Foundations of Networking Part 2 CS 3470, Section 1 Sarah Diesburg

The Big Picture A communication pathway is a link between two or more nodes A network is a collection of path-connected communication pathways Book's definition: A network is a construction of nesting networks, where at the bottom level, the network is implemented by some physical medium. A bit awkward, but motivates “the cloud.”

The Network Nodes on the inside implement the network [switches, routers, and gateways] Nodes on the outside use the network [hosts]

The Internetwork “Clouds helping clouds” would form an internetwork, or—you guessed it—an Internet.

Network Glue A node that is connected to two or more networks and conveying packets from one to another is referred to as a router or a gateway Also does packet switching, but different than a switch

Where’s Waldo Where are the routers/gateways?

Where’s Waldo Where are the switches?

Where’s Waldo Where are the end hosts?

Network Glue Just because there are links doesn't mean that there's a network. Routing requires destination recognition, which requires addressing. Unicast (me send to you) requires addressing, whereas broadcast (me send to all) doesn't. How about multicast? (me send to group)

Multicast Multicast occurs when a node wants to send a message to some subnet of the other nodes Streaming media and Internet TV In actual practice, doesn’t scale well

Multiplexing Much like timesharing of a CPU, data links can be multiplexed over the physical links of a network. Recall the difference between circuit switching and packet switching?

Circuit Switching In circuit switching, there are only so many dedicated links Has anyone ever heard the message “All circuits are busy. Please try your call again later.”?

Circuit Switched Networks All resources (e.g. communication links) needed by a call dedicated to that call for its duration Example: telephone network Under circuit switching, all the resources needed by a call are dedicated to that call for the duration of the call. This is used in telephone networks. Good thing about it is its guaranteed service. You are assured of the required resources for the entire duration of the call. The bad thing is that resources are not utilized efficiently. There may be silent periods during the talk but the dedicated network resources cannot be used by other calls. Whether you are shouting or silent you consume the same amount of network resources. What happens when the resource demands exceed the resources available. For example, there is only one circuit/link. Lets say is A is currently occupying the circuits between C-D and D-E. Now suppose B wants to call E. Then B is blocked. Once call is admitted, you are guaranteed to have good quality of service.

Packet Switching Can handle multiple “streams” at once Sharing system resources among multiple users is called multiplexing. Multiplex onto switch 1 vs demultiplex off switch 2

Multiplexing Synchronous Time-Division Multiplexing (STDM) is when you get the whole link, but only for specific time quantum. If there are 4 sending hosts, the switch cycles through 4 quantums (even if some of the hosts are quiet)! Frequency-Division Multiplexing (FDM) is when you get an always-available stream, but it's only a portion of the entire spectrum (like TV channels) But how do you add a new stream once all the portions are used?

Multiplexing Statistical multiplexing overcomes those challenges Link is shared over time Flow is on-demand, not by necessarily by “reservation” How does the switch choose which packet to send next?

Multiplexing Scheduling Strategies How do we choose which packet to send next? First In, First Out (FIFO) Priority Need to worry about starvation Round Robin How would this be implemented in STDM vs statistical multiplexing?

Multiplexing A lot of multiplexing is logical, statistical or aggregate. For example, in ATM networks, you might get a guaranteed 10% peak rate by earmarking one out of every 10 frames to your channel. The 10% rate is a logical rate (like FDM), but the implementation is through STDM.