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					      Катедра Информационни технологии и комуникации


Lecture 3 – Telecommunications and Networks

What is Telecommunication

•Telecommunication is sending signals on a distance, with a purpose devices to
communicates – computers, telephones, TVs, Radio, etc.

•Telecommunication systems consist of 3 parts:
     oMedia for exchange signals

•The exchanged signals can be analog or digital, but in ICT are used only digital

•History of telecommunications
     oEarly stage of sending signals
     oTelegraph, telephone
     oRadio and television
     oComputer networks and Internet

ISO Reference model with7 layers

       Application layer

     Presentation layer

       Session layer

       Transport layer

        Network layer

        Data Link layer

       Physical layer

What are the computer networks

  •Computer networks connect computers between them

  •The networks link computers for the purpose of exchange of information and
  share resources

  •According to its purpose, Computer networks can be classified as:
      oPersonal Area Networks (PAN) – linking devices near to the human
      oLocal Area Networks (LAN) – linking computers in a short distance
      oCampus Area Network – interconnection of LANs in a limited
      geographical area
      oMetropolitan Area Network (MAN) – interconnection of LANs in a
      territory of a town
      oWide area networks (WAN) – linking of LANs on big distance
      oGlobal Area Networks (GAN) – linking computers located globally on
      different continents (there is no exact technical definition)

 Correspondence between 2 international standards

ISO Reference model    TCP/IP model

   Application layer

 Presentation layer    Application layer
   Session layer
    Transport layer    Transport layer     TCP, UDP – connection oriented protocol

   Network layer       Network layer       IP – Not-connection oriented protocol

    Data Link layer    Network access
     Physical layer

 Different networks according their functioning
•Server based network – uses central resource – Server, managing the communications
between different computers and is used as a shared resource
•The computers in Server based networks communicate as “client” to the Server. The
Servers concentrates the main computing resources
     •Example of Server based network - Microsoft LAN, Novell LAN etc.
•In Peer-to-peer networks – P2P) each computer play the role of “client” and “Server”
•How the network will behave - as Server based network or P2P network, is set when a
communication is needed. There are some networks, in which the possibility for P2P is set
during the installation
     •Example of P2P network – not in general functioning of LANs, but for downloading
     movies, music, between Email servers for exchange emails, etc.

  Server based network                                  Peer-to-peer network            6
    Main cables used in networks
•   Twisted pair cables are pair of twisted wires between them and are
    the most cheap transmission media used in networks. They consist of
    2 wires, each one with isolation and the twisting provide equal
    electromagnetic intensity of both wires. Outside they can be covered
    by a shield for between electromagnetic isolation – Shielded twisted
    pair. In a cable can be a few twister pairs with a single outside shield.
    Their main disadvantage is the shorted distance that they can support.

•   Coaxial cables are used for transmission of high radio-frequency. Their
    are more expensive that the Twisted pairs, but can transmit data on a
    longer distance. The installation of coaxial cables is as easier as for the
    twisted pairs. The coaxial cable consists of Copper wire and Copper
    Mesh with isolation between them and outside. The Copper Mesh
    provides higher electromagnetic isolation. These cable can be used in
    LANs and in TV connections.

•   Optical fiber cables are made from glass or plastic fiber and carry light
    along its lengths. Fibers are used instead of metal wires. Light is kept
    in the core of the fiber by total internal reflection. They are wrapped
    in bundles of 10’ and 100’ cables. They are used for high-speed
    transmissions. The main disadvantage is their high-cost for connection
    between the cables.
Network topologies

                                            •Signal topology
 •Star                                      •Physical topology
 •Mesh and Full mesh                        •Logical topology
 •Line                                      •Hybrid topology
 •Tree                                      •Distributed topology

             Ring      Mesh          Star          Full mesh

                Line          Tree                    Bus

Topology Star
•   Star topology has an approach in which sending and receiving of messages passes
    through a central entity – like in a telephone systems. All messages have to go first to
    the Central entity and after that to be readdressed to the receiver. The central entity is
    a device or a computer – with a name Central communication device, Concentrator or
    Hub. Each computer is connected to the central communication device via 2 single way
    lines – one for sending and one for receiving; or via full-duplex line.

                                                •Easy to install and wire.
                                                •No disruptions to the network then connecting or
                                                removing devices.
                                                •Easy to detect faults and to remove parts.

                                                •Requires more cable length than a linear topology.
                                                •If the hub or concentrator fails, nodes attached are
                                                •More expensive than linear bus topologies because
                                                of the cost of the concentrators.

Bus topology
•   Bus topology uses a bus for data exchange. Connection of a new computer is easy – via
    a link to the Bus. The information security in the Bus topology is not easy. In
    comparison to the other topologies, the length of the cables is the shortest, (in Star
    topology the cables create problems)

Ring topology
•   Ring topology combines the advantages of Star and Bus topologies. One of the
    computer is dedicated to manage the network. When a computer goes out of order, the
    network continue to work. The messages going from one computer to the other are
    only in one direction. There is a check, whether the sent messages are received. When
    a computer receives a message, its store it, mark it as received and send in the ring to
    the sender, confirming its receiving.

Local Area Network (LAN)
•Local Area Network – LAN is a computer network in a scope of limited
geographical area – office, home, small number of buildings, airport, university,
• Historical evolution – Cambridge Ring (1974), ARCNET (1977), Token Ring,
•LANs connect PCs, Desktops, Big servers, Printers, Scanner, Plotters, etc.,
permitting data exchange between those resources
•Based on its big bandwidth capacity (10’s Gbps), LAN are the main network in
Data centers.
•Cables used in the LAN are Twisted pair, Shielded twisted pair, coaxial, fiber
•LAN uses also wireless way of connection (WiFi)
•The connected devices can exchange data
•The connected devices can share devices
•The types of connected devices are in category “network devices” and can include
computers, servers, printers, shared devices
•LAN are made mainly by “Switches”, working on 2nd Layer of ISO Reference

Type of servers used in LAN

 •File servers – server with disks, offering to the computers in the network its shared
 disk space (quick and with large amount)

 •Print servers – server with printer, permitting all computers in the network to print on
 that printer (generally high-quality printers, with high speed)

 •Email server – server with software for email services

 •Data base server – server with installed DBMS, used from many computers

 Role of Switches in LAN
•The Switches are network devices amplifying the electrical signals of the connected to them
devices, providing exchange of data between those devices – using Star topology
•Each device is connected to the Switch via a port
•When a network packet from a devices reaches the Switch, it resend it to the appropriate port of
the device-received of the packet
•The Switches are working on Layer 2 of the ISO Reference model
•In the current LANs, where Ethernet is the predominant Layer 2 protocol, the Ethernet standard is
the main one, using ISO 802.x standards
•On the Layer 2 - Media Access, the devices addressing uses MAC (Media Access Control) addresses
•There are Switches, in which are incorporated Routers, and based on Routers those devices are
becoming Layer 3 devices
•The Switches on Layer 3 use the incorporated Router and work also with IP packets and routing
functions, such as Packet inspection, Packet prioritizing, QoS (Quality of Service), etc.
•The Switch is multi-port device, where each port can define:
      oNetwork bandwidth
      oType of VLAN – Virtual LAN
•VLAN permits to separate the ports as parts of different logical LANs
•The devices connected to a Switch can belong to different VLANs. The devices belonging to the
same VLAN can communicate between themselves
•VLAN is important instrument in the network security, providing logical separation between the
devices connected to a Switch
•Same VLANs can be created between few Switches                                              14
   Connecting Switches in a LAN

   Computer                                                                 Computer
 Application Layer              Communication in Layer 2                  Application Layer

Presentation Layer                  of the ISO model                     Presentation Layer

  Session Layer                                                            Session Layer

  Transport Layer                                                          Transport Layer

  Network Layer         Switch                           Switch            Network Layer

   Data Link Layer    Data Link Layer                  Data Link Layer      Data Link Layer

   Physical Layer    Physical Layer        LAN        Physical Layer        Physical Layer

Wide Area Networks – WAN

• WAN covers big geographical territories

•The purpose of WAN is to connect different LANs, located in big distance, for
exchange information

•WAN is used mainly for corporate need – connecting the distributed offices, as well
as connecting people world-wide

•Mainly TCP/IP protocol is used in WANs

•Most popular WAN is the Internet

•WAN is built via Routers – network devices working on Layer 3 and 4 of the ISO
Reference model

•Router is also used to connect the LAN to the WAN

Using Routers and Switches to build networks


           LAN-1                       LAN-2

   Connecting computers in WAN
                           Router                        nication on              Router
                                                           Layer 3
                         Network Layer                                          Network Layer

                          Data Link Layer                      WAN               Data Link Layer
                           Physical Layer                                         Physical Layer
                                                                                                              Application Layer

                                                                                                             Presentation Layer

                                                                                                               Session Layer

                                                                        Commu-                                 Transport Layer
                                               Switch                  nication on          Switch             Network Layer
                                                                         Layer 2
                                             Data Link Layer                               Data Link Layer      Data Link Layer
   Computer                                 Physical Layer              LAN-2            Physical Layer         Physical Layer
 Application Layer

Presentation Layer

  Session Layer

  Transport Layer                                Commu-
                        Switch                  nication on             Switch
  Network Layer
                                                  Layer 2
   Data Link Layer    Data Link Layer                                  Data Link Layer
                                                 LAN-1               Physical Layer
   Physical Layer    Physical Layer
In the Internet there are few Address Classes – IPv4
• In addresses Class A, the MSB is 0 (zero), the following 7 bits are used for
  the network part and the rest 24 bits are for host address . Therefore,
  there are 128 networks Class A, with more than 16 million computers in
  each network

• In Addresses Class B the two MSB are in “10”, the next 14 bits are
  dedicated for network number, and the rest 16 bits are for host address.
  Therefore, there are 16 000 networks Class B with more than 64 000 hosts
  in each network

• In Addresses Class C the 3 MSB are in “110”, the following 21 bits are for
  network number and the rest 8 bits are used for addressing the hosts.
  Therefore, there are 2 million network Class C and 256 hosts in each

• In Address Class D the 4 MSB are in “1110”, and the following 28 are used
  for addressing groups of computers (multicast address). This permits using
  a single IP address for a “group of host machines”, which are set of
  computers identified by a single IP address.
Example of addressing in the Internet

                                 Class A

     0       Network                             Host

                                      Class B
     1   0       Network    Network                       Host

                                      Class C

    1    1   0    Network   Network             Network          Host

 Example of addressing in Internet
                                       Class B
      1   0   Network        Network                 Host              Host

                           Using a Mask =
      1   0   Network        Network             Sub-Network           Host

•Let’s assume an Co has a network Class B with IP address, e.g. more than 64
000 host addresses.
•This network can be split into 256 Sub-networks, using some bits dedicated to hosts for
creation of a filed “Sub-Network”. For this reason a Mask with value is
•In the Mask, the “1” define to use the bits from the IP address to form the network numbers.
•In the Mask, the “0” define the bits used to address the Host
•In this example, the higher Host bits are used to specify the address of the Sub-Network,
and lower – the address of the Host

Role of Routers in the network
                   LAN                            Router




•The Router transform the LAN message into an IP address for using in the WAM, for the
purpose to rich the destination Host; and reverse
•The part of the transformation is making a Header part of the IP packet, containing IP address
of the sender and IP address of the receiver host
•The Router takes a decision where to send the IP packet (to reach the receiver host), using
Routing Table                                                                                 22
•Router makes also packet filtering
     Router types and Switches types
                   Entrance Router


    •Internal Router
    •Edge Router

                                 LAN-1                                LAN-2

                                            Can be incorporated
                                           into a single device –
                                             Switch of Layer 3
LAN Architecture Layers
                                                   ISO Layer-3

 Edge routing
                                                       ISO Layer-2

Distribution                                                               VLAN1            VLAN2


               Switch                                       Inside there           Access
                                                             is a Switch           Switch

                        Data VLAN
                        VLAN for IP telephone (for voice)                              24
Building packets for using between the Layers –
“Envelope” principle

Layer 7                               Data                   Original application message

Layer 4                               Data         TCP Hdr                ТСР packet

Layer 3                               Data         TCP Hdr     IP Hdr           IP packet (with IP addresses)

                                                                                              Ethernet packet
Layer 2              MAC End          Data         TCP Hdr     IP Hdr     MAC Hdr           (with МАС address)

                     End of packet                                      Start of packet

•IP address is on Layer 3. The Router readdress the packet to find the receiver
•Each device recognizable in the network has an IP address (32 bit for IPv4 and64 bit for IPv6)
•МАС (Media Access Control) address is on Layer 2. Generally, this is a hardware created address (48
bit– 24 to identify the vendor and 24 for the device)
•The Router transforms the МАС address into IP address
•Firewall is used to limit the entrance of one network to the other
       oFrom outside network(Internet) to the internal corporate network
       oBetween internal networks (using different level of security inside the corporate system) –
       e.g. DMZ and Internal network, Development segment and Production segment, etc.
       oBetween different corporate (B2B – Business-to-Business)
•Firewall support security policy:
       oWhich services are accessible for using
       oWhich IP addresses are forbidden to use
       oWhich TCP and UDP ports are available and which stopped
•All traffic passing through the Firewall is under inspection
•Firewall can be created via:
       oSpecialised device (Appliance)
•From functioning point of view, the following Firewalls can exist:
       oFirewall for packet filtering – first generation Firewalls
       oFirewall – Proxy – second generation
       oFirewall keeping session state – third generation
       oFirewall for dynamic packet filtering – forth generation
       oFirewall using Proxy as a Kernel – fifth generation

The Router resends IP packets

 •The Router can be done via hardware or software
 •The Router connects different components of the network, using
 different Interfaces (network cards) – an Interface per a component
 •The Router isolates the different parts of the network
 •The decision – where to resend the packet is made using Routing
 Table, based on the addressing information of the incoming packets
 •Router and Firewall often work together logically
 •There are devices combining both Router and Firewall


Router conceptual architecture

         Network-0            Network-1                  Network-n

       Interface-0           Interface-1               Interface-n

                                     Router Bus

                                           Processor        Configuration


Telnet, SSH, Web interface
Corporate architecture for eBusiness applications

•DMZ is perimeter zone, separating controlled computers from uncontrolled
•Using Router for creation of DMZ is weak solution from Information security
point of view
•It is recommendable to use Firewalls for separation of the zones

                         DMZ                Production zone

                                             server                Intranet

                       Web server –
     Internet          non critical

                                           Server room                         29