lecture3

Hardware Layer Networking: 

Hardware Layer Networking One Layer Lower

Hardware Issues: 

Hardware Issues The previous discussions have been about the IP layer, and mostly ignored the hardware layer. Notice that we didn’t say anything about the wires. From the standpoint of IP, all that is hidden What sorts of things do we have to worry about at the hardware layer? Signal loss/degradation Maximum distance/latency

Latency: 

Latency Ethernet works via the “party line” principle Listen on the wire to see if anyone else is talking If not, begin sending If two people talk at the same time, stop, and wait some random amount of time to try to stat talking again. (Why a random time?)

Latency: 

Latency What if two stations are a long ways away? Host1 Host2 H1 and H2 listen, H1 talks, H2 doesn’t Immediately hear H1 due to latency, and Begins talking too. Ethernet can recover from this, but it’s not good. The delays designed into the protocol depend on A maximum latency between two stations

Repeaters: 

Repeaters Signals can degrade over distance. A simple piece of electronics can just accept the ethernet signal on one side and resend it on the other Repeater Host1 Host2

Repeaters: 

Repeaters Repeaters are also known as “hubs”. These are similar to fan-out devices, popular with twisted pair A popular way to network computers in one house/office Hub Host1 Host2 Host2

Repeater Benefits: 

Repeater Benefits Cheap Can add range to a network In twisted-pair networks an easy way to get hosts talking to each other

Bridges: 

Bridges Bridges are similar to repeaters, but a bit smarter. They also operate at the hardware/data link layer The idea is to not forward traffic across the bridge if it’s not needed on the other side Bridge Host1 Host4 Host2 Host3

Bridges: 

Bridges Bridges listen passively for traffic on either side of the bridge, then keep track of which MAC address is on which side (Remember, the MAC address is not the same as the IP address) When a frame comes in on one side, the bridge decides whether or not to forward it to the other side. This means the bridge must maintain a table of MAC addresses and what interface that MAC address is associated with.

Bridges Vs. Routers: 

Bridges Vs. Routers What’s the difference between a bridge and a router? The bridge operates at the hardware layer (layer 2) while routers operate at layer 3. Bridges work with ethernet frames, while routers work with IP packets. You can have multiple bridges within one IP network Broadcasts will cross a bridge

Bridge Benefits: 

Bridge Benefits Cuts down somewhat on traffic on parts of the network Broadcast traffic still crosses bridges On the other hand, somewhat more expensive than repeaters/hubs. This is not much of an issue these days as the cost of hardware trends to zero for high volume equipment. The difference these days is only a few bucks.

Switches: 

Switches The bridges described so far have had only two ports. What happens if we have more? Bridge H1 H2 H1 H1 H1 H1 The bridge just has a slightly more complex table And forwarding algorithm

Switches: 

Switches If we take this to its extreme, we have one port per computer This is called a switch. This term is rooted mostly in marketing hype—manufacturers needed a way to differentiate their products from other bridges, so they came up with a different term. A switch is a bridge with a marketing program attached.

Layer 3 Switch: 

Layer 3 Switch A layer 3 switch (named from the OSI seven layer model) is just a router Generally the routing is done in hardware rather than is software as with older routers Less flexible than a software-based router, and may not support all protocols. On the other hand they are very fast when compared to conventional routers. L3 switches are MUCH more expensive and complex than L2 switches and are generally sold by major vendors (Cisco, Dell, Foundry) to enterprises.

Protocol Encapsulation: 

Protocol Encapsulation As information goes down the protocol stack from the application it gets “encapsulated” in the lower protocol info TCP Data info TCP Data info TCP Data info IP Data IP Data Ether Data

Encapsulation: 

Encapsulation If you examine packet data in ethereal you can see this principle in action A packet has a ethernet frame, IP, UDP or TCP, and payload section, each corresponding to a different layer in the TCP/IP protocol stack

Protocols: 

Protocols Notice that we can, at the ethernet layer, have multiple higher level protocols at the same time. As far as ethernet is concerned, it doesn’t know whether the packets being passed around contain TCP/IP or Appletalk This means we can have multiple protocols running on ethernet at the same time; Apple machines talk to apple machines, TCP/IP machines talk to TCP/IP machines, and there is no conflict