Computer Networks Introduction

Assessment Breakdown

• Quizzes, assignments, and lab assessments before the Mid-Term Examination contribute 20% to the final grade
• The Mid-Term Examination accounts for 15% of the final grade
• Project work constitutes 20% of the final grade
• Quizzes, assignments, and lab assessments before the Final Exam contribute 20% to the final grade
• The Final Exam is worth 25% of the final grade

Course Resources

• Textbook by Jim Kurose and Keith Ross titled "Computer Networking: A Top-Down Approach," 8th edition, published by Addison-Wesley in 2020
• Integrated Development Environment (IDE) options include Python and Wireshark
• Additional online resources are available

Course Content Overview

• The course covers fundamental concepts related to computer networks
• Topics include the Application Layer, Transport Layer, and Network Layer

Introduction to Computer Networks

• The goal of the first chapter is to get a "feel" and "big picture" introduction to computer networking terminology
• Topics include, what is the Internet, what is a protocol, network edge components, network core functions, performance metrics, security considerations, protocol layers, service models, and historical context

Internet Overview

• The Internet consists of billions of connected computing devices like hosts or end systems that run network applications
• Packet switches forward data
• Communication links include fiber, copper, radio, and satellite
• Networks consist of devices, routers, and links managed by organizations
• Internet standards involve Request for Comments (RFC) and the Internet Engineering Task Force (IETF)

Internet as an Infrastructure

• The Internet provides services to applications that include web, streaming video, multimedia teleconferencing, email, and e-commerce
• A programming interface to applications is provided through "hooks" for sending or receiving data
• Service options analogous to postal service options provided

Protocols

• Human protocols involve specific messages and actions
• Network protocols enable computers (devices) to communicate rather than humans
• Protocols define the format, order, and actions taken during message transmission and receipt among network entities

Chapter 1 Roadmap

• Key topics: what the internet is, protocols, hosts, access networks, the network edge, physical media, packet/circuit switching, internet structure, loss, delay, throughput, security issues, protocol layers, service models, and history

Internet Structure Breakdown

• The network edge includes clients, servers, and data centers
• Access networks involve wired and wireless communication links
• The network core consists of interconnected routers that create a network of networks

Access Network Considerations

• Considerations when connecting end systems to an edge router include the type of access network and transmission rate
• Access networks include residential, institutional (school, company), and mobile networks

Cable-Based Access Networks

• Frequency division multiplexing (FDM), different channels transmit in different frequency bands
• Hybrid Fiber Coax (HFC) is asymmetric with downstream rates of, up to 40 Mbps – 1.2 Gbps and upstream rates of 30-100 Mbps
• Cable and fiber connect homes to the ISP router using a shared access network

Digital Subscriber Line (DSL) Access Networks

• Existing telephone lines connect to a central office DSLAM
• The data over DSL phone line goes to the Internet
• Voice over DSL phone line goes to the telephone net
• Dedicated downstream transmission is 24-52 Mbps
• Dedicated upstream transmission is 3.5-16 Mbps

Home Networks

• The components of home networks often include wireless devices, a single box combining modem functionality, and wired Ethernet connections

Wireless Access Networks

• Wireless access network connects end systems to a router via a base station
• Wireless Local Area Networks (WLANs) with ~100 ft range using 802.11b/g/n (WiFi) at 11-450 Mbps transmission rate
• Wide-area cellular access networks that are mobile, cellular networks operating up to 10's of Mbps using 4G cellular networks

Enterprise Access Networks

• Used by companies, universities etc
• Connects technologies using switches and routers
• Ethernet at 100Mbps, 1Gbps, 10Gbps wired access
• WiFi wireless access points at 11, 54, 450 Mbps

Data Transmission

• Hosts break application messages into packets of length L bits which are then transmitted into the access network at a rate R
• The time needed to transmit a L-bit packet into a link, transmission delay is equal to L (bits) divided by R

Physical Media Links

• Data transmitted as bits propagates between transmitter and receiver pairs over a physical link
• Guided media propagates signals in solid media such as copper, fiber, or coax cables
• Unguided media propagates signals freely through radio waves

Twisted Pair (TP) Cabling

• It consists of two insulated copper wires
• Category 5: 100 Mbps, 1 Gbps Ethernet
• Can support 10Gbps Ethernet

Coaxial Cable Transmission

• Two concentric copper conductors are used
• Transmission is bidirectional
• It is broadband: multiple frequency channels on cable
• 100's of Mbps per channel

Fiber Optic Cable Transmission

• Glass fibers carry light pulses
• High-speed point-to-point transmission ranging from 10's to 100's of Gbps.
• Low error rate due to immunity to electromagnetic noise
• Repeaters are spaced far apart

Wireless Radios

• Carries electronic signals with no physical wire
• Broadcast and half-duplex, between sender and receiver
• Reflects electromagnetic waves
• Subject to obstruction by objects and susceptible for interference

Radio Link Transmission Types

• Terrestrial microwave: Up to 45 Mbps channels
• Wireless LAN (WiFi): Up to 100's Mbps
• Wide-area (cellular): up to 10's Mbps
• Satellite offers up to 45 Mbps per channel with 270ms end-end delay, geosynchronous versus low-earth-orbit

Network Core Function

• The network core is a mesh of interconnected routers that use packet-switching to forward application-layer messages into packets
• Packets are forwarded from one router to the next along the path from source to destination
• Each packet is transmitted at full link capacity

Packet Switching Details

• Taking L/R seconds to transmit push an L-bit packet into a link at R bps

Queuing and Packet Loss

• Packet queuing and loss occurs when transmission rate exceeds the link for a period of time
• Packets will queue, waiting to be transmitted on output link
• Packets are dropped if memory buffer in router is full

Network Core Functions

• Local function: moves arriving packets from router's input link to router output link
• Global function: the algorithm determines the source to destination paths taken by packets

Circuit Switching Alternative

• End-end resources allocated and reserved for "call" between source and designation
• Dedicated resources: no sharing
• Circuit segment is idle if it's not used

Circuit Switching: FDM and TDM

• Frequency Division Multiplexing (FDM) optical and electromagnetic frequencies are divided into frequency bands for allocation
• Time Division Multiplexing (TDM): time is divided into slots, each call allocated periodic slot(s)

Packet Switching vs Circuit Switching

• Packet switching allows more users to use the network
• Circuit switching guarantees resources
• Packet switching is great for"bursty" data with resource sharing and no call setup
• Packet overflow is possible

How to Connect Access ISPs

• It is not feasible that each of the millions of access ISPs connect to each other as it would not scale
• A more viable option is to connect each access ISP to one global transit ISP subject to customer and provider agreement
• A more likely scenario involves multiple global ISPs that want to connect, resulting in Internet exchange points

Internet Structure Progression

• Regional networks arrive to connect access nets to ISPs
• Content provider networks may run their own network in efforts to bring services and content closer to endusers
• Tier-1 commercial ISPs establish national & international coverage
• Content provider networks connect data centers while bypassing tier-1 and regional ISPs

Analyzing Network Performance

• Losses and delays occur when packets queue in router buffers because the packet queue waits for turn

Packet Delay Components

• The components of nodal delay include processing, queuing, transmission, and propagation
• Nodal delay equals processing plus queuing, plus transmission, and propagation delay
• Transmission delay equals packet length(bits) over transmissions rate (bps)
• Propagation delay equals the distance of physical link / the propagation speed

Throughput Rate

• Expressed as bits per time unit, measures the rate at which bits are being sent from receiver to sender

Types of Network Security Attacks

• Malware such as viruses or worms can get in host
• Spyware malware can record keystrokes, web sites visited, upload info to collection site
• Infected hosts can be enrolled in botnet, DDOS for spam

Types of Malware

• Denial of Service (DoS): attackers make resources unavailable to legitimate traffic
• Packet Interception: promiscuous network interface reads/records all packets
• IP spoofing: packets are sent with false source addresses

Protocol Layers

• Networks are complex, so protocol "layers" provide structure and organization
• Layering deals with identification, relationship of complex systems

Internet Protocol Stack Structure

• Application: network applications
• Transport: process-process data transfer
• Network: routing of datagrams from source to destination
• Link: data transfer between neighboring network elements
• Physical: bits “on the wire”

Hybrid Model

• Application Layer
• Transport layer
• Network layer
• Data link layer
• Physical layer

Internet History Highlights

• 1961-1972: Early packet-switching principles
• 1972: ARPAnet public demo
• 1972-1980: Internetworking, new and proprietary nets
• 1974: Cerf and Kahn architecture for interconnecting networks
• Internetworking principles, and defining today's Internet architecture
• 1980-1990: New protocols and proliferation of networks.

More Internet History

• Deployment of TCP/IP and definition of smtp e-mail
• DNS defined name-to-IP-address translation
• ftp protocol defined and TCP congestion control implemented
• 100,000 hosts connected to networks
• Early 1990s: ARPAnet decommissioned, NSF lifts restrictions on commercial use of NSFnet
• Killer apps emerge as more users join

History of Malware Protection

• 2005-present: New applications and the Internet are "everywhere"
• ~18B devices attached in 2017, aggressive broadband deployment, increasing 4G and 5G wireless access
• Online social network and enterprises run services in "cloud"