Lecture 6

 The application layer provides the necessary structure to run an application on a network  The transport layer sets up a connection for the application  However neither the application layer nor the transport layer identify the target machine – this is the role of the network layer Physical Link Network Transport Application

 In order to run Wireshark, you will need to have access to a computer that supports both Wireshark and the WinPCap packet capture library.  Go to -

 The main screen looks like:

 Network Manager Headaches  Signals  Transmission Media  Multiplexing

Network Manager Headaches

 Managed services provider is setting things up for a security company, and after many meetings, everyone agrees that secure FTP is the best way to transfer the files containing sensitive customer and sales information. "They seemed excited by the prospect of encryption," says a Net Adm on the managed services side. "Within a day or two of the decision, we coordinated with their IT department to obtain the log-in information. We received our log- in and password to their 'secure' FTP site -- via standard . I neglected to follow up with them on their own internal security procedures, but we requested a password change within minutes.“  Company changes systems and begins to remove deleted e- mail automatically after two weeks instead of leaving that to users. And that makes one user howl. "Turns out he stores all his s in the Deleted Items folder as soon as he reads them, because he can get them out of his in-box with just a click on the big 'X,'" says the Net Adm there. "I suggested that he create a folder for items he didn't want to disappear. His first reaction: 'You mean I have to make a new folder called Deleted Mail when there already is one?'"

Signals

 To be transmitted the individual bits must be transformed into electromagnetic signals Transmission media -- Buffering Communication Channel Properties: -- Bandwidth -- Transmission and Propagation Delay -- Loss/Error rates

0 1  There are two types of signals: Analog and Digital  Analog signals have a infinite range  Digital signals are discrete

 A typical sine wave looks like: 1 second Period = 0.5 second The frequency is the inverse of the period f = 2 Hz

 baud (D) - the number of changes per second, typically the bits per second  bandwidth (H) - the range of frequencies that is passed by a channel. The transmitted signal is constrained by the transmitter and the nature of the transmission medium in cycles/sec (hertz)  channel capacity (C) – the rate at which data can be transmitted over a given channel under given conditions. {This is also referred to as data rate (R).}

 A signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency?  A signal has a spectrum with frequencies between 1000 and 2000 Hz (bandwidth of 1000 Hz). A medium can pass frequencies from 3000 to 4000 Hz (a bandwidth of 1000 Hz). Can this signal faithfully pass through this medium? 40 Hz no The analog bandwidth of a medium is expressed in hertz; the digital bandwidth, in bits per second.

 In the Digital world, the waveforms define bits: SECOND 8 BITS Bit rate = 8 bps

 For data transmission, the bit rate between the sender and the receiver must match – they must be synchronized This is called unipolar encoding

 In a digital transmission with unipolar encoding, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 Kbps?  How many if the data rate is 1 Mbps? 1000 bits x 0.1% = 1 extra bit 10 6 bits x 0.1% = 10 3 extra bits

 A good encoded digital signal must include a provision for synchronization  For example, in Manchester encoding  There is always a mid-bit transition {which is used as a clocking mechanism}.  The direction of the mid-bit transition represents the digital data. 0 1 ?1???

 Consider how Manchester encoding can be used to synchronize a bit transmission: Notice what happens when the clock is off

Transmission Media

Physical Sender Physical Receiver

 Two insulated wires arranged in a spiral pattern.  Copper or steel coated with copper.  The signal is transmitted through one wire and a ground reference is transmitted in the other wire.  Typically twisted pair is installed in building telephone wiring.

 Unshielded Twisted Pair (UTP)  Ordinary telephone wire  Cheapest  Easiest to install  Suffers from external EM interference  Shielded Twisted Pair (STP)  Metal braid or sheathing that reduces interference  More expensive  Harder to handle (thick, heavy)

CategoryBandwidthData RateDigital/AnalogUse 1very low< 100 kbpsAnalogTelephone 2 < 2 MHz2 MbpsAnalog/digitalT-1 lines 3 16 MHz 10 MbpsDigitalLANs 4 20 MHz 20 MbpsDigitalLANs MHz 100 MbpsDigitalLANs 6 (draft) 200 MHz 200 MbpsDigitalLANs 7 (draft) 600 MHz 600 MbpsDigitalLANs

 10 Mbps baseband transmission over twisted pair.  Two Cat 3 cables, Manchester encoding  Maximum distance meters Ethernet hub

 Most versatile medium  Television distribution  Cable TV  Long distance telephone transmission  Can carry 10,000 voice calls simultaneously  Being replaced by fiber optic  Short distance computer systems links  Local area networks  More expensive than twisted pair, not as popular for LANs

 Analog – Broadband Coaxial Cable  Amplifiers every few km, closer if higher frequency  Up to 500MHz  Cable TV, Cable Modems (~10Mbps)  Digital – Baseband Coaxial Cable  Repeater every 1km  Closer for higher data rates NameTypeMbpsmIn… RG-58Coax Base2, “ThinNet” RG-8Coax Base5, “ThickNet”

 Breakthrough in data transmission systems!  Core: Thin strands of glass  Cladding: Glass with different optical properties than core  Jacket: Plastic/Insulation

 Greater capacity  Data rates of hundreds of Gbps  Tbps demonstrated  Smaller size & weight  Order of magnitude smaller than TP/Coax  Lower attenuation  Electromagnetic isolation  Not vulnerable to interference, impulse, crosstalk!  Greater repeater spacing  Often 10’s of kilometers  Hard to tap

Multiplexing

 Fundamental to networking  General concept  Used in  Lowest level of transmission systems  Higher levels of network hardware  Protocol software  Applications

 Separate pairs of communications travel across shared channel  Multiplexing prevents interference  Each destination receives only data sent by corresponding source MUX DeMUX Shared channel

 Time Division Multiplexing (TDM)  Only one item at a time on shared channel  Item marked to identify source  Demultiplexor uses identifying mark to know where to deliver  Frequency Division Multiplexing (FDM)  Multiple items transmitted simultaneously  Uses multiple “channels”