Layers and Wording
7 |
Application |
Data |
6 |
Presentation |
Data |
5 |
Session |
Data |
4 |
Transport |
Segments/Datagram |
3 |
Network |
Packets |
2 |
Data-Link |
Frames |
1 |
Phyiscal |
Bits |
1. Physical
Function |
transmission of bits across network |
Representation |
Electrical voltage on wires -> 1 or 0 |
- NRZ |
0 volt = 0, +/-5 volts = 1 |
- Transition Modulation |
during a clock cycle, no change = 0, change = 1 |
Topology |
see cheatsheet 1/20 |
Async comm. |
use of start bits and stop bits to indicate when transm. occurs |
Sync comm. |
use of a reference clock to coordinate transm. |
Broadband bandwith |
divides bw into separate channels (ex Cable TV) |
Baseband bandwith |
uses different freqs on a cable & a ref clock to coordinate transm. |
Baseband Multixplexing |
TDM, StatTDM, FDM are ways to allocate time slots and freqs over channels |
ex |
cables, radio freqs, devices (hubs, WAP, converters, ...) |
2. Data-Link
MAC |
48-bit -> Network Interface Card (NIC) / 1st 24bits : manufacturer, 2nd 24bits : unique device value |
LLC |
Logical Link Control |
| |
Reliable transmission of data |
| |
Segmentation & addressing |
| |
Flow and Error control (checksum) |
Syncro |
Isochronous, Syncronous or Asyncronous |
Devices |
NIC, bridges, switches |
3. Network
Function |
forwards traffic with logical adress |
Logical adress |
IPv4, IPv6, IPX, AppleTalk |
Packet switching |
divides data into packets and forward |
Circuit switching |
dedicated comm link |
Message switching |
divides data into storable messages which can be stored and forwarded later |
Routers |
routing table based on IP adress, static or dynamic route |
| |
protocol RIP, OSPF, EIGRP |
Flow control |
regulates data flow/speed |
Packet reordering |
thanks to numbering and sequencing, packets can be sent across multiple routes |
ICMP |
Internet Control Message Protocol |
| |
send error msg & ops info about an IP, uses ping and traceroute |
ex: |
routers, multilayer switches, IPv4, IPv6, ICMP |
|
|
4. Transport
TCP |
Transmission Control Protocol |
connection oriented |
reliable, resend lost segments, acknowlege (3-way handshake) |
| |
UDP |
User Datagram Protocol |
connectionless |
ureliable, no retransmission, faster due to low overhead |
Windowing |
adjust amount of data, based on retransmission reception quantity |
Buffering |
router allocates memory to store segments |
| |
buffer overflow = segments dropped |
ex: |
TCP, UDP, WAN accelerators, load balancers, firewalls |
5. Session
Function |
setup a conversation |
Setting up |
check credentials, session id, services needed and who starts |
Maintaining |
transfer data, reestablish connection, acknowledge data recept° |
Tearing Down |
on mutual agreement or other party disconnecting |
ex: |
H.323/264 (voice/video streaming), NetBIOS (file exchange) |
6. Presentation
Function |
format data for readability, encrypt and secure data |
Data Formating |
for compatibility purposes, readability (ASCII, JPG, etc) |
| |
compression, conversion |
Encryption |
scrambles data, provide confidentiality (TLS) |
ex: |
programmation languages, text formats, pict extentions, protocols like TLS, SSL |
7. Application
Function |
interface user <-> computer |
App. Services |
File transfer, sharing, email, remote access, NW mgmt, cl/srv processes |
Advertisement |
service initiating a service offer to a NW |
ex : |
POP3, IMAP, SMTP / HTTP-S / DNS / FTP-S / Telnet, SSH / SNMP |
Encapsulation & Decapsulation
Function |
"envelopping" data with headers |
PDU |
Protocol Data Unit. ex: L3PDU->Packet |
Flags |
SYN |
initiates synchronization of connection |
ACK |
acknowledges during 3-WSH and packet reception |
FIN |
initates termination of connection |
RST |
when client or server receives a non expected packet |
PSH |
gives priority to data (for sender) |
URG |
gives priority to data (for recipient) |
MAC |
phyiscal adress of a NIC |
EtherType |
identify the protocol used (IPv4/v6) |
from L7 to L1 |
at L4 |
+TCP/UDP header (source & dest ports) |
at L3 |
+IP header (source & dest adresses) |
at L2 |
+MAC+LLC ---- +FCS |
at L1 |
transmit L2 in bits (0 and 1) |
|
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