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					                                                                      Lectures 9-10:
                                                                         DATA
                                                                       NETWORK
                                                                       SUPPORT
                                                                      by Prof. Michael Tse



In this lecture, we will take a look at the available technology infrastructure that connects
people to the Internet. We will review a few common forms of network technology and
study their operations and limitations.

      Connection/access technologies
         1. xDSL (x Digital Subscriber Line)
         2. ISDN (Integrated Services Digital Network)
      Switching technologies
         1. Frame relay
         2. SMDS (Switched Multi-megabit Data Service)
         3. ATM (Asynchronous Transfer Mode)

Reading materials

A very useful source of reference is http://www.gare.co.uk/technology_watch/index.htm
About ADSL, see http://www.cs.tut.fi/tlt/stuff/adsl/pt_adsl.html
About ISDN, see http://www.ralphb.net/ISDN/ and http://www.isdnzone.com/
About ATM, see http://www.atmforum.com/
About frame relay, see http://www.frforum.com/basicguide/toc.html




xDSL (Digital Subscriber Line)
DSL is a technology to employ existing telephone network (copper wires) to transmit
                                                      high-speed data for the Internet
                                                      users.

                                                          In fact, POTS (plain-old-
                                                          telephone service) uses just a
                                                          little bit of what the copper wires
                                                          can do. The fact is that POTS
                                                          uses 4kHz bandwidth while the
                                                          copper twisted pair has up to
                                                          1.5MHz over a distance of 3km.

                                                          A few basic forms have
                                                          emerged for different
applications/performance. So, the "x" can be "A", "S", "H" and "V" as defined below:

   1.   ADSL Asymmetric DSL: 1.5 Mbps-384kbps/384-128kbps
   2.   HDSL High-bit-rate DSL: 1.5 Mbps/1.5 Mbps (4Wire)
   3.   SDSL Single-line DSL: 1.5 Mbps/1.5 Mbps (2Wire)
   4.   VDSL Very high DSL: 13 Mbps-52 Mbps/1.5 Mbps- 2.3 Mbps.
   5.   IDSL ISDN DSL: 128 Kbps/128 Kbps.
   6.   RADSL Rate Adaptive DSL: 384kbps/128kbps

Note: T1 line is an earlier technology that uses the same old copper wires. Typically T1
is leased to a company (usually provided by the phone company) for high-speed data
transmission. No voice can be used simultaneously. But, the latest ADSL can put voice
and data together, and at a more affordable price. It is now penetrating our homes! You
will see how and why later.




Overview of frequency spectrum of DSL
Spectrum of the copper wires
(conventional twisted pair in
existing telephone networks)

The copper wire bandwidth can
be up to 1.5MHz for a distance of
3 km. For shorter distance, it can
be even wider.

Spectrum of DSL

Basically the DSL spectrum
extends from 20kHz up to
1.1MHz. Thus, DSL can be
handled by traditional telephone copper wires for a few kilometers.

One important observation is that the attenuation increases as frequency goes higher.
So, DSL will experience poor performance for the high-frequency part. For example,
ADSL uses a discrete multi-tone (DMT) technique (to be discussed later) which
allocates the high-frequency part mainly for downstream transmission. Hence, for longer
distance transmission where the high-frequency attenuation is severe, the downstream
data rate can be much affected.

[Note: Downstream -- towards the user. Upstream -- towards the ISP.]




Asymmetric DSL (ADSL)
ADSL is widely used for Internet connection via existing telephone lines. The "A" stands
for asymmetric, which means the upstream and downstream bandwidths are unequal.
For most Internet applications, like poping mail messages, web browsing, downloading
files, the downstream data rate is more demanding. Also, for video-on-demand, the
traffic is mainly downstream. So, it makes sense to partition a much larger bandwidth for
downstream traffic than for upstream.

Advantages --- up to 140 times faster than VF modems; simultaneous voice and data
transmission; using same existing copper wires

Installation --- a special ADSL modem is installed at the user's premises.

                                                Discrete Multi-Tone (DMT) DSL

                                                The ADSL technology uses a discrete
                                                multi-tone (DMT) approach to send data
                                                through the copper wire. Simply put,
                                                there are 256 modems turning on at the
                                                same time and transmitting data at
                                                different frequency slots. The modulation
                                                technique for each tone partition is QAM
                                                (Quadrature Amplitude Modulation).

      Upstream -- 20 - 138 kHz
      Downstream -- 140 - 1100 kHz

For long distance, the downstream data rate is drastically reduced because of the high
attenuation at the high-frequency region.


Other line coding schemes
Other forms of DSL may use other coding
schemes. The different coding schemes
have different impact on the frequency
spectrum.

For example, some HDSL use a 1B2Q (1-
binary-2-quaternary) scheme, and the T1
line uses a AMI (alternate mark inversion)
scheme. Obviously, as shown, the AMI
needs a much wider spectrum (more high
frequency components) than the 1B2Q
scheme. Both schemes require no additional
filters.

For ADSL, additional hardware (ADSL modem) is needed for the DMT scheme, and
special filtering is needed to compensate the high-frequency attenuation.

Question: What else does ADSL offer over the T1 or HDSL? Look at the spectrum in an
earlier slide!!
                                            Integrated Services Digital
                                            Network (ISDN)
                                            ISDN is another technology that uses existing
                                            copper-wire telephone lines for combined
                                            digital telephone, data, video plus other
                                            services. The most common type of ISDN that
                                            serves homes and most private companies is
                                            the Basic Rate Interface (BRI). For larger
                                            business, Primary Rate Interface (PBI) is
                                            used.

ISDN divides the telephone line into a number of digital "B" channels and one "D"
channel, each of which can be used simultaneously. (BTW, "B" stands for bearer, and
"D" stands for data.)

          Rate
Channel              Function   Number
          (kbps)
                                2 for BRI
                     carrying
B         64                    23 for
                     data
                                PRI
          16 for BRI
D                    control    1
          64 for PRI

To access BRI service, it is necessary to subscribe to an ISDN phone line. Customer
must be within 18000 feet (about 3.4 miles or 5.5 km) of the telephone company central
office for BRI service; beyond that, expensive repeater devices are required, or ISDN
service may not be available at all. Customers will also need special equipment to
communicate with the phone company switch and with other ISDN devices. These
devices include ISDN Terminal Adapters (sometimes called, incorrectly, "ISDN
Modems") and ISDN Routers.




ISDN installation
From the telephone company to NT-1

The phone company usually provides a U-interface, which is a two-wire interface
directly out of the phone switch. This U-interface is connected to Network Termination-
1 (NT-1), which then gives an S/T-interface.
From NT-1 to
NT-2 inside the
home

NT-1 simply
changes the 2-
wire interface to
a 4-wire S/T-
interface. This
S/T-interface can
connect up to 7
devices/equipme
nt. (Note: Inside
an ISDN
equipment, there is an NT-2 which handles the lower-layer ISDN protocols.)

ISDN terminal equipment (TE)

Some terminal equipment can interface with S/T-interface directly, but some don't.
Terminal equipment 1 (TE-1) refers to those that can be connected to S/T-interface (the
ISDN capable), and terminal equipment 2 (TE-2) refers to those that cannot be
connected directly to S/T-interface but have a POTS phone jet. For the latter, we need a
terminal adaptor to connect it to the S/T-interface.

For details of protocols, see http://www.ralphb.net/ISDN.




Frame relay
What is frame relay?

Basically frame relay is a high-
speed communications
technology that is used to
connect LAN and Internet. A
frame relay network consists of
endpoints (e.g., PCs, servers,
host computers), frame relay
access equipment (e.g.,
bridges, routers, hosts, frame
relay access devices) and
network devices (e.g., switches, network routers, T1/E1 multiplexers).

Why frame relay?

The main purpose is to provide high-speed data transfer for packet-switched systems
such as ones that are based on the old X.25 and TCP/IP. The existing switching method
in telephone network (PSTN) is based on circuit-switched technology; the frame relay
technology is developed to bridge this gap.
Brief operation of frame relay

Frame relay technology is based on the concept of using virtual circuits (VCs) --- 2-way
software-defined transmission paths that replace private lines in the network. It is a way
of sending information over a wide area network (WAN) that divides the information into
frames or packets. The frames travel through a series of switches within the frame relay
network and arrive at their destination.

NEXT LAST


Frame relay -- virtual circuits
Virtual circuits

Two types of virtual circuits are

   1. Permanent virtual
      circuits (PVCs) --
      original standard; more
      commonly used
   2. Switched virtual
      circuits (SVCs) --
      getting popular now

PVC

PVCs are set up by a network
operator (e.g., a private [PolyU]
network or a service provider).
A PVC is

      defined as a connection between two sites or endpoints;
      fixed path, not to be set up on a call-by-call basis;
      pre-configured by the provider or network manager with given bandwidth
       allocated packet-by-packet (i.e., no bandwidth is allocated to the path until actual
       data is transmitted.)

Note that actual path taken through the network may change from time to time, such as
when automatic rerouting takes place after a new site is added. Also, since the
beginning and end of the circuit will not change, the PVC is like a dedicated point-to-
point circuit.

SVC

SVCs are more complex than PVCs. They are available on a call-by-call basis. When
setting up a SVC, the user specifies a destination address similar to a phone number.
The network dynamically establishes connections based on requests by many users (as
opposed to PVCs where a central network operator configures the network). The
network then quickly establishes the connection and allocates bandwidth based on the
user's requests.
Frame relay -- operation overview
   1. Virtual point-to-point
      circuit is established first.
   2. The frame's destination
      is specified in a data link
      connection identifier
      (DLCI) header, which is
      added to the packet.
   3. If there is an error
      handling a frame
      (because of congestion
      usually), it just discards
      the frame.
   4. The frame relay network
      has no error correction; it
      relies on the higher layer
      protocols in the user
      devices to recover the
      lost frames by retransmitting them.
   5. Clearly, without error correction, frame relay requires lines with low error rates to
      achieve good performance.
   6. Usually, congestion is the main cause of discards; thus, the network's ability to
      avoid and react to congestion is extremely important in ensuring satisfactory
      performance.




SMDS (Switched Multi-megabit Data Service)
SMDS is another high-speed communication technology, and its function is similar to
frame relay network. So, SMDS is a competitor of frame relay.

SMDS is considerably faster (up to 34Mbps now; 155Mbps will be available), and
operates in a different way.

   1. It is a connectionless service, i.e., there is no traditional call set-up/tear-down
      process such as encountered with X.25. Like the LAN on customer's premises,
      every SMDS cell contains the FULL address of the destination in the header.
      Local exchange cell relay switches route each packet to the destination LAN and
      then onto the destination machine. The shared bandwidth ensures high speed
      routing.
   2. When a local PC or workstation wants to send data to a remote machine, the
      LAN creates data packets and forwards these to the SMDS compatible router.
      The router, which is equipped with an SMDS interface card, converts the variable
      length frames generated by the LAN into fixed length SMDS cells, which are then
      sent to the local public exchange via a bearer link. On receipt, the destination
      SMDS router converts the cells back into the LAN packet format and forwards
      them to the destination PC of workstation.
   3. It consists of routers which embrace IP packets in SMDS packets (this is done by
      SMDS Interface Protocol (SIP)).

Other features:

   1. It has additional provision for restricted access to certain paths; it has added
      security.
   2. It has multicast capability (e.g., one-to-many broadcast of video).
   3. It can create virtual private network (specially arranged and charged).
   4. It has a mechanism to keep track of users' activity and network usage for
      accounting.
   5. Access speed is from 4 Mbps up to 32 Mbps. Hence, it can directly link to
      10Mbps Ethernet LANs and 16Mbps token ring LANs.

Reference: http://www.gare.co.uk/technology_watch/smds.htm




Hardware Switch Technology -- ATM (Asynchronous
Transfer Mode)
The hardware switching technology that supports very fast switching network is the
Asynchronous Transfer Mode (ATM).

Why ATM is fast?

Using hardware --- It achieves
very high speed by using static
hardware (not software) to route
packets of uniform length (cells).
Each cell is 53 octets long (1 octet
= 8 bits) which includes 48 octets
of data and 5 octets of addressing
and control information. Because
of uniform length, hardware
switching is possible. ATM
switches are designed to switch data streams in parallel so that where cells belonging to
one call are within the switch this does not delay cells passing through the switch
belonging to another call.

Being asynchronous --- The other crucial feature is it being asynchronous. Unlike
traditional synchronous switching which uses Time Division Multiplexing (where the data
position within a group of data is synchronized to the channel it belongs to), ATM does
not synchronize with the time slots, but uses an identifier in each cell to indicate which
channel it belongs to.

For details of switcher design, see
http://www.atmforum.com/atmforum/library/notes1.html.
                                                          NEXT LAST


                                                          Comparison
                                                          Switching technologies

                                                          The aforediscussed network
                                                          technologies differ in
                                                          operation, cost and speed.

                                                           From what we see at the
                                                           present time, frame relay and
                                                           SMDS seem to transient
technologies for switching networks which will probably still be there for some time. In
future, when speed is crucial, ATM will be the ultimate way to go, unless a new powerful
technology emerges in the near future.

Access technologies

ADSL will still the main technology for accessing Internet from homes and small
businesses. This will obviously sustain a rather long transient since the traditional
copper wires are there and it will take some time before optical fibe can penetrate and
totally replace the existing telephone network.




Summary
In this lecture, we have reviewed some current technologies for supporting network
access from homes, businesses and corporations. We have seen how the conventional
telephone hardware can be used to provide relatively wideband transmission. In addition
we have reviewed some network connection technologies such as frame relay, SMDS
and ATM.

				
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