Telecommunication is the electrical transmission of data among systems (analog, digital, wireless...). Systems uses protocols to communicate with each others.

Some standards drives the telecommunication: FCC (Federal Communications Commission), ITU (International Telecommunication Union), ISO (International Standards and Organization).

ISO built the OSI model (ISO 7498) as an open interconnection system (open systems). OSI model has 7 layer:

• Application (examples: HTTP, FTP, SMTP, LPD, TFTP)
• Presentation (syntax and format of data, adds encryption and compression, examples: GIF, TIFF, Word file)
• Session (maintain connection session between applications in simplex, half-duplex or full-duplex mode, examples: NFS, SQL, NetBIOS, RPC)
• Transport (handshake between 2 computers to agree about communication, error detection, correction, flow control, examples: TCP, UDP, SSL, SPX)
• Network (add information to route the packets, examples: IP, ICMP, RIP, OSPF, BGP, IGMP)
• Data link (LAN or WAN binary transformation in electrical signal for ethernet, token ring, ATM of FDDI, examples: SLIP, PPP, RARP, L2F, L2TP, ISDN)
• LLC (logical link control communicate with the network layer IEEE 802.2)
• MAC (connect to the physical: ethernet IEEE 802.3, WLAN 802.11)
• Physical (convert bits in voltage, examples: HSSI, X.21, EIA/TIA-232)

Each layer communicate logically with another system's same layer (encapsulation: headers and trailers are added to the information).

TCP/IP is a suite of protocols:

• IP is a connectionless protocol at network layer responsible to provides addresses to packet to be properly routed
• TCP is a connection-oriented protocol at transport layer that makes sure packets are correctly transmitted
• UDP is a connectionless protocol (best-effort, is faster but no control is made upon reception)

TCP and UDP uses ports to communicate (so the packets knows where to go): it opens a socket.

Ports 0-1023 are well-known ports used by known applications (20-21=FTP, 23=telnet, 25=SMTP, 80=HTTP, 161-162=SNMP...)

The TCP handshake works in these phases:

1. The host sends a synchronous packet to the receiver (SYN)
2. The receiver acknowledge it (SYN/ACK)
3. The Host acknowledge it (ACK)

The data structure is made of a message (original data), split in segments at transport layer, then datagrams at network layer and frame at data link layer. All these are called packets.

IP Addressing:

• IPv4 uses 32 bits addresses
• Class A from 0.0.0.0 to 127.255.255.255 (1st byte is network, 16'777'216 hosts)
• Class B from 128.0.0.0 to 191.255.255.255 (byte 1 and 2 are network, 65'536 hosts)
• Class C from 192.0.0.0 to 223.255.255.255 (byte 1, 2 and 3 are network, 256 hosts)
• Class D from 224.0.0.0 to 239.255.255.255 (multicast)
• Class E from 240.0.0.0 to 255.255.255.255 (reserved for research)
• IPv6 uses 128 bits addresses (or IPng IP next generation)
• Scoped addresses (reserved for server...)
• Auto configuration
• Do not need NAT
• QoS

Analog transmission use modulation of amplitude or frequency of a signal. Digital signals are built on binary digits. Bandwidth refers to the number of pulses that can be transmitted within a second.

Asynchronous communication happens when the devices are not synchronized (examples: terminal, modem. Synchronous when they are (usually with clocking, better for long messages).

Baseband uses all the communication channel (Ethernet), broadband divides it in individual channels (Cable TV: multiple TV channels).

The physical arrangement of computers and devices is called network topology:

• Ring topology
• Bus topology (linear = single cable, tree = branches)
• Star topology (devices connected to a switch or hub: ethernet)
• Mesh topology (each device is connected to all others: internet is a partial mesh)

A LAN is a network with a limited number of devices. Connection between LANs are made by WANs through routers.

Ethernet has evolved from 10Mbps, 100Mbps, 1Gbps to 10Gbps and has these characteristics:

• Shares media
• Broadcast and collision domains
• uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
• support full-duplex
• use coaxial or twisted-pair media
• IEEE 802.3

Several implementations: 10Base2 (coax), 10Base5 (thick coax), 10Base-T (twisted pair, RJ-45 connectors), Fast ethernet...

Token Ring was developped by IBM (IEEE 802.5) and uses a token which travels from computer to computer in a logical ring (4Mbps or 16Mbps). Each computer is connected to a central hub Multistation Access Unit (MAU). The system in posession of the token can transmit data. Active monitor and beaconing are used for common errors (computer blocking...).

FDDI (Fiber Distributed Data Interface) is a high speed token-passing technology (IEEE 802.8, 100Mbps) with fault tolerance (uses 2 rings). FDDI was mainly used for backbones (up to 100km). Extensions are CDDI (on UTP cabling) and FDDI-2.

The cabling is important to transmit data without loss. The bandwidth indicates the highest frequency (10Base-T uses 10MHz, 100Base-TX uses 80MHz). Data throughput rate is after compression (10BaseT = 10Mbps, 100Base-TX = 100Mbps).

Coaxial cable uses a shielded copper core are of types 50-ohm or 75-ohm (faster), 10Base2 (thin) or 10Base5 (thick).

Twisted pair cables are shielded (STP) or unshielded (UTP). UTP categories are:

1. Voice
2. 4 Mbps
3. 10 Mbps (ethernet)
4. 16 Mbps
5. 100 Mbps
6. 155 Mbps
7. 1 Gbps

Fiber-optic uses glass to transmit data with laser lights. But it's more expensive and difficult to work with.

Most common cabling problems are:

• Noise (motors...)
• Attenuation (too long lines > 185m for UTP)
• Crosstalk (data goes from one pair to the other)

Cables must follow fire ratings to not produce harmful smoke while burning. In plenum space (where it can affect people) they have to follow different rules than nonplenum spaces. On sensible portions it is possible to use pressurized conduits to detect attacks.

Multiple transmission methods are used:

• unicast: send to 1 computer
• multicast: send to specific group of computers (the user must tell the router he wants this kind of multicast, so the corresponding packets goes through to him: IGMP protocol)
• broadcast: send to everyone