Computer Networks: Chapter 1 Introduction

• This is chapter 1, an introduction to computer networks.
• The goals are to get a feel and a big-picture introduction to networking terminology, which will be covered in later detail.
• The approach is to use the Internet as an example.
• An overview of the topics covered:
• What the Internet is and what a protocol is
• The network edge, including hosts, access networks and physical media
• The network core: packet/circuit switching and the internet structure
• Performance metrics: loss, delay, throughput
• Protocol layers and service models
• History

The Internet

• The Internet is viewed as a collection of billions of connected computing devices.
• hosts are end systems
• End systems run network applications at the "edge" of the Internet.
• Packet switches forward packets (chunks of data)
• Routers and switches are used
• Communication links consist of:
• Fiber, copper, radio, and satellite connections
• They have a transmission rate known as bandwidth
• Networks are collections of devices, routers, and links, usually managed by an organization.
• Internet-connected devices and appliances:
• Amazon Echo, Internet Refrigerator, IP Picture Frame, Security Camera
• Slingbox (remote control cable TV), Pacemaker & Monitor, Internet Phones, Gaming Devices
• Web-enabled toaster and weather forecaster, sensorized bed mattress, AR Devices
• Fitbit, Tweet-a-watt monitor etc.
• The Internet is a "network of networks" and has interconnected ISPs.
• Protocols are essential everywhere
• They control the sending and receiving of messages
• Examples: HTTP (Web), streaming video, Skype, TCP, IP, WiFi, 4G, Ethernet
• Internet standards include things like:
• RFC: Request for Comments
• IETF: the Internet Engineering Task Force
• The Internet can be viewed as an infrastructure that provides services to applications:
• Web, streaming video, multimedia teleconferencing, email, games, e-commerce, social media etc
• Provides a programming interface to distributed applications
• Allows sending/receiving apps to "connect" to, use the Internet transport service
• Offering service options analogous to the postal service.

Protocols

• Protocols define the format and order of messages and actions taken on message transmission/receipt between network entities.
• Human protocols include:
• "What's the time?"
• "I have a question"
• Introductions
• Network protocols function between devices rather than humans.

Network Edge

• The network edge consists of hosts, access networks and physical media
• Hosts are clients and servers, with servers often in data centers
• Access networks involve wired and wireless communication links.

Connecting to the Edge Router

• Connecting to the edge router can be done through:
• Residential access networks
• Institutional access networks (schools, companies)
• Mobile access networks (WiFi, 4G/5G)

Digital Subscriber Line (DSL)

• DSL uses the existing telephone line to central office DSLAM.
• Voice and data are transmitted at different frequencies over a dedicated line to central office
• Data over DSL phone line goes to the Internet, while voice goes to telephone net
• 24-52 Mbps is used for dedicated downstream transmission rate.
• 3.5-16 Mbps is used for dedicated upstream transmission rate.

Cable Based Access

• Cable based access uses frequency division multiplexing (FDM): where different channels are transmitted in different frequency bands.
• Hybrid fiber coax (HFC) cable is asymmetric.
• EuroDOCSIS 3.0 provides 50Mbps downstream and 27Mbps upstream per channel
• EuroDOCSIS 3.1 offers >1.5Gbps downstream and >0.8Gbps upstream per channel.
• Cable/fiber attaches homes to ISP router
• Share access network to cable headend, unlike DSL which has dedicated access to central office

Fiber (PON)

• Gigabit Passive Optical Network (GPON) includes asymmetry up to 2.488 Gbps downstream and 1.244 Gbps upstream transmission rates
• Attaches homes to ISP router
• Homes share access network to fiber headend (most common)
• Each ONT (Optical Network Terminal) gets its timeslot in order to send data upstream to Optical Line Termination

Home Networks

• Home networks feature wireless and wired devices often combined in a single box.
• Utilizes WiFi wireless access points (54, 450 Mbps)
• Connects to headend or central office and has a cable or DSL modem.
• Includes a router, firewall, and NAT
• Uses wired Ethernet at 1 Gbps

Wireless and Enterprise Networks

• Wireless access networks use a shared wireless access network that connects an end system to a router via a base station aka "access point”.
• Wireless local area networks (WLANs) are typically within or around a building (~100 m).
• 802.11b/g/n/ac/ax WiFi has 11, 54, 450, 600 Mbps or more (6.77Gbps) transmission rate.
• Wide-area cellular access networks are provided by mobile, cellular network operators (10's km)
• Provide between 1 Mbps and 1Gbps using 3G, 4G (LTE) and 5G
• Enterprise networks have companies, universities, etc. with mixed wired, wireless link technologies
• Ethernet is used with wired access at 100Mbps, 1Gbps, 10Gbps
• WiFi is used with wireless access points at 11, 54, 450 Mbps

Data Center Networks

• Data center networks have high-bandwidth links (10s to 100s Gbps) connecting hundreds to thousands of servers together and the Internet.

Physical Media links

• A bit propagates between transmitter/receiver pairs
• A physical link lies between transmitter & receiver
• Guided media: signals propagate in solid media such as copper, fiber, coax
• Unguided media: signals propagate freely using radio waves

Twisted Pair Cable

• Twisted pair cable has two insulated copper wires
• Category 5 is 100 Mbps, 1 Gbps Ethernet
• Category 6 is 10Gbps
• Category 8 is 40Gbps

Coaxial Cable

• Coaxial cable has two concentric copper conductors
• Has a bidirectional setup, and is broadband, with multiple frequency channels on cable for HFC

Fiber Optic Cable

• Fiber optic cable carries light pulses to represent each bit as a pulse.
• High-speed operation provides point-to-point transmission at 10's-100's Gbps.
• Presents low error rates with repeaters spaced far apart, because it’s immune to electromagnetic noise.

Wireless Data

• Wireless radio transmits a signal carried in various electromagnetic spectrum "bands"
• Broadcast, "half-duplex" sends from (sender to receiver) and has no physical “wire”
• The propagation environment effects includes:
• Reflection, obstruction by objects and interference/noise

Radio Link Types

• Radio link types are:
• Wireless LAN (WiFi): 6.77 Gbps (802.11ad) or 866 Mbps (802.11ac) or 150 Mbps (802.11n)
• Wide-area (e.g., 4G cellular): 10's Mbps over ~10 Km
• Bluetooth: cable replacement over short distances, limited rates
• Terrestrial microwave: point-to-point; 45 Mbps channels
• Satellite: up to 45 Mbps per channel, with 270 msec end-end delay, geosynchronous versus low altitude

Network Core

• The network core consists of interconnected routers.
• Packet switching which breaks application-layer messages into packets
• Networks forward packets from one router to the next, across links on path from source to destination, with each packet transmitted at full link capacity.

How Data Transmission Works

• A host sends packets of data, where the host sending function:
• Takes application message and breaks it into smaller chunks, known as packets, of length L bits
• Packets are transmitted into access network at transmission rate R
• Packet transmission delay is the time needed to transmit L-bit packet into link; packet transmission delay = L (bits) / R (bits/sec)

Packet switching

• Packet switching uses store-and-forward: entire packet must arrive at router before it can be transmitted on the next link
• Packet transmission delay: takes L/R seconds to transmit (push out) L-bit packet into link at R bps
• End-to-end delay is therefore 2L/R assuming zero propagation delay

Packet-Switching with Store-and-Forward

• L = 10 Kbits, R = 100 Mbps means one-hop transmission delay is 0.1 msec

Packet-Switching

• In packet-switching the Queueing occurs when the rate of work arriving is faster than it can be serviced

Congestion

• Packet queuing and loss occurs when the arrival rate (in bps) to a link exceeds transmission rate (bps) of link for some 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
• Two key network-core functions include:
• Forwarding: aka "switching," local action to move arriving packets from router's input link to appropriate router output link
• Routing: A global action determines source-destination paths taken by packets using routing algorithms

Circuit Switching

• Circuit switching dedicates end-end resources allocated to, reserved for "call" between source and destination
• Each link has circuits like getting the 2nd circuit in top link and 1st circuit in right link
• Dedicated resources do not allow for no sharing and provide guaranteed performance.
• Circuit segment is left idle if not used by call, and are commonly used in traditional telephone networks.

Multiplexing

• Circuit switching uses Frequency Division Multiplexing (FDM)
• Where optical and electromagnetic frequencies are divided into (narrow) frequency bands
• Each call allocated its own band and transmit at max rate of that narrow band
• Time Division Multiplexing (TDM) is used to divide time into slots.
• Each call allocated periodic slot(s), can transmit at maximum rate of (wider) frequency band (only) during its time slot(s)
• As an example, with 1 Gb/s link; each user has 100 Mb/s when “active” (active 10% of the time)
• Circuit-switching supports 10 users
• Packet-switching with 35 users, probability > 10 active at same time is less than .0004

Resources

• great for "bursty" data – sometimes has data to send, but at other times not
• simpler with no call setup Excessive congestion is possible with packet delay and loss due to buffer overflow Protocols are needed for reliable data transfer and congestion control

Internet Structure

• Hosts connect to Internet via an access Internet Service Provider (ISPs)
• Access ISPs must be interconnected and economic agreements through customer and provider ISPs
• Evolution is driven by economics and national policies

Internet Structure

• A complex structure forms with global ISPs
• There are Internet exchange points
• It forms peering links among networks

Links

• High-bandwidth links (10s to 100s Gbps) connect hundreds to thousands of servers together and to the Internet.

Performance

• Packets queue in router buffers, waiting for turn for transmission causing queue length when arrival rate to link (temporarily) exceeds output link capacity which leads to delay, or even loss
• Packet loss occurs when memory to hold queued packets fills up

Packet Delay Sources

• Packet delay has four sources
• Nodal processing: check bit errors, determine output link and typically less than microseconds
• Queueing delay: time waiting at output link for transmission, depends on congestion level of router
• Transmission delay: L: packet length (bits) R: link transmission rate (bps) dtrans = L/R
• Propagation delay: d: length of physical link s: propagation speed (~2x108 m/sec) dprop = d/s
• Car ~ bit, caravan ~ packet; toll service ~ link transmission

Transmission Rate

• If the toll booth takes 12 sec to service car (bit transmission time), and propagate 100 km/hr to push the entire caravan through toll booth highway = 12*10 = 120 sec, it takes 62 minutes to wait
• If the suppose cars "propagate" at 1000 km/hr instead and suppose toll booth now takes one min to service a car Cars arrive to 2nd booth before all cars serviced at first booth after 7 minutes

Queueing

• Average packet arrival rate is measured by a, packet length (bits) is measured by L,,link bandwidth (bit transmission rate) is measured by R
• Queueing delay varies based on traffic intensity where La/R determines traffic intensity

Measuring Delay

• Traceroute program provides delay measurement from source to router along end-end Internet path towards destination of i
• Sends three packets that reach router i on path toward destination
• Router i returns packets to sender, sender measures time interval between transmission and reply
• Packet is dropped if arriving to full queue, lost packets may be retransmitted by previous node, by source end system, or not at all

Throughput

• Throughput is the rate (bits/time unit) at which bits are being sent from sender to receiver
• measured in instantaneous values, and/or average over longer period of time
• Is determined by bottleneck link in the stream
• Per-connection end-end throughput can be computed as "min(Rc, Rs, R/10) when there are 10 connections

Protocol "layers"

Networks are complex, with: Hosts, routers, links (various media), applications, protocol, hardware/software

• Explicit structure helps identify and relate pieces when discussing the reference model for discussion
• Modularization eases maintenance as updates are transparent to rest of system; eg: doesn't affect the procedure in the rest of the network when one gate is updated

Protocol Stack

• The layered Internet protocol stack features:
• Application: supporting network applications; HTTP, IMAP, SMTP, DNS
• Transport: the process-process data transfer; TCP, UDP
• Network: routing of datagrams from source to destination; IP, routing protocols
• Link: data transfer between neighboring network elements; Ethernet, 802.11 (WiFi), PPP
• Physical: bits “on the wire”
• While the ISO/OSI reference model features two layers which are missing from the stack
• Presentation; handles allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions
• Session; handles synchronization, checkpointing, recovery of data exchange

Services

Services, layering and encapsulation:

• Application layer exhanges msgs to implement some application service using the services of the transport layer. Transport-layer protocol transfers msgs reliably from one process to another to other using the services of network layer.
• The TL encapsules the AL with the TL header to create the transport layer segment. The header is used to implement the service.
• The NW layer protocol transferrs the segment and encapsulates it with a NW layer header to create the network layer datagram.
• The LL protocol transfroms datagrams to a neighboring host and encapsulates the datagram with an LL header to create an LL frame.

Internet History

• 1961-1972 saw early packet-switching principles
• 1961: Kleinrock - queueing theory shows effectiveness of packet-switching
• 1964: Baran - packet-switching in military nets
• 1967: ARPAnet conceived by Advanced Research Projects Agency
• 1969: first ARPAnet node operational
• 1972 saw improvements to:
• ARPAnet that is set up as a public demo
• NCP (Network Control Protocol) introduced; the first host-host protocol
• Introduces the first e-mail program, and the 15 node ARPAnet is born
• 1972-1980 established internetworking, and new and proprietary networks
• 1970: ALOHAnet satellite network in Hawaii
• 1974: Cerf and Kahn - architecture for interconnecting networks
• 1976: Ethernet at Xerox PARC
• late70's: proprietary architectures: DECnet, SNA, XNA
• 1979: ARPAnet has 200 nodes

Internetworking

• 1980-1990 focused on new protocols, a proliferation of networks
• 1983 deployment of TCP/IP
• 1982: was when the smtp e-mail protocol was defined
• 1983 saw DNS defined for name-to-IP-address translation
• 1985: ftp protocol was defined
• 1988: TCP congestion control
• New national networks like CSnet, BITnet, NSFnet, Minitel and 100,000 hosts are connected to confederation of networks
• 1990, 2000 saw commercialization, the Web, new applications emerge.
• early 1990s: ARPAnet decommissioned
• 1991 when NSF lifts restrictions on commercial use of NSFnet became 1995
• early 1990s ushered in the Web, with hypertext (Bush 1945, Nelson 1960's) and HTML, HTTP: Berners-Lee
• 1994: produced Mosaic, later Netscape, followed by late 1990s and commercialization of the Web
• More killer apps in the late 1990s, the 2000s included: instant messaging, P2P file sharing, network security to forefront and est. 50 million host, 100 million+ users emerged. Backbone links began running at Gbps

Today's Internet

• 2005-present: the Internet has continued to grow scaling and incorporating software designed networking, mobility, and Cloud computing
• The aggressive deployment of broadband increases home access 10-100's Mbps
• 2008: was when software-defined networking (SDN) emerged and service providers (Google, FB, Microsoft), allowed networks to create their own network and bypass commercial Internet to connect "close" to the end user

Wireshark

• Wireshark is a tool to capture and analyze network packets.