OSI model encapsulation

Encapsulation is the process of breaking a message into packets, adding control and other information, and transmitting the message through the transmission media. You need to know the following five-step data encapsulation process:

1. The Physical layer converts the frames into bits for transmission across the transmission media.
2. The Data Link layer converts the packets into frames, adding physical device addressing information.
3. The Network layer converts the segments into packets, adding logical network and device addresses.
4. The Transport layer breaks the data into pieces called segments, adding sequencing and control information.
5. Upper layers prepare the data to be sent through the network.

Layer	description & keywords	protocols	device	encapsulation
application	* provides interface for a service to operate * communication partner identification	HTTP, Telnet, FTP, SNMP		user info & data
presentation	* file format * encryption, translation, compression * data format & exchange	JPEG, BMP, TIFF, PICT, MPEG, WMV, ASCII, EBCDIC, MIDI, WAV		data
session	* separates data streams * setup, maintina, tear down communication sessions	SQL, NFS, ASP, RPC, X window		data
transport	* reliable & unreliable * end to end flow control * port and socket #s * segmentation, sequencing, combination.	TCP(connection-oriented) UDP(connectionless)		segment
network	* logical address * path determination * routing packets	IP(ICMP, ARP), IPX, AppleTalk, DECNET	routers, layer 3 switches	packet
data link (logical link control, media access control)	* convert bits to bytes and bytes to frames * MAC address, hardware address * logical network topology * media access * flow control (ack, buffering, windowing) * parity & CRC	* LAN: 802.2(LLC), 802.3(Ethernet), 802.5(Token-ring), 802.11(Wireless) * WAN: HDLC, PPP, Frame relay, ISDN, ATM	NIC, switch, bridge	frame
physical	* move bits across media * cables, connectors, pin position * electrical signals (voltage, bit sync) * physical topology	EIA/TIA 232 (serial signaling) V.35(modem) Cat5 RJ45	media (cable, wire), connector, transceivers, modem, repeater, hub, multiplexer, CSU/DSU, wireless access point	bit