CS2302 Computer Networks Unit - I (Physical links)

UNIT I

Network architecture – layers – Physical links – Channel Access on links– Hybrid multiple access techniques – Issue s in the data link layer –Framing – Error correction and detection – Link-level flow control

LINKS

·       All practical links rely on some sort of electromagnetic radiation propagating through a medium or, in some cases, through free space

·       One way to characterize links, then, is by the medium they use

– Typically copper wire in some form (as in Digital Subscriber Line (DSL) and coaxial cable),

– Optical fiber (as in both commercial fiber-to-the home services and many long-distance links in the Internet’s backbone), or

–  Air/free space (for wireless links)

·       Another important link characteristic is the frequency

– Measured in hertz, with which the electromagnetic waves oscillate

·       Distance between the adjacent pair of maxima or minima of a wave measured in meters is called wavelength

–  Speed of light divided by frequency gives the wavelength.

– Frequency on a copper cable range from 300Hz to 3300Hz; Wavelength for 300Hz wave through copper is speed of light on a copper / frequency

–  2/3 x 3 x 108 /300 = 667 x 103 meters.

·       Placing binary data on a signal is called encoding.

·       Modulation involves modifying the signals in terms of their frequency, amplitude, and phase. 

Fig: Electromagnetic spectrum

 

CABLES

 If the nodes you want to connect are in the same room, in the same building, or even on the same site (e.g., a campus), then you can buy a piece of cable and physically string it between the nodes. Exactly what type of cable you choose to install depends on the technology you plan to use to transmit data over the link. Eg: twisted pair, coaxial, fiber optical 

                                         
                                        Common types of cables and fibers available for local links.

LEASED LINES

·    If the two nodes you want to connect are on opposite sides of the country, or even across town, then it is not practical to install the link yourself.

• Your only option is to lease a dedicated link from the telephone company.

                                                Common bandwidths available from the carriers.

LAST-MILE LINKS

• If you can’t afford a dedicated leased line—they ra nge in price from roughly a thousand dollars a month for a cross-country DS1 link
• If you have to ask, you can’t afford it
• Then there are less expensive options available. We call these “last-mile” links
• They span the last mile from the home to a network service provider.

These services are summarized below in Table

                                             Common services available to connect your home.

 

 

 

POTS (plain old telephone service)

• Uses conventional modem
• Transmits data at 56 Kbps over a standard voice-grade line for less than a hundred dollars 

 ISDN (Integrated Services Digital Network)

• An ISDN connection includes two 64-Kbps channels, one that can be used to transmit data and another that can be used for digitized voice.
• A device that encodes analog voice into a digital ISDN link is called a CODEC, for coder/decoder. 
• When the voice channel is not in use, it can be combined with the data channel to support up to 128 Kbps of data bandwidth.

  
XDSL (DIGITAL SUBSCRIBER LINE)

• Collection of technologies that are able to transmit data at high speeds over the standard twisted pair lines

  
ADSL (ASYMMETRIC DIGITAL SUBSCRIBER LINE)

• ADSL provides a different bandwidth from the subscriber to the telephone company’s central office (upstream) than it does from the central office to the subscriber (downstream).
• The exact bandwidth depends on the length of the line running from the subscriber to the central office. This line is called the local loop.
• Downstream bandwidths range from 1.544 Mbps (18,000 feet) to 8.448 Mbps (9000 feet). 
• Upstream bandwidths range from 16 Kbps to 640 Kbps.

 

VDSL (VERY HIGH DATA RATE DIGITAL SUBSCRIBER LINE)

• Data rates ranging from 12.96 Mbps to 55.2 Mbps.
• VDSL runs over much shorter distances—1000 to 4500 feet— which means that not typically reach from the home to the central office.
• The telephone company would have to put VDSL transmission hardware in neighborhoods, with some other technology (e.g., STS-N running over fiber) connecting the neighborhood to the central office, this is sometimes called “fiber to the neighborhood”

CABLE TV (CATV)

• CATV channels are made available for transmitting digital data, where a single CATV channel has a bandwidth of 6 MHz.

• The technology is currently able to achieve 40 Mbps downstream on a single CATV channel, with 100 Mbps as the theoretical capacity.

• The upstream rate is roughly half the downstream rate (i.e., 20 Mbps) due to a 1000-fold decrease in the signal-to-noise ratio.