  • Collision Detection works by picking up the voltage jumps of colliding signals, waits random period of time, then tries transmitting again
  • Hubs have bad latency; switches avoid need to wait, allow multiple simultaneous conversations

Things to Consider When Buying Switches

  1. How many ports?
  2. How fast each port
  3. do you want store-and-forward (sends whole packet together) or cut-through (starts shooting packet before whole thing has come through). Cut-through reduces latency somewhat, but advantage flattening.
  4. Simple Network Management Protocol (SNMP): controls many managed switches, notifies you if the network is having trouble (reduces need for human sys admins… alas!)
  5. Physical size: how big a rack do you have? (Just that physical container can cost! Looking good costs money!)
  6. Port flexibility:
    1. fixed-port switches have no flex; set number of ports; 1-2U tall
    2. stackable switches: fixed # ports, 1-2U tall, hi-speed interconnect bus connects stacked switches; ports can be added in increments of as few as 12
    3. Modular switches: 1-2U tall; contain one or a few slots, each slot module contains 1-4 ports; can swap out copper/fiber connectors; spendier
    4. Chassis switches: several U tall, several expansion slots, several slots per board; high-end, usu. used for core switches; may have duplicate/back-up power supply; Craig Miller pegged a replacement model for DSU at $60K-80K
    5. Uplink Ports: create crossover cable (pins 1&2 to pins 3&6) to connect two separate switches (you can also connect two computers via their network card this way)
  7. Power: Switches require power, but they can also provide power over UTP! Power over Ethernet (802.3af) lets us power our wireless access points and other devices via switch instead of having to install more electrical outlets! Now right now, the juice is up to 13 watts, enough for wireless access points, VoIP phones, little doodads. 802.3at may allow boost to 30-60 watts, more doodads, but still not enough for PCs.

Security: The original Ethernet developers didn’t worry about it (ah, the good old days). DSU currently has a database of all MAC addresses allowed on the network. You bring a machine from off-campus, it won’t connect.

802.1X standardizes authentication of everyone who connects to an access switch. This addresses the possibility of someone walking in the building and plugging straight into the network to avoid having to hack the firewall from outside.

Routed LANs: switches and routers getting more alike, so some orgs just go to routers


Back from the break: We work with OPNET IT Guru. Mmmm… fun! Just curious: anyone ever have cause to change the propagation speed?


Panko Chapter 5: Wireless LANs!

DSU has wireless everywhere, even the football stadium. 140 wireless access points total. SDSU would need 800 access points @$300

Note that Bob Metcalf, one of the Ethernet developers, built his network on radio packets (AlohaNet).

802.11: popularly known as “Wi-Fi” (wireless fidelity). Not developed to replace wired networks! Expectation was that you’d have wireless devices plugging into a wired backbone. Wired still has fewer interference issues.

Wi-Fi transmission speed: up to 300 Mbps; usu. 10Mbps-100Mbps

Interference sources: EM, rapid inverse-square law attenuation, absorptive attenuation (e.g., moisture!), shadow zones (dead spots, like our quartzite buildings create with all the iron in the stone!), multipath interference. You also have to check what channel your access points are using: if two adjacent access points run on the same channel, they will interfere with each other. (So visit your neighbors, see what channel they’re on!) 802.11n will help with that problem, since it operates in the 5 GHz band, where it has 11 to 24 non-overlapping channels.

Higher frequency means more absorptive interference and deader shadow zones (think about waves: this makes sense!)

Shannon’s Equation governs distance limitations, interference, etc. (See the book! It’s cool!)

Golden Zone: 800 MHz to 6GHz

The little Linksys at home can easily support 10 users. The Cisco outfits will support up to 50 ($300).

Wireless uses a different frame format; the access point translates.

Ad hoc wireless network: turn your computer into an access point!

802.11 uses CSMA/CA (Collision Avoidance): tries not to transmit when anyone else is transmitting; will also kick in ACK (acknowledgment) if things are really tight.

Lots of flavors of 802.11: g (54 Mbps, every access point supports it) is the dominant standard at the moment. n will bring us 100Mbps to 300Mbps (overkill for most of us) and works well at longer distances

n uses Multiple Input Multiple Output (MIMO) transmissions, bigger channels (40 MHz instead of g‘s 20 MHz)

Main problem for wireless: security. Note that the security does not extend back past the access point. WEP came out in 1997: not good, but better than nothing. WPA (Wireless Protected Access) came from WI-Fi Alliance, created as stopgap until 802.11i (a.k.a. WPA2), which came in 2002. If you’re running WPA, you’re pretty secure. If your equipment can’t be upgraded to WPA, ditch it! WPA2 is more secure.

Bluetooth: Personal Area Network! 10m, 3Mbps! Uses less power than Wi-Fi card. Not secure, but hacker needs to be within 10m of you to hack.

UWB: ultrawide band

Zigbee: used for almost-always-off sensors; low speed (250 Kbps)

RFID tags get power from the radio signals beamed at them! No battery needed! (though you can include one to allow the tag to transmit signal further)

Hey, watch for Mark’s Camtasio recording on NetStumbler!