Week 1: Introduction to Computer Networking
What does the Ethernet do, beyond helping network devices communicate?
It defines a protocol responsible for getting data to nodes (i.e. devices) on the SAME network or link.
What is the activity LED?
It flashes when data is being transmitted along the cable. It used to correspond to the 1s and 0s.
How is binary data transmitted across a wire?
It is represented by two different voltages that are sent across the wire. The system on the receiving end translates them into 1s and 0s.
What is broadcast Ethernet transmission?
It is sent to every single device on a LAN using a broadcast address as a destination.
What is a patch panel?
Often used in wall Ethernet ports. It is a device containing many network ports. Just a container. Additional cables run from the patch panel to different switches or routers.
A unicast transmission is meant for how many receiving addresses?
One.
A MAC address is split into two sections. What are they?
Organizationally Unique Identifier (OUI) and the miscellaneous stuff (can be assigned in any way).
What is CSMA/CD?
Part of Ethernet protocol, used to prevent data collision. Used to determine when the communications channels are clear, and when a device is free to transmit data.
What is an internetwork, and what is the most famous internetwork?
A collection of networks connected through routers. The most famous is the Internet.
What is an Ethernet frame?
A data packet at the Ethernet level. Ethernet frames are structured collections of information, presented in a specific order.
What is a protocol?
A defined set of standards that computers must follow in order to communicate properly.
What is a hub?
A device (on the physical layer) that allows for connections from many computers at once.
What is a router, what layer is it on, and why?
A device that knows how to forward data between *independent networks*. It's a layer 3 device, the Network Layer. It inspects IP data to know where to send things.
What is a Media Access Control (MAC) address?
A globally unique identifier attached to an individual network interface. a 48-bit number represented by 6 groupings of 2 hexadecimal numbers.
What is a collision domain?
A network segment where only one device can communicate at a time. Otherwise you will have electrical interference. If interference is encountered, the device has to wait for a quiet period again.
What is VLAN (Virtual LAN)?
A technique that lets you have multiple logical LANs operating on the same physical equipment. Any frame with a VLAN tag will only be delivered out of a switch interface that is configured to relay that specific VLAN tag. That way, we can have 1 physical network that operates like multiple LANs.
What is modulation?
A way of varying the voltage of the constant charge that moves across a copper cable. This gives us 1s or 0s.
What is hexadecimal?
A way to represent numbers using 16 digits. Uses A - F to represent 10 - 15.
What is the Ethernet broadcast address, used as a destination, in x:x:x:x form?
All Fs.
What is a VLAN header?
An optional part of the Ethernet frame that indicates that said frame is a VLAN frame. If it exists, the EtherType comes after it.
What is an octet in computer networking?
Any number that can be represented by 8 bits.
What is a data packet?
Any set of binary data being sent across a network link. From A to B.
What are some basic requirements for computer networking?
Computers need to hear each other, speak understandable protocols, and repeat messages that weren't fully understandable the first time.
What are the two most common types of networking cables?
Copper and fiber.
What is unicast?
Data transferred from one device to another device.
What is a network switch?
Similar to a hub, with many devices connected. Data link layer, not physical. So it can read Ethernet data, and ONLY sends it to the requisite computer (instead of sending it to all of them and having each computer filter out).
What is the third part of an Ethernet frame?
Source MAC address. 48 bits.
What is the SFD?
Start frame delimiter. It signals that the preamble is over, and the frame will follow.
What is the main different between TCP and UDP?
TCP provides mechanisms to ensure that data is reliably delivered. UDP does not.
What is the most common protocol in the transport layer?
TCP.
What are 2 models that describe the layers of computer networking?
TCP/IP 5-layer, and OSI model 7-layer.
What is the purpose of home routers?
Take traffic from the LAN (home/office) and forward it to the ISP. From the ISP, a much more sophisticated router (core router) takes over.
What is the fourth part of an Ethernet frame?
The EtherType field. 16 bits long. Describes the protocol of the frame's contents. Unless there is a VLAN header, in which case Ethernet frame is 5th.
What is a payload in networking?
The actual data being transferred -- anything that isn't a header. 46-1500 bytes.
How do protocols in computer networking work?
The protocols in each layer *carry the layers above them* in order to get data from one place to the next. The protocol used for data going across cables is different than the protocol for data going across the planet.
What is a bit?
The smallest representation of data a computer can understand; a 1 or a 0.
Why do we have VLANs?
To segregate different types of traffic. Like phones vs. desktops
What is TCP?
Transmission Control Protocol.
What is the most common type of cabling? Describe it.
Twisted pair cable. It has pairs of copper wires that are twisted together. They act as one single unit, their twists helping to protect against interference and crosstalk.
What is a cyclical redundancy check?
Used all over computing. A mathematical transformation that uses polynomial division to create a number to represent a larger set of data. Every time you do a CRC, you should end up with the same checksum number.
What is UDP?
User Datagram Protocol. It's important to realize that UDP, just like TCP, can use IP in the network layer. So IP isn't unique to TCP.
What is half-duplex communication?
Usually happens when a network cable degrades. The two can take turns talking to each other.
What is full-duplex communication?
When 1 or 2 pairs are reserved for communicating in one direction, and the other 1-2 are used for communicating in the OTHER direction. Thus, the server and client (or devices on either side) can communicate with each other at the same time.
What is crosstalk?
When an electrical pulse on one wire is accidentally detected on another wire.
When a network segment is a collision domain, it means that...
...all devices on that segment receive all communication across the entire segment. So we need to figure out which node the transmission was actually meant for.
Ethernet uses MAC addresses to...
...ensure that the data it sends has both an address for the machine that sent the transmission, as well as the one that the transmission was intended for.
How many potential MAC addresses are there?
2^48
How many octets are in a MAC address?
6.
What is the physical layer?
Bottom (first) layer of the stack. Represents the physical devices that interconnect computers.
How are twisted pair network cables terminated?
By a plug at the end that takes the individual internal wires and exposes them.
How do different networks communicate with each other (in the network layer)?
By using routers.
How do routers know where to send information? (High-level)
By using routing tables!
What are the 3 most common forms of copper twisted-pair cables used in networking? And how are they different?
Cat5, Cat5e, Cat6. (Cat = Category) Variables include number of twists. Cat5 is the oldest one and least used now. Differences in how the cables are twisted influence the speed at which data is transferred. And how subject they are to interference. Cat6 are the best at avoiding crosstalk, but have a shorter max distance.
What are servers and clients?
Clients request data; servers fulfill that request.
What is a disadvantage to a hub?
Each computer communicates with every single other computer on the hub. There are ways to filter which message is your computer's. However, this creates lots of noise on the network. (Collision Domain)
How many wires does a Cat6 cable have?
Eight wires, so four twisted pairs, inside of a single jacket.
What protocol is most widely used to send data across individual links?
Ethernet. It allows software higher up on the networking stack to send and receive data.
What protocols exist at the data link layer, to help network devices communicate with each other using these lower-level signals?
Ethernet. Wifi.
What is the last part of the Ethernet frame?
FCS, or Frame Check Sequence. It's a 4-byte number that represents a checksum value for the entire frame.
What is the session layer in the OSI model?
It creates and manages SESSIONS between the operating system and servers. So that way they can interact with each other.
What hardware do we use to connect computers on a single LAN?
Hubs/switches
What is the most common protocol used at the network layer?
IP (Internet Protocol). It is the "heart of the Internet".
How does CSMA/CD work?
If there's no data currently being transmitted on the network segment, a node will feel free to send data. If 2 or more computers try to send data at the same time, the computer detects this, and stops it. Then each computer waits for a random interval before trying to transmit again.
What is simplex communication?
Information can only flow in one direction across the cable. (i.e. baby monitor)
What is the ISP?
Internet Service Provider
Why is the data link layer important?
It abstracts away the physical layer and hardware from the other layers.
How does a transmission know that it's unicast?
It looks at a special bit in the destination MAC address. This bit is the LEAST SIGNIFICANT BIT of the FIRST OCTET. If this special bit is set to zero, it means that the ethernet frame is intended for ONLY the destination address. So all devices in the collision domain are sent the message, but only the destination address receives and processes it.
How does the FCS (Frame Check Sequence) calculate its checksum value?
It performs a cyclical redundancy check (CRC) against the frame.
What is the link LED?
It will be lit when the cable is properly connected to two devices, both of which are on.
What is Border Gateway Protocol (BGP)?
It's the protocol by which routers share data with each other. This helps them learn about the optimal paths to forward traffic.
What are the 2 types of LEDs on an RJ45 port?
Link LED and Activity LED.
What is a LAN?
Local Area Network
What is line coding?
Modulation specifically for computer networks. Helps the computers on either end understand which voltage is 0 and which is 1.
What are copper cables?
Most common form of networking cable. Made up of multiple pairs of copper wires inside of a plastic insulator.
How do computers communicate with each other?
Much like how humans do: Following a series of rules.
If the least significant bit in the first octet of a destination address is set to one, it means you're dealing with what type of frame?
Multicast. Similar to unicast, but each individual device will decide whether or not to accept the frame.
What are the 5 layers in the 5-layer network model?
Physical, Data Link, Network, Transport, Application.
What are the 7 layers in the OSI model?
Physical, Data Link, Network, Transport, Session, Presentation, Application
What is the data link layer?
Second layer of the stack. Defines a common way of interpreting lower-level signals (from the physical layer) such that network devices can communicate with each other. So here we get abstract.
What is duplex communication?
The concept that information can flow in BOTH directions across the cable. (i.e. phone call)
What is the second part of an Ethernet frame?
The destination MAC address -- the hardware address of the intended recipient. 48 bits.
Network ports are generally attached to what?
The devices that make up a computer network.
What is the application layer?
The fifth layer, consisting of applications. Application-specific protocols. The actual data that we are delivering!
What is organizationally unique identifier (OUI)?
The first 3 octets of a MAC address. They are assigned to individual hardware manufacturers.
What is the transport layer?
The fourth layer. It determines WHERE THE DATA GOES. Specifically, which client and server programs get the data (which we received from the network layer).
What is an RJ-45 (Registered Jack 45) plug?
The most common type of plug for twisted pair network cables. It has a network port that goes along with it, the RJ45 port.
What is computer networking?
The name we've given to the full scope of how computers communicate with each other.
What is the presentation layer in the OSI model?
The operating system!
What comes after the Ether-type in an Ethernet frame? (Either the 5th or 6th section)
The payload.
What is the first part of an Ethernet frame?
The preamble. It is 8 bytes long, and is split into 2 seconds. First 7 bytes are a buffer between frames and clock synchronization. The last byte is the SFD.
What is the network layer?
The third layer. It allows DIFFERENT NETWORKS to communicate with each other.
What do cables do?
They connect different devices to each other, allowing data to be transmitted over them.
What are fiber optic cables?
They contain individual optical fibers, which are tiny tubes made out of glass. They transport pulses of light that represent binary data. Generally better than copper but more expensive.