Posting a letter to a pen friend
You write a letter in English. A friend translates it into French and seals it in an envelope. A clerk notes that it is the third letter in a long chat. A courier splits a big parcel into boxes and numbers them.
Each helper has one job. Computers share a job list in just the same way. It is called a model with seven layers.
Key words
| Word | What it means | Example |
|---|---|---|
| Layer | one level of a model, with one job | |
| OSI model | a model that splits network work into seven layers | |
| Protocol | a set of rules computers follow to talk to each other | HTTP |
| Session | one conversation between two programs, from start to end | |
| Port | a number that says which program gets the data | 443 for HTTPS |
| Encryption | scrambling data so only the right people can read it | |
| DNS | domain name system: it turns a name into an address computers use | |
| ASCII | a code that gives each letter and symbol a number | |
| Network | devices joined so they can share things |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can name the top four OSI layers in order and say what each one does.
Starter
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Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
A model splits the job into layers
The OSI model has seven layers. Each layer does one job, and talks only to the layer above and the layer below. You can change one layer without changing the rest.
Number 7 is at the top, next to the user. Number 1 is at the bottom, next to the cable. Today we cover 7, 6, 5 and 4.
The top four layers
| Layer | Name | Its job | Protocols and examples |
|---|---|---|---|
| 7 | Application | Gives network services to the program you use | HTTP, HTTPS, FTP, SFTP, SMTP, POP, IMAP, DNS, DHCP |
| 6 | Presentation | Changes the form of data, scrambles and unscrambles it, makes it smaller | TLS (the S in HTTPS), JPEG, ASCII |
| 5 | Session | Starts, keeps and ends a conversation between two programs | RPC, NetBIOS |
| 4 | Transport | Splits data into parts, numbers them, and delivers them to the right program | TCP, UDP |
The cells students fill in are left blank in this preview.
Check
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Check
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.