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Jan 16, 20256 min read

The Internet: A Nuts and Bolts View

*The idea of networking is to transfer arbitrary data between two or more computers over the internet/LAN.

  • Host: Named end-systems running apps at the internet’s edge
  • Packet Switching: Forwards packets over routers and switches
  • Communication lines: Media. i.e. Radio, LTE, Fiber
  • Internet: A network of networks via interconnected ISPs
  • Protocols: control the sending and receiving of messages.
  • Internet Standards: RFCs, IETF
  • Infra: provides services to applications, web streaming, email, gaming

Important

Protocols define the format, order, of messages Tx and Rx among network entities, and actions taken on message transmission, receipt.

OSI model

#LayerEx
7ApplicationHTTP
6PresentationSSL/TLS
5SessionSockets, TCP
4TransportTCP, UDP
3NetworkIPv{4,6}
2Data linkEthernet
1PhysicalDSL

Network Edge

  • Hosts: clients, servers
    • Servers in data centers
  • Access networks, physical media
    • wired and wireless network links
  • Network Core:
    • interconnected routers
    • network of networks

DSL

  • Uses existing telephone line to central office DSLAM
    • data over DSL phone line goes to Internet
    • voice over DSL phone line goes to telephone net

Info

  • A high-speed downstream channel, in the 50 kHz to 1 MHz band
  • A medium-speed upstream channel, in the 4 kHz to 50 kHz band
  • An ordinary two-way telephone channel, in the 0 to 4 kHz band
  • 24-52 Mbps – DL transmission rate
  • 3.5-16 Mbps – UL transmission rate
  • Asymmetric access

Wired Cable

Frequency division multiplexing (FDM): different channels transmitted in different frequency bands.

Info

  • HFC: hybrid fiber coax
  • Asymmetric: up to 40 Mbps – 1.2 Gbs downstream transmission rate, 30-100 Mbps upstream transmission rate
  • Network of cable, fiber attaches homes to ISP router
  • homes share access network to cable headend.

Home Networks

Enterprise Networks

Deployed at companies, universities, etc.

Note

  • Mix of wired, wireless link technologies, connecting a mix of switches and routers (we’ll cover differences shortly)
  • Ethernet: wired access at 100Mbps, 1Gbps, 10Gbps
  • WiFi: wireless access points at 11, 54, 450 Mbps

Wireless Networks

Shared wireless access network connects end system to router via base station aka “access point”.

  • Wireless local area networks (WLANs)
    • Typically within or around building (~100ft)
    • 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate to Internet
  • Wide-area cellular access networks: provided by mobile, cellular network operator (10’s km).
    • Throughput ~10 Mbps
    • 4G cellular networks (5G coming)

Physical Media

  • bit: propagates between transmitter/receiver pairs
  • physical link: what lies between transmitter & receiver
  • guided media: signals propagate in solid media > copper, fiber, coax
  • unguided media: signals propagate freely > radio
  • Twisted pair (TP)
    • two insulated copper wires (STP & UTP)
    • Category 5: 100 Mbps, 1 Gbps Ethernet
    • Category 6: 10Gbps Ethernet
  • Coaxial cable:
    • two concentric copper conductors
    • concentric rather than parallel
    • bidirectional
    • broadband:
      • multiple frequency channels oncable
      • 100’s Mbps per channel
  • Fiber optic cable:
    • glass fiber carrying light pulses, each pulse a bit
    • high-speed point-to-point transmission (10’s-100’s Gbps)
    • low error rate:
      • repeaters spaced far apart
      • immune to electromagnetic noise
  • Wireless radio:
    • signal carried in electromagnetic spectrum
    • no physical “wire”
    • broadcast and “half-duplex” (sender to receiver)
    • propagation environment effects:
      • reflection
      • obstruction by objects
      • interference
  • Radio link types:
    • terrestrial microwave: up to 45 Mbps channels
    • Wireless LAN (WiFi): Up to 100’s Mbps
    • wide-area (e.g., cellular): 4G cellular: ~ 10’s Mbps
    • satellite:
      • up to 45 Mbps per channel
      • 280 msec end-end delay
      • geosynchronous vs. low-earth-orbit

Circuit Switching

End-end resources allocated to, reserved for “call” between source and destination (eg: telephone)

  • dedicated resources: no sharing
  • circuit-like (guaranteed) performance
  • circuit segment idle if not used by call (no sharing)
  • commonly used in traditional telephone networks

Packet Switching

  • Hosts send pkts of data
  • Takes application message
  • Breaks into smaller chunks, known as packets, of length L bits
  • Transmits packet into access network at transmission rate R
  • Link transmission rate, aka link capacity, aka link bandwidth

Important

Packet Transmission Delay is: The time needed to transmit an L-bit packet into link =

Great for “bursty” data – sometimes has data to send, but at other times not

  • resource sharing
  • simpler, no call setup
  • excessive congestion possible: packet delay and loss due to buffer overflow
  • protocols needed for reliable data transfer, congestion control
  • Q: How to provide circuit-like behavior?
  • bandwidth guarantees traditionally used for audio/video applications

Example Problem

How long does it take to send a file of 640,000 bits (1 byte = 8 bits) from host A to host B over a circuit-switched network?

  • All links are 1.536 Mbps
  • Each link uses TDM with 24 slots/sec
  • 500 msec to establish end-to-end circuit

Solution:

  • Each circuit has a transmission rate of (1.536 Mbps)/24 = 64 kbps
  • It takes (640,000 bits)/(64 kbps) = 10 seconds to transmit the file
  • To this 10 seconds we add the circuit establishment time, giving 10.5 seconds to send the file

Store & Forward

  • Transmission delay: takes L/R seconds to transmit (push out) L-bit packet into link at R bps.
  • Store and forward: entire packet must arrive at router before it can be transmitted on next link.
  • End-end delay: 2L/R (above), assuming zero propagation delay (more on delay shortly)

One-hop numerical example:

  • L = 10 Kbits
  • R = 100 Mbps

Therefore, one-hop transmission delay = 0.1 msec

Queuing delay, loss

If arrival rate (in bps) to link exceeds transmission rate (bps) of link for a period of time:

  • packets will queue, waiting to be transmitted on output link
  • packets can be dropped (lost) if memory (buffer) in router fills up

Routing & Forwarding

  • Forwarding: local action: move arriving packets from router’s input link to appropriate router output link
  • Routing: global action: determine source-destination paths taken by packets

Multiplexing in Circuit Switched Networks

Frequency Division Multiplexing

  • optical, electromagnetic frequencies divided into (narrow) frequency bands
  • each call allocated its own band, can transmit at max rate of that narrow band

Time Division Multiplexing

  • time divided into frames → slots
  • each call allocated periodic slot(s), can transmit at maximum rate of (wider) frequency band, but only during its time slot(s)

Internet Structure: “network of networks”

End systems connect to Internet via access ISPs (Internet Service Providers)

  • Residential, company and university ISPs
  • Access ISPs in turn must be interconnected.
  • So that any two hosts can send packets to each other
  • Resulting network of networks is very complex
  • Evolution was driven by economics and national policies

Given millions of access ISPs, how to connect them together?

Connecting each access ISP to each other directly doesn’t scale: connections.

Tiered ISP

CDNs

The content provider networks (e.g., Google, Facebook): are private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs

Packet Delay

Performance Metric: Packet Delay – 4 Sources

dnodal = d proc + queue + trans + prop

  • dproc: nodal processing
    • check bit errors
    • determine output link
    • typically < 1 msec
  • dqueue: queueing delay
    • time waiting at output link for transmission
    • depends on congestion level of router
    • microseconds to milliseconds
  • dtrans: transmission delay = L/R
    • L: packet length (bits)
    • R: link transmission rate (bps)
  • dprop: propagation delay = d/s
    • d: length of physical link
    • s: propagation speed (~2x108 m/sec)
Transmission DelayPropagation Delay
Time required for the router to push out the packetTime it takes a bit to propagate from one router to another
A function of packet length and transmission rate of the linkA function of distance
L/Rd/s
Nothing to do with the distance between the routersNothing to do with packet length or transmission rate

Graph View

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