The Problem with Shared CSMA/CD

Carrier Sense Multiple Access / Collision Detection

Hubs and Switches perform the same function, but are significantly different.

• Hubs send all incoming signals to all connected PCs.
• Hubs work at the Physical Layer, sees the signal and sends it out.
• Hub Speeds are very fast – Wire Speed.
• No difference between Unicast, Broadcast or Multicast
• 1 Collision Domain: Only one device can send at a single time
• 1 Broadcast Domain: One broadcast will travel through every Hub connected to it.
• The more PCs you connect, the more collisions you’ll get and transmissions rates slow significantly.

Understanding Collisions

• One of the colliding devices will send a “Jam Signal”
• It will halt all communication for a random amount of time (<1 second) before attempting to resend.
• After both devices have sent their data, the Jam is released and traffic can resume as normal.

Understanding Bridges

• Bridges connect between hubs and learn MAC addresses.
• By using a Bridge, you divide your network into 2 Collision Domains (or more, depending on # of ports on the bridge.)
• This allows both Collision Domains to sent amongst themselves without creating collisions.
• Bridges were popular between Mid ’80s to early 90’s
• Bridges were slow.  MAC address learning done via software.

Understanding Switches

• Switches operate at the Data Link Layer because it “Sees” MAC addresses and can learn them.
• Each Switchport is a Collision Domain – All devices can Transmit at the same time!
• Allow Full Duplex Communications (Can Transmit and Receive Simultaneously.)
• MAC Learning done via ASIC chip (Application Specific Integrated Circuit) so very fast!
• Switches are as fast as Hubs – Wire Speed!

Understanding Collision Domains and Broadcast Domains

Collision Domain: How many devices can send or receive at the same time.

Broadcast Domain: How far a signal can travel before it stops.

How a Switch…Switches!

• Switches ONLY Learn a MAC Address when that connection Transmits
• If PC-A sends an ARP, looking for PC-B:
  • It sends its MAC as the Source MAC
  • It sends FFFF:FFFF:FFFF as the Destination MAC
  • This designates that ALL recipents should LISTEN to the Packet!
• The Switch receives the ARP and learns the location of PC-A
• The Switchport and MAC are then stored in the CAM Table (Content Addressable Memory)
• When PC-B receives the ARP, it replies with its MAC, which is then stored.
• It takes a Switch <15 Seconds to learn all MAC addresses connected to it.

If a Switch DOES NOT KNOW a MAC Address:

• Assume PC-A already knows the MAC address of PC-C, it will encode both the Source MAC and Destination MAC in the Packet.
• The Switch receives the Packet and not knowing which Switchport owns the Destination MAC, so it sends to ALL Switchports (Except the Sending) with FFFF:FFFF:FFFF (Broadcast)
• Only the correct owner of the IP address embedded as the Destination Address will respond and in doing so will send the correct MAC for the Switch to learn.