Passing to mobile communications Telecommunication ...calin.comm.pub.ro/Didactice/ARI/Notite curs/Prez/En... · • Access Networks (AN) = networks that connect domestic users and

0. Introduction

• Communications era: telephones, radios and televisions, computer

terminals to access Internet; senses for ships, aircrafts, rockets and satellites

→ Passing to mobile communications;

• Telecommunication (computer) network composed of (HW+SW∼ graphs):

– Data terminals (stations / computers)– Data terminals (stations / computers)

– Network devices: repeaters, bridges, switches, routers, and gateways;

– Transmission lines and systems.

• Communication protocol = a set of rules for exchanging informationbetween the network components (data terminals).

Chapter 1

Fundamentals of Fundamentals of

Communications Networks

Architectures and Internet

FW1

1.1 The components of a broadband

network

Core Net R1

TN1

R2

LAN

FW2

AN2

AN3

TN3

Network topology ~ GraphMobile Net

Core Net

(CN)R1 LAN1

AN1

SAN1

TN2

SAN2 LAN2

TN – Transport Net

AN – Access Net

LAN – Local Area Net

T

SAN – Storage Area Net

R – Router

FW – Firewall

T - Terminal

Data NetIntegrated

Services Net

� A global broadband network (e.g., Internet is the largest one) = a network

composed by smaller networks with different technologies, different

transmission media, and thus, with different performances;

� Broadband network components (having different covered geographical


network (contin.)

� Broadband network components (having different covered geographical

areas and different functions):

• Access Networks (AN) = networks that connect domestic users and

corporations to the Internet infrastructure. Examples of ANs: Digital

Subscriber Lines (DSL), mobile telephony (3G, 4G), and cable modems (TV);

• Local Area Networks (LAN) = networks that interconnect several users inside

a reduced area, such as a corporation building or a campus. The most used

LAN technology = Ethernet;


network (contin.)

� Broadband network components (contin.):

• Storage Area Networks (SAN) = networks used by corporations for data storage,

which are formed by interconnecting some memory discs of high capacity using

high data rate interfaces transmitting over optical fibers (OF);

• Metropolitan Area Networks (MAN) = networks that transfer the data and voice

traffic (integrated services), covering a town area, and using OF technology =

Synchronous Optical NETwork (SONET, in USA) rings; in Europe, Synchronous

Digital Hierarchy (SDH);

• Wide Area Networks (WAN) = networks that interconnect several corporation

networks or MANs, covering large distances. These WANs are also called core

networks or long-haul networks. WANs use exclusively OFs.

1.1.1 Access Networks

� ANs = networks that allow the (mobile) users to access a broadband

infrastructure (e.g., Internet); users can be domestic or corporate ones.

� In a switched public network (e.g., PSTN – Public Switched Telephone

Network), the AN is the region that is used to connect the local switching nodes

(PBX – Private Branch Exchange) to individual subscribers.(PBX – Private Branch Exchange) to individual subscribers.

� Types of AN links for domestic users (different data rates and prices):

• analog telephony (dial-up);

• digital telephony, Digital Subscriber Line (DSL);

• cable modems (TV);

• wireless

o WiFi (IEEE 802.11),

o satellite,

o third/fourth generation mobile communications systems (3G – UMTS, 4G –

LTE );

• OF;

1.1.1 Access Networks (contin.)

� All these (multiple) access links, accommodating several users, are managed by

companies named Internet Service Providers (ISPs), which offer Internet access

services in terms of the content.

� In general, ANs offer pure-data, video or voice connections for any subscriber,

from any location, by means of specific technologies and protocols.from any location, by means of specific technologies and protocols.

1.1.2 LANs and SANs

� A LAN interconnects PCs, workstations, printers, and other equipments inside a

building or a campus, using (in almost all cases) the Ethernet protocol;

� In (almost) all cases, a LAN (a private network) is connected to a public

network through a security mechanism called firewall. Therefore, the firewalls

represent the delimitation points between LANs and MAN/WAN infrastructure.represent the delimitation points between LANs and MAN/WAN infrastructure.

� LAN connections use metallic cables (UTP or coaxial copper cables), due to

small distances of a few hundreds of meters;

� SANs are specialized LANs, which are exclusively dedicated to data storage.

These include magnetic tapes, hard disks, and servers for storage.

1.1.2 MANs and WANs

� Internet = the largest (the most complex) WAN;

� LANs / MANs are connected to a WAN by a network equipment named Router;

� One WAN can be composed by several MAN cores or long-hauls;

� Long-hauls networks offer transport services over large distances, of the order� Long-hauls networks offer transport services over large distances, of the order

of hundreds of km;

� Cores cover town-size areas, i.e., a maximum distances between 80 km and 120

km. Usually, the core MANs data rates are smaller and offer more services as

compared to long-haul MANs;

1.1.2 MANs and WANs (contin.)

� In WANs → transmissions over OFs using the Wavelength-Division

Multiplexing (WDM) technology. One common solution is Dense-WDM

(DWDM) with data rates of Tb/s (Terabytes per second) over one OF.

� As compared to LANs that belong to private organizations or domestic users,

most WANs (including Internet) do not have a unique owner; a WAN is amost WANs (including Internet) do not have a unique owner; a WAN is a

collective entity that is managed in a distributive manner.

� Examples of WAN technologies that are specific to public data networks (with

packet switching):

• ATM (Asynchronous Transfer Mode),

• Frame Relay,

• X.25.

• A computer communication schematic:

1.2 Communications Nets Architectures

1.2.1 Cooperation and data exchange between computers.

The functions of the Communications Subsystem

• The communication subsystem (CS) = hardware and software; is concernedwith the exchange of information between APs running in computers;

• The CS complexity depends on the physical distance between computers;

• Bit-serial transmission - parallel-to-serial conversion at the computer interfaceprior to outputting data, and the reverse serial-to-parallel function on input;

1.2.1 Cooperation and data exchange between

computers. The functions of the Communications

Subsystem (contin.)

• Bit errors - error control function; is necessary to detect and to correct theseerrors; methods: Forward Error Correction (FEC) and Automatic RepeatreQuest (ARQ).

• Flow control is used to regulate the rate at which data is transferred androuting is necessary to choose and to establish a communication path acrossthe network.

• Other functions: addressing, routing, syntax translation, OS compatibility, etc.

• Conclusion: It is very difficult to implement the software for CS with a single,complex, unstructured program with many interacting components. Theresulting program is very difficult to test and to modify for adding newfacilities→ The solution is …

... a layered approach for the reference model of the complete CS.

� This is known as the ISO (International Standards Organization) ReferenceModel for Open Systems Interconnection (OSI).

� CS is broken down into 7 layers (or levels) each of which performs a well



Subsystem (contin.)

� CS is broken down into 7 layers (or levels) each of which performs a welldefined function.

� Each layer performs a well defined function in the context of the overallcommunication system. It operates according to a defined protocol byexchanging messages, both user data and additional control information,with a corresponding peer layer in a remote system.

� Each layer has a well defined interface between itself and the layersimmediately above or below.

� The implementation of a particular protocol layer is independent of allother layers.

• Layer N can be viewed as a service provider and layer N+1 can beviewed as a service requester or service user.

• The layer N protocol uses the services of layer N−1 to provide adefined set of services to layer N+1 above it.



Subsystem (contin.)

Relations between OSI model layers

Layer N-1 connection

N++++1

N−−−−1

N Protocol

N-PDU

N Service

Transmission medium

N-SAP Layer N service

primitives

E (N)N-1

SAP

N++++1

N−−−−1

E1 (N) E2 (N)Ei (N) – Layer N

protocol entity

PDU – Protocol

Data Unit

• The interactions of two adjacent layers at their common interface (SAP –Service Access Point) are defined in terms of a set of service primitives,each of which has a defined set of parameters:

– Request - Issued by a service user to request that a particular service beperformed by a service provider and to pass parameters needed to fully



Subsystem (contin.)

performed by a service provider and to pass parameters needed to fullyspecify the requested service.

– Indication - Issued by a service provider to notify a service user that asignificant event has occurred.

– Response - Issued by the service user to acknowledge or completesome procedure previously invoked by the service user through anindication primitive.

– Confirm - Issued by a service provider to notify the service user of theresults of one or more request primitives the service user previouslyissued.

Source system

(A)

Destination system

(B)

Layer N+1 Layer N Layer N+1

N-SAP, ARequest



Subsystem (contin.)

time

a) Non-confirmed service = incomplete service, CL

– ConnectionLess

N-SAP, A

N-SAP, B

Request

Indication• MSC – Message

Sequence Chart for themessages (primitives)exchange



Subsystem (contin.)Source system

(A)

Destination system

(B)

Layer N+1 Layer N Layer N+1

N-SAP, A N-SAP, B

time

MSC

b) Confirmed service =

complete service, CO –

Connection Oriented

N-SAP, A N-SAP, B

Request

Indication

Response

Confirm

N-SAP

N-SAP – N-layer Service Access

Point = Layer N address

PCI(N) – N-layer Protocol Control

Information = layer N header;

includes supplementary (control)

Layer N+1

PDU (N+1)

SDU (N)PCI (N)



Subsystem (contin.)

Layer N

Layer N-1

Data units encapsulation for the layer N protocol

includes supplementary (control)

information needed to implement the

functions of the layer N protocol.

PDU – Protocol Data Unit

SDU – Service Data Unit(N-1)-SAP

SDU (N)PCI (N)

PDU (N)

• The 7 layers OSI-RM:



Subsystem (contin.)

• The physical layer defines the electrical and mechanicalcontrol required to transmit data bits across a physical circuit;controls the generation and detection of signals that areinterpreted as 0 bits and 1 bits.

• The data link layer controls data transfer over the physical



Subsystem (contin.)

• The data link layer controls data transfer over the physicallinks. It handles the transmission of data units, often calledframes, including frame delimiting (synchronization), errordetection and (optionally) error correction, flow control, framesequency, and recovery from transmission errors.

• The network layer is concerned with making routingdecisions and relaying data from one device to another throughthe network. Intermediate systems perform routing andrelaying functions.

• The transport layer provides a reliable end-to-end datatransport service and acts as the interface between the higherapplication-oriented layers and the underlying network-dependent protocol layers. Functions: end-to-end integritycontrols to recover from lost, out-of-sequence, or duplicatemessages, and flow control.



Subsystem (contin.)

messages, and flow control.

• The session layer is responsible for organizing the dialogbetween two application programs and for managing the dataexchanges between them. In addition to organizing the dialog,session layer services include establishing synchronizationpoints within the dialog, allowing a dialog to be interrupted,and resuming a dialog from a synchronization point.

• The presentation layer is concerned with the representation(syntax) of data during transfer between two communicatingapplication processes. Functions: syntax translation,encryption.

• The application layer provides the user interface - normally

an application program/process - to a range of network-wide

distributed information services. Communication services

provided by the application layer hide the complexity of the



Subsystem (contin.)

provided by the application layer hide the complexity of the

layers below the communicating programs.

1.2.2 TCP/IP Stack of Protocols

� TCP/IP architecture:

� The network interface layer accepts the messages from the internet layer and prepares

them for the transmission over a specific type of data link and physical network.

� The internet layer implements the routing and relay functions for transmitting the packets

from the source system to the destination one. Several protocols are used at this layer, among

which the Internet Protocol (IP) is the most relevant one, offering a connectionless data

transmission service. IP assures the data blocks transmission between systems identified in

the network by a fixed length address.

1.2.2 TCP/IP Stack of Protocols (contin.)

� The Internet Control Message Protocol (ICMP) is used for transferring control messages

in the network. This protocol uses the IP services (the ICMP message is transferred inside the

IP data field) assuring a mechanism used by all routers and systems in the network to inform

each other about abnormal functioning situations. ICMP includes several diagnostics

functions and can transmit packets announcing several events, as: network route

modification, transmission speed negotiation between two hosts with different maximum data

rates (the flow control function), etc.

� The Address Resolution Protocol (ARP) is used only by Ethernet networks and allows a

system to determine the Medium Access Control (MAC) address of another system from the

same physical network when it knows the IP (network layer) address of that system.

� The Reverse Address Resolution Protocol (RARP) allows a system to obtain its own IP

address, when the system doesn’t know it.

�The transport layer assures the end-to-end communication between the application

programs. The transport layer can adjust the data flow, transfer data without errors, and

keeping the same order in the sequence. At the transport layer the flow of data that have to be

transmitted is split into packets, and each packet is passed down to the internet layer together

with the destination address. When more application programs running in the same system

are requiring the network services, then the transport layer must accept the data from all and


are requiring the network services, then the transport layer must accept the data from all and

to pass them down to the inferior layer, adding to each message some overhead for

application programs identification. There are two transport protocols used in Internet:

� User Datagram Protocol (UDP) - offers a connectionless service using IP for the

messages transport. This protocol, much simpler than TCP, doesn’t guarantee the

messages delivery to the receiver with no errors, no duplicates, and in the same order

as they were issued. The application programs that use UDP must solve themselves

these transmission problems.

� Transmission Control Protocol (TCP) - assures a connection oriented service,

offering a reliable transfer based on error correction, sequence control and duplicate

removal strategies. When elaborating a new application program the underlying

transport protocol must be considered taking into account the application Quality of

Service (QoS) requirements.

� The application layer assures a variety of services for the network users, through dedicated

application programs. Among these application programs the most popular ones are:

� Simple Mail Transfer Protocol (SMTP) is used for electronic mail (e-mail) messages

transfer. This protocol sends routes, and receives e-mail messages over any size

networks, becoming the de facto protocol in Internet.

� The File Transfer Protocol (FTP) is used for a both ways (duplex) files transfer

between any two distant systems. The files may contain text characters (represented

using ASCII or EBCDIC codes) or pure binary data.


using ASCII or EBCDIC codes) or pure binary data.

� Telnet Remote Login allows a user to identify himself onto a distant system from the

local system. This protocol establishes a client-server relation between the local

system (client) and the Telnet application distant system (server), hence allowing the

operation of a local system as a virtual terminal connected to the distant system. The

Secure SHell (SSH) protocol offers Telnet similar services, and other additional

services. Even if SSH is an enhanced version of another protocol Remote SHell

(RSH), it is more used as a replacement for Telnet because it offers a more reliable

authentication, and data encryption, too.

� The Simple Network Management Protocol (SNMP) is used for network

interconnecting equipments remote management.

� The Domain Name System (DNS) implements a client-server application that keeps

the correspondence and translates the systems names given by users into their IP

addresses.

� HyperText Transfer Protocol (HTTP) assures the information transfer service in the

global network (WWW – World Wide Web), using a specific HyperText Markup

Language (HTML). The application implemented by this protocol is a client-server

one, too, and the servers Web pages are identified using a special addressing scheme


one, too, and the servers Web pages are identified using a special addressing scheme

called Uniform Resource Locator (URL).

� Packet InterNet Groper (PING) offers a service for testing the possibility to connect

two systems in Internet.

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Passing to mobile communications Telecommunication ...calin.comm.pub.ro/Didactice/ARI/Notite curs/Prez/En... · • Access Networks (AN) = networks that connect domestic users and