EE 450 Computer Networks Discussion Session 3 1 Some Terminology n Bit b n n n Byte B n n Basic unit of information in computers Binary 0 or 1 8 bits in one byte Bit Rate n n n Number of bits transmitted in a time unit Typical unit is bits per second bps Used to measure transmission speed in digital transmissions 2 Terminology continued 1K Bytes 1000 Bytes 8000 bits n Similarly 1M Bytes 1 000 000 Bytes However n 1 Kbps 210 bps 1 Kbps 1000 bps n Similarly 1 Mbps 106 bps n In this course the approximation 1KB 1000 Bytes is always allowed 3 Terminology n n n Delay Latency Time it takes a message to travel from one end of a link to another It is a very important performance parameter End to End delay consists of several components n n n n Transmission time Propagation delay Nodal Processing time Queuing delay 4 Transmission time n n How long does it take to transmit a message usually in KB over a link with bit rate usually in Mbps Steps n 1 Convert message size to bits n n n n 1KB 1000 bytes 1MB 1 000 000 bytes 1 Byte 8 bits Key is the difference between B and b 5 Transmission time ctd n 2 To obtain the transmission time divide the message size in bits by the bit rate a k a bandwidth in bps Transmission time Message size Bit rate 6 Transmission time example Ex How long does it take to transmit a 4KB file over a link with 1Mbps bandwidth Solution Step 1 Convert the file size to bits 4 KB 4 x 1000 Bytes 4000 Bytes 32000 bits 1Mbps 106 bps n Step 2 Transmission Time file size bandwidth ttrans 32000 bits 106 bps 32ms n 7 Propagation delay n Propagation delay The time it takes for a bit to traverse from one end of the link to the other end tprop Link length m Vprop m s Where Vprop is the speed with which the bit travels in the medium same as the speed of light in the given medium 8 Propagation delay example Ex What is the propagation time of a message in a link of 2 5 Km long The speed of light in the cable is 2 3 x 108 m s Solution tprop Link length Vprop 2500 m 2 3 x 108 m s 10 9 s Attention tprop is independent of message size and bit rate of the link 9 Message Transfer Time n Message transfer time txfr Time taken from the point when the sender starts transmitting the message till the receiver receives the entire message Also known as end to end delay txfr ths ttrans tprop tqueuing processing Where n ths is the handshake time time it takes for the initial connection establishment phase n tqueuing processing is the queuing and processing delay in the network n We will assume the latter as zero most of the time 10 Round Trip Time RTT n n n Round Trip Time The time to send a message from a sender to the receiver and receive a response back Depends on the message size length of link direction of propagation propagation velocity speed node processing delay network traffic load etc We will assume RTT 2 x tprop n n May not be true if the message and the response choose different links to traverse The other delay components are ignored here 11 Bit Duration n n Bit Duration duration in time of a pulse representing a bit depends on bit rate bandwidth of the link Bit Duration 1 Bandwidth n n A bit is 1 s wide in a 1 Mbps channel 1 106 bps 1 x 10 6 seconds per bit A bit is 0 5 s wide in a 2 Mbps channel 12 Bit Length bit Width n n Bit length The length occupied by a bit on a transmission link Bit length Bit durationx Prop Speed sec x meters sec meters 13 Bandwidth Delay Product n n n Product of Bandwidth and link latency propagation delay Represents the maximum number of bits present in the link at given time Analogy n n A Pipe delay is the length bandwidth is the width Bandwidth Delay product gives the volume 14 Example 1 Ex A terminal sends a 1 MB file to another computer through a link of 10 Mbps The distance between the two terminals is 2000 Km and the propagation speed in the cable is 2x108 m s a What is the RTT b What is the Bandwidth Delay Product Use RTT as the delay c What is the bit duration d Assume a handshake period of 2 RTT s and no processing queuing delay what is the total transfer time of the file 15 Example contd a RTT 2 tprop tprop 2 x 106 m 2 x 108 m s 10 msec Therefore RTT 20 msec b Bandwidth X Delay 10 Mbps x 20 ms 200000 bits 25000 Bytes 25 KB 16 Example contd c Bit duration 1 Bandwidth 1 10 Mbps 10 7 sec bit 0 1 s bit d txfr ths ttrans tprop ttrans 1 MB 10 Mbps 8 Mb 10 Mbps 800 msec ths 2RTT 40 msec tprop 10 msec txfr 40 800 10 850 msec 17 Example 2 Bandwith or Delay Sensitive n For each of the Following operations on a remote file server discuss whether they are more likely to be delaysensitive or bandwidth sensitive n n n n Open a file Read the contents of a file List the contents of a directory Display the attributes of a file 18 Solution n n n n Delay sensitive the messages exchanged are short Bandwidth sensitive particularly for large files Technically this does presume that the underlying protocol uses a large message size or window size stop and wait transmission as in Section 2 5 of the text with a small message size would be delaysensitive Delay sensitive directories are typically of modest size Delay sensitive a file s attributes are typically much smaller than the file itself even on NT file systems 19 Example 3 n Hosts A and B are each connected to a switch via 10 Mbps links as shown in the figure The propagation delay on each link is 20 s S is a store and forward device it begins transmitting a received packet 35 s after it has finished receiving it Calculate the total time required to transmit 10 000 bits from A to B n n As a single packet As 2 5000 bit packets sent one right after another 20 Solution BW 10 Mbps A S Dprop 20 s B Dprop 20 s a Per link transmission delay is 104 bits 107 bits sec 1000 s Total transfer time 2 1000 2 20 35 2075 s 21 Solution continued b When sending as two packets here is a table of times for various events T 0 start T 500 A finishes sending packet 1 starts packet 2 T 520 packet 1 finishes arriving at S T 555 packet 1 …