Computer Networks and OSI Model

• Computer networking connects computers for communication and data exchange.

Computer Networks: Key Terms

• Computers: PCs, hosts, servers, hubs, switches, routers, and network-enabled devices like printers, phones, and cameras.
• Net: Physical mediums like wires and fiber optics that connect computers.
• Work: Protocols that enable computer networks to function.

OSI Model

• Networking is structured into layers to facilitate technology implementation by modifying specific layers without affecting others.
• The Open Systems Interconnection (OSI) model standardizes communication functions in computing systems, irrespective of their internal structure.
• The OSI model has seven layers:
• Physical: bit representation.
• Data link: controls medium transmission.
• Network: establishes pathways.
• Transport: error correction and flow control.
• Session: manages connection APIs.
• Presentation: compression and encryption.
• Application: interfaces with applications like web browsers.

OSI Model Protocols

• Physical and Data Link Layers (1 and 2):
• Ethernet (over fiber or UTP).
• 802.11 (Wi-Fi).
• Bluetooth.
• Network Layer (3):
• IPv4: predominant internet protocol.
• IPv6: the future internet protocol.
• Transport Layer (4):
• TCP: transmission control protocol.
• UDP: user datagram protocol.
• Application Layer (5-7):
• HTTP: hypertext transfer protocol for websites.
• DNS: domain name system for domain to IP address conversion.
• SMTP: simple mail transfer protocol for email servers.

Physical Mediums

• The physical medium includes cables and components for data transmission and reception.

Unshielded Twisted Pair (UTP)

• UTP is a common physical medium where wires are twisted to minimize interference.
• UTP Categories:
• Category 5e: 100 meters at 1 Gbps.
• Category 6: 100 meters at 1 Gbps, 55 meters at 10 Gbps.
• Category 6a: 100 meters at 10 Gbps.
• Category 7: 100 meters at 10 Gbps.
• Category 6 or higher is recommended for new UTP installations, with costs increasing for higher categories.

Shielded Twisted Pair (STP)

• STP is similar to UTP but with additional shielding to further reduce interference.

Fiber Optic

• Fiber optics transmit light pulses through a thin glass tube.
• Types of Fiber Optics:
• Multi-mode: Light bounces inside the cable; at 10 Gbps, the maximum distance is about 550m.
• Single-mode: Has a smaller core, allowing one light path for high speeds over long distances (40 km at 10 Gbps), but is more expensive.
• Fiber optics are immune to electromagnetic interference and can achieve higher speeds over longer distances than copper.
• However, fiber optic cables are typically more expensive due to termination costs.
• Fiber optics is cost-effective for distances over 100 m or when speeds of 10 Gbps are needed.

Coaxial Cable

• Coaxial cable transmits radio frequency signals.
• Used in computer networks and for cable television signal distribution.

802.11 Wi-Fi

• 802.11 Wi-Fi is a wireless technology to connect devices to the internet.

Ethernet Protocols

• Ethernet uses copper wire or fiber optic cable for data transmission.
• It includes over 50 protocols for different mediums and speeds.
• Copper Ethernet cables typically use an eight-pin connector and unshielded twisted pair (UTP) cables.
• Early Ethernet used a bus topology with a single coaxial cable.
• Bus Topology: local area network (LAN) setup where all nodes connect to a single cable; failure of the cable results in network failure.
• Problems with Bus Topology:
• Network failure from a single point of failure
• Expensive coaxial cables.
• Limited signal transmission range.
• Data collisions from simultaneous transmissions.
• Collision Detection:
• A collision signal is sent to notify hosts when a collision occurs.
• Hosts wait a random time (back-off time) before resending.
• Half-Duplex: data is sent in both directions but not simultaneously.
• Star Topology: Replaced bus topology with a central hub.
• Star Network: Hosts connect to a central hub; one of the most common network topologies.
• Star topology benefits:
• Unplugging one host will not bring the network down
• Cheaper unshielded twisted pair cables
• Collision: Still an issue with the star topology.

Ethernet Hubs

• Ethernet hubs accept data on one port and send it out to all other ports.
• Hubs participate in collision detection and extend network distances.
• Ethernet hubs do not have memory and are limited to 10 Mbps.

Frames

• A packet of data at layer 2 is called a frame and includes:
• Header: contains metadata like destination, sender, and payload type.
• Payload: the data being sent.
• CRC Checksum: verifies data integrity.
• MAC (Media Access Control) addresses identify hosts on the network.
• Each network interface card (NIC) has a unique MAC address.
• Hosts ignore packets if the destination MAC address does not match their own.
• Security and performance are common problems.
• Security – all hosts can read all the traffic on the network
• Performance – limited to 10 Mbps

Ethernet Switches

• Ethernet switches have memory to store frames until the medium is clear.
• Switches eliminate collision problems and allow full duplex communication.
• Switches store MAC addresses in a MAC table to learn host ports.
• Data is only sent to the intended port, solving collisions.
• Key Points of Switches:
• Break up collision domains.
• Can be managed or unmanaged.
• Managed switches can be configured via a console port.
• Switches do not have MAC addresses unless managed.
• Support different speeds on different ports.

Collision Domain

• Collision Domain: A network segment where simultaneous data transmissions cause collisions.
• Hubs:
• The whole network is a single collision domain.
• All hosts need to wait during a collision.
• Switches:
• Collision domains are limited to individual connections.

Ethernet Speeds

• Copper Wire Medium:
• 10BASE-T: 10 Mbps.
• 100BASE-T: 100 Mbps.
• 1000BASE-T: 1,000 Mbps (Gigabit).
• 10GBASE-T: 10 Gbps.
• Fibre Optic Cable:
• 10GBASE-LR: 10,000 Mbps.

802.11 (Wi-Fi)

• 802.11 allows wireless devices to participate in Ethernet networks.
• 802.11 is a short-range wireless with a maximum distance around 50 metres.
• Ethernet and 802.11 share compatible layer 2 MAC address schemes.
• Wi-Fi Protocol Standards:
• 802.11b: up to 11 Mbps.
• 802.11g: up to 54 Mbps.
• 802.11n: up to 300 Mbps, 5 GHz or 2.4 GHz.
• 802.11ac: over 1,000 Mbps.
• Wireless Access Point (WAP):
• Connects wireless devices to wired devices.
• Emits Wi-Fi signals.
• Wireless devices connect to a switch.
• Wi-Fi lacks collisions because WAPs allocate transmission time for each client.
• Limitations:
• Ethernet is efficient for LANs but struggles with long distances or extremely large networks like the internet.
• Ethernet broadcasts become too noisy with many hosts.
• MAC tables have size limitations; most switches can only support around 8,000 MAC addresses.

Internet Protocol Version 4 (IPv4)

• IPv4 offers global addressing.
• IP addresses are in dotted decimal format (e.g., 192.168.0.1). Each number is an octet (8-bit byte) and ranges between 0-255.
• IP address assignment is managed by ICANN.
• IPv4 functions within layer 2 protocols like Ethernet; layer 3 packets are treated as data by layer 2.

Router

• Routers' characteristics:
• Have different MAC and IP addresses on each port.
• Can make complex networking decisions and enforce security rules based on IP addresses.
• Fewer ports than a switch.
• Does not forward broadcasts or ARP messages.
• Home routers typically combine a switch, router, and wireless access point.

Broadcast Domain

• Broadcast Domain: the area over which a broadcast message can be propagated.
• Routers split up broadcast domains.
• MAC addresses are only useful within a network segment.

Routing Table

• Routing Table Contains
• Known networks
• Router used to get to them
• Systems usually also have a default gateway or default route.

Subnet Masks

• Subnet mask determines if an IP address is within a network segment.
• Determines a range of IP addresses using bitwise OR, AND, and NOT.
• Bitwise AND:
• Compares bits; if both are 1, the result is 1; otherwise, 0.
• Bitwise OR:
• Returns 1 if either or both bits are 1; otherwise, 0.
• Bitwise NOT:
• Inverts bits (1 becomes 0, 0 becomes 1).
• IPv4 Network Addresses
• Every network has an address.
• The first IP address of the network
• Network Address Calculation:
• Bitwise AND of the IP address and the subnet mask. Mask Representation:
• Expressed as a slash followed by the number of 1s (e.g., /24).

Broadcast Address Calculation

• The highest IP address in a network is the broadcast address.
• Computing the broadcast address:
• Invert subnet mask bits.
• Perform a bitwise OR with the IP address.

IP Range

• Using 192.168.0.11 with a mask of 255.255.255.0 (/24):
• Highest IP address: 192.168.0.255 (broadcast address).
• Lowest IP address: 192.168.0.0 (network address).
• Maximum IPs: 256.
• Usually the first and last IP addresses are reserved, leaving 254 usable host addresses.
• Host Calculation:
• Number of addresses = 2^(32-MaskBits).
• Subtract two for reserved addresses.

Static Routes

• To fix an issue, a default route must be set.
• Setting the R1 router to send any unknown packets to the R2 router will create a default route.
• Manually configured routes.
• A route with a mask of 0 will match every possible IP.

Layer 4 Protocols

• Reliability: making sure the packets arrive in the correct order
• Flow Control: making sure packets are not being sent too fast
• Multiplexing: allows multiple applications to run on the same IP, and each application is allocated a port number.
• TCP and UDP packets are parts of an IP packet, and the IP packet is a part of the link frame data.

Transmission Control Protocol (TCP)

• TCP enhances the reliability of IP packets, and every packet is assigned a sequence number to identify missing packets and rearrange out-of-order packets
• TCP Advantages:
• Reliability – TCP identifies lost packets and attempts to resend them.
• Reordering – TCP can recognize and correct out-of-order packets.
• Flow control – TCP can identify if the receiver is overwhelmed and reduce speed.
• Error detection – TCP includes checksums on all segments for error detection.
• Security – The sequence numbering of TCP makes it challenging to forge a TCP packet.
• Connection-oriented – Each connection must undergo a three-way handshake.
• Protocols based on TCP
• HTTP (Hypertext Transfer Protocol) uses port 80
• HTTPS (Hypertext Transfer Protocol Secure) uses port 443
• FTP (File Transfer Protocol) uses port 21
• SMTP (Simple Mail Transfer Protocol) uses port 25
• POP3 (Post Office Protocol) uses port 110
• SSH (Secure Shell) uses port 22.

TCP Handshake

• TCP also has acknowledgment numbers.
• The receiver sends an acknowledgment back to the sender that says, ‘Yes I received segment number xxxxx’. Sequence numbers are 32-bit numbers and help improve the security of the system.
• TCP creates a secure process through a three-way handshake:
• SYN: initiating a connection with a SYN packet with the sender's starting sequence number.
• SYN-ACK: the receiver will then send back a SYN-ACK with its sequence number.
• ACK: acknowledgment of the connection start.

User Datagram Protocol (UDP)

• UDP: unreliable, if a packet goes missing, it will not be resent automatically, it does not have sequence numbers.
• UDP Benefits:
• Excellent for small requests that need low latency
• UDP Limitations:
• Has no tracking number
• UDP Packet Transfers:
• You can send UDP packets to the broadcast address of a network and all devices in that network will receive it
• TCP connections can only have two hosts involved
• Popular Protocols based on UDP:
• DNS: the Domain Name System (port 53)
• DHCP: dynamic host configuration protocol (ports 67 and 68)
• OpenVPN (port 1194)
• VOIP: voice over IP.