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					                       Modem Signaling & Cabling

Number of different standards defines the signaling over a serial cable.
It includes:
              EIA/TIA-232
              X.21
              V.35
              EIA/TIA-449
              EIA-530
              EIA-613 HSSI

       Each slandered defines the signals on the cable and specified the connector at the
        end of the cable

       With DB-25 pin connector of EIA/TIA-232 standard, only eight pins are actually
        used for connecting DTE to DCE

       The other 17 signals are ignored and are not interesting

       The eight interesting signals (pins) can be grouped into three categories by their
        functionality:

   1.      Data transfer group.
   2.      Hardware flow control group.
   3.      Modem control group.

                                 DB-25
     DTE                                              DCE
(Computer or router)           RS-232 or            (modem)
                               EIA / TIA

TXD        2                                  2
                                                      Data
RXD        3                                  3       Transfer
                                                      Group
GRD        7                                  7



RTS        4                                  4       Data
                                                      Transfer
CTS        5                                  5       Group
DTR      20                              20
                                                   Modem
CD       8                               8         Control
                                                   Group
DSR      6                               6


DATA TRANSFER GROUP
 SIGNALS                                 DESCRIPTION
   TXD               Stands for transmit data.
                     The DTE transmit data to the DCE.

      RXD            Stands for receive data.
                     The DTE receives data from the DCE.

      GRD            Stands for ground.
                     Provides ground reference for voltage measurements

FLOW CONTROL GROUP
 SIGNALS                                          DESCRIPTION
   RTS             Stands for request to send.
                   The DTE has buffers available to receive form the DCE.
                   This is the signal form the computer or router telling the modem
                   when to send data.

      CTS          Stands for clear to send.
                   The DCE has buffers available to take data from the DTE
                   The signal is used by the modem to tell computer or router when to
                   send data.

MODEM CONTROL GROUP
     Signals                                 Description
      DTR      Stands for Data terminal ready.
               This signal is controlled by DTE.
               The DTE indicates to the DCE that the equipment (computer or router)
               is connected and available to receive data

      CD       Stands for carrier detect.
               This signal is controlled by DCE.
               It indicates that it has established an acceptance carrier signal with the
               remote DCE. (it is a DCE-to-DCE connection)
      DSR      Stands for Data Set Ready
               It gets activate as soon as a modem is turned on.
MODEM COMMUNICATION TERMINATION
Either the DTE device pr the DCE device may signal for the connection to be terminated.
The signals that are used for this function are DTR from DTE or the modem recognizing
the loss of CD signal
The modem connection can be terminated in two ways:

DTE Initiated
      The access server or computer can chop the DTR signal
      The modem must be programmed to terminate the connation on loss of DTR and
       restore to save settings in its NVRM.

DCE Initiated
      The access server detects Carrier Detect (CD) low and terminates the connection.


Modem Operation
       DCE                                         DCE

 TxD                                                      RxD

 CTS                                                      RTS

          Buffer                                        Buffer
                             Flow Control


       Compressor            Compression             Compressor
                             Decompression


        Packetizer           Checksum                 Packetizer
                             Retransmit


       Modulator /                                   Modulator /
       Demodulator                                   Demodulator
UART
     Stands for Universal Asynchrous Receiver Transmitter.
     A computer component (IC) built-in on the motherboard to handle serial
      commutation.
     Every computer contains an UART to manage serial ports.
     External modems have their own UART IC on the circuit.
     It is (UART) controlled by a clock usually running at 1.84 MHz and has
      maximum throughput of 115kbps.
     UART have a buffer to temporarily hold incoming data.
     Some of the models of UART’s are:
                  16 C450
                  16 C550 uses 16-byte buffer.
                  16550 af
                  16750 uses 64-byte transmit & 56-byte receiver buffer.

DTE
     Acronym for data terminal equipments.
     Device at the user end of a user network interface that serves as a data source,
      destination or both.
     DTE includes such devices as computers, protocol translators, multiplexers etc.

DCE
     Acronym for Data Circuit-Terminal Equipments
     Devices and connections of a communications network that comprises the
      network end of the user-to-network interface.
     Modems and interface cards are the examples of DCE.

DB Connector
     Stands for Data Bus Connector.
     Type of connector used to connect serial and parallel cables to a data bus.
     DB connector names are in the format DB-X, where ‘X’ represents the number of
      wires within the connector.
     Although each line is connected to a pin on the connector, not all pins are always
      assigned a function.
     DB Connectors are defined by carious EIA / TIA standards like,
                  EIA / TIA -232
                  EIA / TIA -449 etc.

      EIA / TIA  Electronic industry Association /
                  Telecommunication Industry Association
                 TRANSMISSION CONTROL PROTOCOL
                        HEADER FORMAT.
    TCP is a connection protocol.
    It is a three way handshake protocol.
    Works at transport layer of an OSI model.

                    Source Port (16)            Destination Port (16)
                               Sequence Number (32)
                            Acknowledgement Number (32)
                  Header       Reserved       Code Bits      Window          20
                 Length (4)       (6)            (6)           (16)          bytes
                     Checksum (16)                   Urgent (16)
                                Options (0-32) if any
                                     Data (varies)

             0                                                        31 (bits)


The TCP header length =       20 bytes.
    TCP header size =         32 bytes.

Source port  Shows the port number of the source or calling network devices
                                                                                     16-bits

Destination Port  Shows the port number of destination ort called network device.
                                                                                16-bits

Sequence Number  A number that shows that correct sequence of arriving data at the
                  destination.                                               32-bits

Acknowledgement Number  It shows the next expect TCP segment from the source.
                                                                         32-bits

Header length  The complete length of the header id TCP.
                                                                                     4-bits

Reserved  They are for future use and set to zero in the TCP header.
                                                                                     6-bits

Code Bits  It controls the functions of setup and termination of a specific session.
                                                                                   6-bits

Window  The window is being calculated which the device is willing to accept at
         destination                                                          16-bits
Checksum  Calculates the security measures within the header and data fields.
                                                                                     16-bits

Urgent  It indicates the end of the urgent data. The urgent data depends upon the
         priority set at any of the ends where the data is transmitted to and form.
                                                                                    16-bits

Options  The extra options, features and / or service(s) installed with the TCP header.
                                                                                0-32 bits
                                                                                 (if any)

Data  The data from upper layers.
                                                                                     Varies.

   Q.       Segment to be transmitted from source to destination with the window size of
            4. Show the procedure using TCP for complete three way handshake
            transmission.

   Source                                                    Destination
                            WINDOW 1
                            1   2    3   4

   Send                                                             Receive       Window1
      5                                                                        Segments 1,2,3,4

                                 ACK
                            1   2    3   4
  Send                                                              Receive      Window2
                                                                               Segments 1,2,3,4
        9

                                 ACK
                            1   2    3   4

                                                                    Receive     Window 3
                                                                              Segments 1,2,3,4
UDP Header Format

      UDP stands for User Datagram Protocol.
      It is a Connectionless protocol.
      It is a two way handshake protocol.

            0                             bits                           31
                   Source Port (16)              Destination Port (16)
                     Length (16)                    Checksum (16)             8 bytes
                                      Data (if any)



Source Port  Shows the port of the source or calling network device.
                                                                          16-bits

Destination Port  Dhows the port of the destination or called Network Device
                                                                        16-bits

Length  The complete UDP header length.
                                                                          16-bits

Checksum  Calculates the security features for reliable communication with in the
           UDP header format.
                                                                       16-bits

Data  The upper layer data.
                                                                          Varies.



IP Header Format.
      IP stands for Internet Protocol
      There are two versions of IP address,
            o IP v4
            o IP v6
      IP works at network layer of an OSI model.
      Data transmits form source to destination using source and destination IP address.
      IP provides connectionless, best fit delivery routing of datagram.
      IP is not concern with the contents of the datagram.
      It looks for a way to move the datagram to their destinations.
         0                              bits                                31
              Version      Header Length      Priority &        Total Length
                (4)               (4)          Type of              (16)
                                             Services (8)
                       Identification                Flags       Fragment
                             (16)                     (3)        Offset (13)
                Time to live              Protocol                Header
  20               (8)                       (8)                 Checksum
 bytes                                                              (16)
                                Source IP Address (32)
                               Destination IP Address (32)
                               IP Option (0 or 32 ) if any
                                   Data (varies) if any




Version  Version number of IP.
                                                                                       4-bits

Header Length  The length of the IP Header.
                                                                                       4-bits

Priority & type of service  It shows that number of services that are handled for
                              any IP packet. The first 3 bits are the priority bits.
                                                                                       8-bits

Total Length  The total length of the header + the data attached wit the header.
                                                                                       16-bits

Identification  The Identification of unique IP packet.
                                                                                       16-bits

Flags  It shows weather the fragmentation should occur.
                                                                                       3-bits

Fragment Offset        Provides fragmentation of packets to allow different MTUS in the
                        WAN.                                                     13-bits

TTL  Time to Live. The time for which the header will live.
                                                                                       8-bits

Protocol  The upper layer protocol (transport layer) used and senduip segments to the
           network.                                                             8-bits
Checksum  The internal security and integrated check of the header.
                                                                                    16-bits

Source IP address  The 32-bit IP address of the source network device.
                                                                                    32-bits

Destination IP address  The 32-bit IP address of the destination network device.
                                                                                    32-bits

IP Option  The options like testing, debugging, security etc. (IP SEC)
                                                                               0-32 bits
                                                                                 (if any)

Data  it varies from upper layer protocol.
                                                                                    Varies.




                         SWITCHING NETWORKS.
      For transmission of data beyond a local area, communication is typically achieved
       by transmitting data from source to destination through a network of intermediate
       switching nodes.
      The switching network is also implemented on LANS.
      The end stations that wish to communicate with each other using switched
       network are referred to as stations.
      These end stations may be computers, terminals, telephones or other
       communicating devices.
      The switched networked are a type of networks in which data entering the
       network from a station are routed to the destination by being switched from node
       to node.

Types of Switched Networks.
There are two main types of switched networks which are:
   o Circuit Switching
   o Packet Switching

Circuit Switching Networks.
              Communication via circuit switching implies that there is a dedicated
               communication path between two stations.
              This path is a connected sequence of links between networks.
              Communication via circuit switching involves three phases, which are as
               follows:
                      Circuit Establishments.
                      Data Transfer
                      Circuit Disconnect.

   1. Circuit Establishment
          Before any signals can be transmitted, an end-to-end circuit must be
             established.
          The circuit between end-to-end network devices first establishes then data
             between those devices will flow in both directions.
   2. Data Transfer
          Information can flow now, after the circuit establishment.
          This data can be half-duplex (one way at a time) or full-duplex (in both
             directions at the same time).
   3. Circuit Disconnect.
          After some period of data transfer, the connection is terminated, by the
             action of one of the two stations.

Examples of CIRCUIT SWITCHING
    The well-known example of circuit switching networks is the PSTN (Public
     Switched Telephone Network).
    Another example of the PBX (Public Branch Exchange). It is used to interconnect
     telephones within the building or office.



                      Routing In circuit Switched Network

      In a large circuit switched network link PTCL (Pakistan Telecommunication
       Limited), many circuit connections will require a path through more than one
       switch.
      When the call is placed, the network determines a route through the network form
       calling subscripts (e.g. QTA PTCL) to called subscriber (e.g. KHI PTCL) that
       passes through some number of switches and trunks.
      Two main requirements for the networks architecture that bear on the routing
       strategies are:
           o Efficiency
           o Resilience
      There are two main classes of routing algorithms used in Circuit Switching
       networks, which are:
           o Alternative Routing
           o Adaptive Routing

   Alternative Routing.
   In this type of routing the possible routs to be used between two end offices are
   predefined.
Each switch is given a set of preplanned routes for each destination, in order of
preferences.
The preferred choice is a direct trunk connection between two switches.
If the trunk is unavailable, then the second choice is to be tried and so on.
If there is only one routing sequence defined for each source destination pair, the
scheme id known as a fixed alternate routing scheme.
                                S

             Route 1                               S


                                                                              Destination
   S
                            Route 2


 Source                                     Route 3
                                                         S


Adaptive Routing
An adaptive routing scheme is designed to enable switches to react to changing traffic
patterns ion the network.
This routing scheme requires great management overhead as the switches must
exchange information to learn network condition.

				
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