Networks II: The OSI Model

What is the OSI model? The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology. https://en.wikipedia.org/wiki/OSI_model Basically, it is a reference model of how applications communicate over a network.

The Physical Layer This is all of hardware. Computers, laptops, routers, cell phones, tablets, and anything else that interacts with the internet.

The Data Link Layer This is the “access” layer. This layer is the transmission medium. For example, Wifi uses 802.11 a/b/c/n. That is the access medium to Wifi. Cell phones use GSM, CDMA, and others to access network. Bluetooth is an access medium. You access your network using Bluetooth technology.

The Network Layer Essentially, this is where IP and DNS live. Network layer is responsible for determining where everything is going to go. We need proper destinations.

The Transport Layer How are things going to get there? How do we guarantee that things don’t get lost? How do we guarantee that files don’t become corrupted? How do we guarantee that files arrive in order and quickly?

The Session Layer Whenever we start to communicate, we create a session. We need to make sure that this session is uninterrupted and reliable.

The Presentation Layer This layer ensures data reliability and security, and formats data back to its original state.

The Application Layer All of your software. Period. Your software runs on your hardware and executes programs that you use.

Application Layer: OS, apps, software. Presentation Layer: This exchange needs to be secure. Additionally, format everything back to original. Session Layer: I don’t want to lose signal while in the middle of a sessions. Transport Layer: How do I get there? Is it safe? Is it reliable? Network Layer: Must get addresses. Who do I communicate to and where do they live? Data Link Layer: How am I to access my network? Wifi, Bluetooth, cellular, fiber optic? Physical Layer: What are the devices I’m going to use to communicate? Phone, tablet, PC?

When we send data across the internet, we cannot send the entire piece of data at one time. They must be broken down into smaller parts. We refer to these smaller parts as internet packets. Each packet contains some portion of the data. How these packets are sent and received will depend on the protocol that is running.

TCP/IP Protocol TCP is the transport layer protocol IP is the network layer protocol. Together, they form the primary means of transmission and delivery system in the internet. Why is this protocol so preferred? Guarantees delivery Has flags that check if data has been tampered with Has timers that set and reset if packet is not delivered properly Fully encrypted

UDP Protocol It hardly has any of the checks and balances of TCP/IP. It’s advantage is that it is lightning fast because unlike TCP/IP, these packets require very little processing.

TCP/IP Packet

UDP Packet

Review for Exam I

From CS Basics (AP) - binary logic: what is it - transistor: basic building blocks of technology - what is an integrated circuit. - what is an high level language

From Architecture - what is a cpu - what is inside a cpu - what does it mean to be a synchronous circuit - Understand the unit of Hz, or cycles/second - what is cache memory - what is ram, static ram, dynamic ram - how does pipelining work, what are its limitations? - understand how access times increase as we move further away from the CPU - how is a solid state different from a standard hard drive - basic properties of RISC vs CISC architecture

From Operating Systems: - what are some of the tasks an OS does From Operating Systems: - what are some of the tasks an OS does? - what is deadlock? - what is virtual memory? - what is paging memory, how does it work? - what is memory fragmentation? - what is the OS Kernel? - what operating systems are proprietary / open source - what is the size of the OS kernel is respect to the other OSs - Seek and latency times

From Networks: - what is an IP address - what is a DNS query - explain channel capacity: how do we measure bandwidth? - what is a firewall - what is a router - OSI Model: know the 7 layers of the OSI model