Untitled Quiz

Computer Networks Lecture 01: Introduction

• Course title: Computer Networks
• Instructor: Dr. Sahar M. Ghanem, Associate Professor
• Department: Computer and Systems Engineering
• University: Alexandria University
• Textbook: Computer Networking: A Top-Down Approach, 8th ed., Kurose & Ross
• Course materials and discussions on MS Teams
• TA: Eng. Mohamed Essam

Course Outline

• Grading:
  • Attendance & participation: 5-7%
  • Assignments & quizzes: 40%
  • Midterm: 15%
  • Final: 40%
• Join code: 142tcab
• Course outline:
  • What is the Internet?
  • The Network Edge
  • The Network Core
  • Delay, Loss, and Throughput in Packet-Switched Networks
  • Protocol Layers and Their Service Models

Chapter 1: Computer Networks and the Internet

• The Internet is a computer network connecting billions of devices.
• These devices are called hosts or end systems.
• In 2017, approximately 18 billion devices were connected to the Internet.
• By 2022, the estimated number of connected devices is expected to reach 28.5 billion.
• End systems are connected by communication links and packet switches.
• A packet switch takes a packet, forwards it to an outgoing communication link.
• Transmission rate of a link is measured in bits per second (bps).
• Two prominent types of packet switches are routers and link-layer switches.
• The sequence of communication links and packet switches a packet traverses is termed route or path.
• End systems access the Internet through Internet Service Providers (ISPs).
• Each ISP is a network of packet switches and communication links.
• The Internet runs on protocols, with TCP and IP being important ones.
• TCP/IP protocols collectively are used.
• Internet standards are documented as requests for comments (RFCs).
• There are nearly 9000 RFCs.
• Other bodies also specify network component standards (e.g., IEEE 802 LAN Standards Committee).

A Nuts-and-Bolts Description

• The Internet is a computer network connecting billions of devices.
• All these devices are referred to as hosts or end systems.
• The number of Internet-connected devices is growing rapidly.
• End systems are connected through communication links and packet switches.
• These devices use protocols to communicate.
• The sequence of communication links and packet switches a packet traverses is its route or path.
• Users access the Internet through ISPs.
• ISPs are networks of packet switches and communication links.

A Services Description

• The Internet provides services to applications.
• Applications run on end systems, not packet switches.
• End systems provide a socket interface for applications to interact with the underlying network infrastructure.
• The Internet offers multiple services to applications..

What is a Protocol?

• A protocol defines the format and order of messages exchanged between entities.
• A protocol also defines the actions taken during the transmission or receipt of a message or event.

The Network Edge

• End systems include desktop computers, laptops, smartphones, servers, and other devices connecting to the Internet.
• Non-traditional "things' are also being introduced as end systems..
• End systems are also referred to as hosts as they host applications.
• Hosts can be further categorized as clients and servers.
• Most servers are located in large data centers.

Access Networks

• Home Access: DSL, Cable, FTTH, and 5G Fixed Wireless
• Access in the Enterprise (and the Home): Ethernet and WiFi
• Wide-Area Wireless Access: 3G, LTE 4G, and 5G

Home Access: DSL

• A customer's telco is also its ISP.
• DSL modems use existing telephone lines to exchange data.
• Data and traditional phone signals are encoded on the same line at different frequencies.
• There's a downstream (carrier to user) and upstream (user to carrier) channel.
• DSL is designed for short distances.
• Households can connect to a single DSLAM.

Other Home Access

• Cable Internet uses cable TV infrastructure.
• It's a shared broadcast medium.
• FTTH provides gigabit speeds.
• 5G fixed wireless offers high-speed access without physical cabling.

Access in the Enterprise/Home: Ethernet

• LAN is used to connect end systems to the edge router.
• Ethernet users use twisted-pair copper wire to connect to the Ethernet switch.
• Typical user access is 100 Mbps to tens of Gbps, and servers can have 1 to 10 Gbps access.

Access in the Enterprise/Home: WiFi

• Wireless LAN access based on IEEE 802.11 technology (WiFi).
• A user must be within tens of meters of the access point.
• Shared transmission rates up to 100 Mbps are common.

Wide-Area Wireless Access

• Mobile devices use the same wireless infrastructure as cellular telephony.
• Users need to be within tens of kilometers of the base station.
• 4G wireless offers up to 60 Mbps download speeds.
• 5G will offer higher speeds.

Physical Media

• Bits travel from source to destination by electromagnetic waves or optical pulses.
• Physical media are categorized as guided (fiber optic, twisted-pair, coaxial) or unguided (wireless LAN, digital satellite).

The Network Core

• Packet Switching
• Circuit Switching
• Multiplexing

Packet Switching

• In network applications, end systems exchange messages.
• Sources break messages into smaller chunks called packets.
• Packets travel through communication links and switches (routers, link-layer switches).
• Most switches use store-and-forward transmission —receiving the entire packet before forwarding.

Packet Switching (continued)

• Each packet consists of L bits. Transmission rate R bits/sec.
• For N links, end-to-end delay = NL/R

Packet Switching (continued)

• Each link has an output queue.
• Packets endure queuing delays due to congestion.
• Packet loss occurs when the buffer space is full.

Circuit Switching

• Traditional telephone networks use circuit switching.
• Resources (buffers, link transmission rate) are reserved.
• A dedicated end-to-end connection is established between hosts.
• Data transfer is guaranteed at a constant rate

Multiplexing in Circuit-Switched Networks

• Implementing circuits uses frequency-division multiplexing (FDM) or time-division multiplexing (TDM).

Delay, Loss, and Throughput

• Physical laws introduce delay and loss, constraining throughput.

• Throughput is the rate of data transfer between end systems.

• Packets experience processing delays (microseconds / less), queuing delays (microseconds-milliseconds).

• Transmission delay = L/R. Propagation delay = d/s.

Nodal Delay

• d_nodal = d_proc + d_queue + d_trans + d_prop

Queuing Delay and Packet Loss

• Queuing delay varies between packets.
• Factors affecting queuing delay are arrival rate of packets, transmission rate of the link, and traffic patterns.
• Trafic Intensity = La/R; if intensity > 1, queue will increase indefinitely, causing infinite queuing delay.

End-to-End Delay

• If there are N-1 routers and no queuing delay: d_end-to-end = N(d_proc + d_trans + d_prop)

Throughput

• Speed test applications measure end-to-end delay and throughput.
• Throughput = F bits/T seconds = F/T bits/sec.
• In a two-link network, throughput is the minimum link rate.
• In a network with N links, throughput is the minimum of the individual link rates.

Example #4

• 10 servers and 10 clients have the same rates (R_s=R_c).
• R (shared link) is very large.Throughput is the minimum of the rates.

Protocol Layers

• The Internet's complexity is managed by a layered architecture.
• Each layer provides a service to the layer above, using the services of the layer below.
• This modularity simplifies design, implementation, and changes.

Encapsulation

• The transport layer encapsulates application layer messages, adding header information.
• The network layer adds more header information for routing, creating a datagram.
• The link layer encapsulates the datagram, creating a frame.

Protocol Layering

• Protocols can be implemented in software, hardware, or a mix.
• The physical layer deals with transmitting bits across the physical medium.
• The application layer provides services for network applications.
• The transport and network layers are responsible for handling communication between end systems (segments).