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Giacinto Gelli » 1.Basic definitions


Telecommunication services:

  • basic definitions and classification

Telecommunication networks:

  • basic definitions and classification
  • circuit switching and packet switching

Protocol architecture:

  • OSI and TCP/IP models

Services and networks

Telecommunications service: a set of procedures for transmission and remote utilization of information:

  • e.g. telephony

Telecommunications services are provided by means of telecommunications networks.

Basic information types

Voice information: destined to the ear:

  • voice, audio

Video information: destined for the eye:

  • still/moving images (photos, movies)

Data information: abstract information not belonging to the first two categories:

  • text, e-mail, e-banking, etc

Due to the digital paradigm shift, the current trend is to regard all kinds of information as being “data”.

Classification of services

Single-media: carries only one kind of information:

  • telephony (voice)
  • terminal session (data)

Multi-media: carries at least two different kinds of information:

  • video-telephony (video + voice)
  • MMS (video + data)

Classification of services (cont’d)

Point-to point: the information emitted by one user is delivered only to one user:

  • telephony, SMS

Point-to-multipoint: the information emitted by one user is delivered to many (but not to all) users:

  • video-conference, mailing list

Broadcasting: the information emitted by one user is delivered to all users:

  • television, radio

Classification of services (cont’d)

Unidirectional: one information flow from the source to the user (or the users):

  • tele-surveillance, television

Bidirectional: two information flows in both directions:

  • symmetric (telephony)
  • asymmetric (web-browsing)

The ITU I.211 service taxonomy

Interactive services: characterized by interaction between source and destination:

  • Conversational services: real-time interaction (e.g., telephony, video-telephony, video-conference)
  • Messaging services: non real-time interaction, some form of storage at the destination is needed (e.g., e-mail)
  • Retrieval services: the user accesses at will remotely-stored information (e.g., web-browsing, e-learning, video on demand, database access)

ITU (International Telecommunications Union) is the largest global standards body in telecommunications.
ITU-T Recommendation I.211 “Integrated services digital network (ISDN) – B-ISDN service aspects”.

ITU (International Telecommunication Union)

The ITU I.211 service taxonomy (cont’d)

Distributive services: characterized by lack of interaction between source and destination:

  • Without presentation control: the user cannot control start and order of information (e.g., television, radio)
  • With presentation control: the user can control start and order of presentation, since the information is transmitted cyclically by the source (e.g. televideo)

Main service requirements

Blocking probability:

  • the probability that the service is refused by the network

End-to-end delay:

  • the time required by the network to deliver the information from source to destination

Average bit-rate (bit/s):

  • average speed of information flow through the network

Error/loss probability:

  • percentage of information in error or lost by the network

Main service requirements: example

The aggregate of service requirements defines the Quality of Service (QoS)

The aggregate of service requirements defines the Quality of Service (QoS)

Telecommunication network

An aggregate of devices (hardware) and instructions (software) designed to provide telecommunication services.
Originally, different networks were separately deployed for providing different services:

  • television network for TV broadcasting
  • telephone network for telephony services
  • data network for data transmission
  • etc …

The current trend is towards a single integrated network for providing all different services (Next Generation Network, NGN) → network convergence.

Access vs. backbone networks

Access network:

  • interfaces with end users to provide communication services

Backbone network:

  • transfers huge quantities of information through internal network nodes (routers/switches) and high capacity links
  • core/transport network

Wired vs. wireless networks

Wired network:

  • twisted-pair, fiber, coaxial cable
  • the backbone network is mainly wired

Wireless network:

  • radio, IR, optical
  • mainly in the access network

Networks by coverage area

Personal Area Network (PAN):

  • range <1 m (Bluetooth)

Local Area Network (LAN):

  • range 1 m-1 km (Ethernet, WLAN)

Metropolitan Area Network (MAN):

  • range 1 km-10 km (cable TV)

Wide Area Network (WAN):

  • range > 10 km (telephone network)

What is Internet?

Internet is a huge collection of WAN, MAN, and LAN connected through routers.
Network interoperability is based on the widespread adoption of the common TCP/IP protocol suite.

Circuit switching

A communication resource (circuit) is allocated for all the duration of the transmission:

  • Three phases: setup, data transfer, release
  • The allocated resource is busy even if there is no information to transfer
  • The circuit acts as a FIFO queue with guaranteed speed and delay
  • QoS guaranteed, inefficient use of resources
  • Typically used in telephone networks

Packet switching

The information is fragmented into small units (packets), which are individually transmitted by the network:

  • Network nodes with storage capabilities (store & forward networks)
  • The order of delivery of the packets is not necessarily guaranteed, some packets can be lost
  • QoS critical, efficient use of resources
  • Both connectionless and connection-oriented services
  • Typically used in data networks

Protocol architecture

To reduce their design complexity, telecommunication networks are organized as a stack of layers or levels:

  • the purpose of each layer is to offer certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented

A protocol is an agreement between the communicating parties in each layer.
The interface between layers defines which primitive operations and services the lower layer makes available to the upper one.

Reference models: OSI vs. TCP/IP

A hybrid reference model

The session and presentation layers in OSI model are not well defined.
The TCP/IP does not distinguish between physical and data-link layers.
It is common practice to adopt an “hybrid” reference model, which captures the best characteristics of both models.

I materiali di supporto della lezione


A. Pattavina. Reti di telecomunicazioni (seconda edizione). McGraw-Hill, 2007 (chap. 1)

A.S. Tanenbaum. Computer Networks (4th edition). Prentice-Hall, 2003 (chap. 1)

ITU-T Recommendation I.211 “Integrated services digital network (ISDN) – B-ISDN service aspects”

ITU (International Telecommunication Union)

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