Computer Networking Chapter 1

Introduction to Computer Networking

• Computer Networking: A Top-Down Approach by Jim Kurose, Keith Ross, 6th edition, Addison-Wesley, March 2012
• The book uses PowerPoint slides freely available for students, faculty, and readers.
• Slides can be modified, added to, and deleted.
• Users should acknowledge the source and copyright when using the slides.

Chapter 1: Roadmap

• 1.1 What is the Internet?
• 1.2 Network Edge: End systems, access networks, and links.
• 1.3 Network Core: Packet switching, circuit switching, and network structure.
• 1.4 Delay, Loss, Throughput in Networks.
• 1.5 Protocol Layers, Service Models.
• 1.6 Networks Under Attack: Security.
• 1.7 History.

What's the Internet: "Nuts and Bolts" View

• Millions of connected computing devices (hosts = end systems) running network apps.
• Communication links: fiber, copper, radio, satellite; transmission rate: bandwidth.
• Packet switches: forward packets (chunks of data); routers and switches.
• Internet: "network of networks" interconnected ISPs.
• Protocols control sending/receiving of messages (e.g., TCP, IP, HTTP, Skype, 802.11).
• Internet standards: RFC (Request for comments), IETF (Internet Engineering Task Force).

What's the Internet: A Service View

• Infrastructure for applications: Web, VoIP, email, games, e-commerce, social nets.
• Provides a programming interface to applications.
• Hooks for sending/receiving app programs to connect to the internet.
• Provides service options analogous to postal service.

What's a Protocol?

• Human Protocols: "What's the time?", "I have a question" (introductions, etc.)
• Network Protocols: Machines communicate via protocols. All communication activity in the Internet is governed by protocols.
• Protocols define format, order of messages among network entities. Actions taken on message transmission and reception.

A Closer Look at Network Structure

• Network Edge: Hosts (clients and servers), often in data centers.
• Access Networks: Physical media (wired, wireless communication links).
• Network Core: Interconnected routers, a network of networks.

Access Networks and Physical Media

• Connecting end systems to edge routers: residential access nets, institutional access networks (schools, companies), mobile access networks.
• Bandwidth (bits per second) of access network; shared or dedicated.

Access Net: Digital Subscriber Line (DSL)

• Uses existing telephone line to central office DSLAM.
• Data over DSL phone line to internet.
• Voice over DSL phone line to telephone networks.
• Transmission rates (upstream: <1 Mbps, typically; downstream: <10 Mbps, typically).

Access Net: Cable Network

• Frequency division multiplexing: Different channels transmitted in different frequency bands.
• HFC (hybrid fiber coax): Asymmetric, up to 30Mbps downstream transmission rate, 2 Mbps upstream.
• Network of cable, fiber connecting homes to ISP router.
• Homes share access network to cable headend. Unlike DSL, cable has shared access.

Access Net: Home Network

• Wireless devices often combined into a single box.
• Wireless access point (54 Mbps).
• Wired Ethernet (100 Mbps).
• Cable or DSL modem connects to network.

Enterprise Access Networks (Ethernet)

• Ethernet switch used in companies, universities etc.
• Transmission rates (10 Mbps, 100Mbps, 1Gbps, 10Gbps).
• End systems typically connect to Ethernet switch now.

Wireless Access Networks

• Shared wireless access network connects end systems to router via base station (access point).
• Wireless LANs: within a building (100 ft).
• 802.11b/g (WiFi): 11, 54 Mbps transmission rates.
• Wide-area wireless access: provided by telco operators (e.g. cellular), 10s of km between end systems (~1-10 Mbps).
• 3G, 4G, LTE

Host: Sends Packets of Data

• Host sending function: breaks application messages into smaller chunks (packets).
• Transmits packets into access network at transmission rate R.
• Transmission delay = packet length (bits) / link bandwidth (bits/sec).

Physical Media

• Bit propagates between transmitter/receiver pairs.
• Physical Link: What lies between transmitter and receiver.
• Guided Media: Signals propagate in solid media (e.g., copper, fiber, coax).
• Unguided Media: Signals propagate freely (e.g., radio).
  • Twisted pair (TP): Two insulated copper wires.
  • Category 5: 100 Mbps, 1 Gbps Ethernet.
  • Category 6: 10 Gbps Ethernet

Physical Media: Coax, Fiber

• Coaxial Cable: Two concentric copper conductors, bidirectional broadband, multiple channels.
• Fiber Optic Cable: Glass fiber carrying light pulses, high-speed operation, low error rate.

Physical Media: Radio

• Signal carried in electromagnetic spectrum.
• Bidirectional; propagation environment effects (reflection, obstruction, interference).
• Radio link types: microwave (terrestrial, up to 45 Mbps), LAN (e.g., WiFi, 11, 54 Mbps), wide-area (e.g., cellular, ~few Mbps), satellite (Kbps to 45Mbps).

How Do Loss and Delay Occur?

• Packet arrival rate to link exceeds output link capacity.
• Packets queue in router buffers; wait for turn.
• Packet being transmitted (delay).

Four Sources of Packet Delay

• Transmission Delay: (L/R). Length (bits) / bandwidth (bps).
• Propagation Delay: (d/s). Distance (length) / propagation speed (e.g. ~ 2x10⁸ m/s).
• Nodal Processing: Check bit errors and determine output link. Typically < msec.
• Queuing Delay: Time waiting at output link. Depends on congestion level of router.

Caravan Analogy

• Cars (packets) propagating at a speed.
• Toll booth represents a router.
• Time to push entire caravan through a toll booth.
• Time for last car to propagate.

Queuing Delay (Revisited)

• Link bandwidth (R). Packet length (L). Average packet arrival rate (a).
• La/R ~ 0: Average queuing delay is small.
• La/R > 1: Average queuing delay is infinite.

“Real” Internet Delays and Routes

• Traceroute program measures delay from source to destination via routers.
• Sends three packets. Routers return to sender.
• Sender times interval between transmission and reply.

“Real” Internet Delays, Routes (Example)

• Example of traceroute output showing delays along the path from one source to a destination. Shows some examples, specifically trans-oceanic links.

Throughput

• Throughput = rate (bits/time unit).
• Instantaneous rate at a given point in time.
• Average rate over a longer period of time.

Throughput (More)

• If Rs < Rc = average end-end throughput = Rs. (Sender speed constrains throughput)..
• If Rs > Rc = average end-end throughput = Rc. (Receiver speed constrains throughput)..
• Bottleneck link on end-end path constraining throughput.

Throughput: Internet Scenario

• Per-connection end-end throughput = min(Rs, Rc, R/10).
• In practice, R or Rs is often bottleneck.

Explore the fundamentals of computer networking with Chapter 1 of 'Computer Networking: A Top-Down Approach'. This quiz covers key concepts including the Internet, network edges, core, delay, loss, throughput, protocol layers, security, and the history of networking.