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LAN

Introduction

• LAN = Local Area Network
• It just means a local network—doesn’t have to be connected to any other networks.
• The term LAN is independent of the medium, or the technology used to transmit data over the medium.
• An implicit assumption is that the number of computers on a LAN is relatively small.
• LANs far predate the Internet.

Historic perspective

• For the moment, don’t think of a LAN as just being a way to get to the Internet.
• Instead, think of a business office, back in the before times. You might have:
  • the engineer’s computer
  • the boss’s computer
  • the secretary’s computer
  • a printer.
• You’d want them all connected together, on a LAN.
• You don’t really need to be connected anywhere else.
  • There is no Internet.
• To get something to another department, you carry or mail a floppy disk.

Types of LANs

• Ethernet
  • Cat-5 cable
  • Wi-Fi
• Bluetooth
• AppleTalk
• Token Ring
• ARCNET
• FDDI
• And many, many more, fallen upon the ash heap of history.

The basic problem

• Here’s the basic problem: most LAN technologies use a medium that is common to all computers in the LAN.
• They’re all listening to the same radio frequency, sensing voltage on the same wire, etc.
• What if computer A and B both want to talk at the same time?
• They’ll both try to transmit radio waves at the same time, or put voltage on the wire at the same time, and neither one wins.

Human Solutions

• We humans have the same problem. We all speak and listen on roughly the same range of frequencies.
• If two nearby people try to talk at the same time, neither can be heard.
• How do we solve this …
  • on a date?
  • in a classroom?
  • in a meeting?
  • at a party?

Token Ring

┌───┐     ┌───┐
│ A │·····│ B │
└───┘     └───┘
  :         :
  :         :
┌───┐     ┌───┐
│ D │·····│ C │
└───┘     └───┘

• Token Ring is a LAN technique developed by IBM in 1984.
• Imagine computers connected in a ring, with data flowing clockwise.
  • A can send data only to B.
  • B can send data only to C.
  • C can send data only to D.
  • D can send data only to A.

Token Ring

┌───┐     ┌───┐
│ A │·····│ B │
└───┘     └───┘
  :         :
  :         :
┌───┐     ┌───┐
│ D │·····│ C │
└───┘     └───┘

• A small packet, or token, is constantly flying around the ring.
• It’s usually empty.
• When a given computer receives the token, it examines it.
• If it contains data for that computer, it consumes the data, and sends an empty token to the next computer.
• If it doesn’t contain data for this computer (or is empty), send it on to the next computer.
• If a computer wishes to send data to another computer:
  • Wait until it sees an empty token.
  • Fill the token with our data, along with a To: address.
  • Send it to the next computer, which may or may not be the target.

Token Ring

• What are the advantages and disadvantages of Token Ring?
  • Simplicity?
  • Latency?
  • Throughput?
  • Robustness?

Ethernet

• Ethernet, on the other hand, has all computers on a single wire.
• We hope that they don’t all talk at once!
• If two computers try to talk at the same time (called a “collision”), they detect it and back off for a random amount of time.

            ┌───┐     ┌───┐     ┌───┐     ┌───┐
            │ A │     │ B │     │ C │     │ D │
            └───┘     └───┘     └───┘     └───┘
              :         :         :         :
              :         :         :         :
              ·······························

Star Configuration

       ┌───┐  ┌───┐  ┌───┐
       │ A │  │ B │  │ C │
       └───┘  └───┘  └───┘
          ·     :     · 
           ·    :    ·  
            ·   :   ·
    ┌───┐   ┌────────┐   ┌───┐
    │ H │···│ Switch │···│ D │
    └───┘   └────────┘   └───┘
            ·   :   ·
           ·    :    ·  
          ·     :     · 
       ┌───┐  ┌───┐  ┌───┐
       │ G │  │ F │  │ E │
       └───┘  └───┘  └───┘

• With modern wired Ethernet, all computers are wired directly to a switch.
• It’s like many small Ethernets.
• The only possible collisions are if the computer and the switch start to talk at the same time—much less likely.
• However, the switch could get overwhelmed.
• More expensive—you have to buy a switch.

Hub vs. Switch

• An Ethernet hub, also called a repeater, simply connects everything to everything.
• That is, if A were sending data to B via a hub, C & D would also see the data.
• If A were sending to B at the same time that C were sending to D, a collision would occur on a hub.
• An Ethernet switch, on the other hand, only sends data where it needs to go.

                ┌───┐  ┌───┐  ┌───┐  ┌───┐
                │ A │  │ B │  │ C │  │ D │
                └───┘  └───┘  └───┘  └───┘
                  :       :    :       :
                  ······  :    :    ····
        	       :  :    :    :
        	      ┌───────────────┐
        	      │ Hub or switch │
        	      └───────────────┘

MAC addresses

• When using Ethernet, each device has a unique 48-bit MAC (Media Access Control) address
  • 2⁴⁸ = ~281 trillion possible MAC addresses
  • Really, they should only have to be unique on a given LAN, but they’re now also used to identify computers for licensing.
  • Official style: 00-00-5E-00-53-42
  • Linux style: 00:00:5E:00:53:42

Ethernet Packet

• Note the lack of IP addresses.
• What if you want to send more than 1500 bytes?