CS 164: Slide Set 2: Chapter 1 -- Introduction (continued).

Roadmap Performance metrics –bandwidth –latency –bandwidth delay product –throughput What are sockets ? APIs.

Bandwidth Bandwidth: Number of bits that can be transmitted over a certain tiem -- typically per unit time. Some people also refer it to the spectrum -- example 10 Ghz. This typically translates to a maximum data rate.

Transmission Time A function of bandwidth If bandwidth is B, transmission time is 1/B. If bandwidth is 10 Mbps, the transmission time is 1/(10 x 10 6 ) = 1  s.

Propagation Delay Once a bit is put on a link, the time it takes to go across the link. Depends on the speed with which the electromagnetic signal (light) travels in the medium -- 2 x 10 8 m/s in fiber. Propagation delay = distance/speed of signal.

Queuing Delay At each intermediate node or router, a packet is queued. Thus, it has to wait prior to transmission. How long does it have to wait ? Dependent on the load on the network -- how many packets are traversing that router ?

Latency How long does a packet take to go from one host to another. Also called “Delay”. Latency = Propagation Delay + Queueing Delay + Transmission Delay

Round Trip Time Packet is sent from sender to receiver. Receiver sends ACK (assume immediately) to sender. Total time delay incurred between the instance the packet is set to the time the ACK is received. Note if forward delay = backward delay, RTT = 2 * Latency (typically assumed -- although not always accurate).

Bandwidth Delay Product Think of this as a pipe. How much does the pipe hold in bits ? In essence, if I send a stream of bits, the first bit traverses the bit in “Delay” seconds. In the meantime, how many more bits can I send ? Bandwidth Delay

Example For a transcontinental channel -- latency = 50 milliseconds. Bandwidth = 45 Mbps. Bandwidth delay product = 50 x x 45 x 10 6 = 2.25 Mbits We can transmit 2.25 M bits before the first bit reaches the other end of the channel !

What if ACK is expected ? Note if ACK is expected, how many bits can the user transmit before he expects to have an ACK ? RTT X Bandwidth For symmetric channels 2 X Delay X Bandwidth.

Throughput Defines how efficiently channel is being used. Throughput = Transfer size/ Transfer time. What is the transfer time ? RTT + (Transfer size/Bandwidth) (ignoring queuing delays).

Example 1 MB file over a 1 Gbps network with RTT 100 milliseconds. Transfer time = 100 ms + (1 MB/1Gbps) = 100 ms + 8 ms = 108 ms. Effective throughput = 1 MB/108 ms = 74.1 Mbps.

Impact of data size If data size increases, (Transfer size/Bandwidth) increases. Could become much larger than RTT. In that case, Throughput ~ (Transfer size /(Transfer size/Bandwidth) ~ Bandwidth! Bottomline : Throughput increases with data transfer size.

APIs and Sockets Network protocols implemented as a part of the OS. API -- Interface that OS provides to the networking subsystem. This interface is exported to the user or the application process.

Socket Interface Session Presentation Application Transport Link Network Physical Appl TCP/UDP IP Device Drivers/HW User Process Kernel Process Application Details Communication Details Socket

A note about sockets Data through sockets -- similar to writing to a file. Sockets were originally in BSD (Berkeley software development) but now are popular in almost all OS. Rest of sockets: Read book, Labs.