Cabling and Packet Flows

How does data flow in hubs, switches and routers

Types of Communication

• Unicast - one-to-one

• Broadcast - one-to-all

• Multicasts - One-to-some - Only sends traffic if they have subscribed.

• Ping broadcast address to get CAM table fill.

  • eg. #ping 255.255.255.255

• Multicast traffic is "flooded" and NOT broadcast - they are different hexadecimal addresses (FF:FF:FF:FF)

Auto MDI/MDIX

• Normally Routers and PC's use MDI

• Normally Switches and Hubs use MDIX (Medium Depemdent Interface Crossover)

• Auto MDI/MDIX allows for automatic switching once a cable is connected

Cable Cateogories

• The higher the category the more twists it has within the cable. Its also less suscebtible to electromagnetic interference and the more stringent the specifications for cross-talk and system noise.

DAC Cables

Direct Attachment Cable (DAC) uses copper Twinax and comes up to lengths of up to 15m and has SFP's on each end.

Hubs

Physically its a star topology but logically its a bus. It lives at the PHYSICAL LAYER and is a SINGLE collision and SINGLE broadcast domain. Traffic is received by everyone and not uni-cast like switches etc. Remember the HUB is a multi-port repeater and if its a 10Mbs its SHARED amongst all ports. If you had 4 ports it would be calculated as 10/4*30% = 0.75Mbps per port. The 30% is maximum utilisation.

Bridges

Remember wireless networks operate like Hubs - for example a 54Mbps wireless network is actaully 54/by tjhe number of devices connected to the access point!! A bridge is a L2 device and uses MAC address table to intelligently send traffic. Brigges are slower as they do the processing in software whereby switches process frames using ASIC's (Application Specific Intergrated Circuits). MAC addresses are learnt by traffic coming through the bridge. With a bridge, EACH port is in its own COLLISION domain, but a SINGLE broadcast domain. Remember aslo, a HUB is a SINGLE collision domain and SINGLE broadcast domain.

Switches

Similar to bridges in a sense that it resides on Layer 2, it also has a MAC address table to forward traffic but it processes the traffic in ASIC's has more ports available than a bridge. There is no degradation of traffic unlike a bridge, they operate at much higher speeds (wire speed switching) and each port gets the full speed (10Mbs or 1Gbps)