Computer Networks Chapter 1: Introduction

• Computer Networks Chapter 1 introduces the terminology, big picture concepts, and the Internet.

Chapter 1 Goals and Approach

• The goal is to "get a feel" and "big picture" understanding.
• The approach uses the Internet as an example.

Overview and roadmap

• What is the Internet? What is a protocol?
• Network edge: hosts, access network, physical media
• Network core: packet/circuit switching, internet structure
• Performance: loss, delay, throughput
• Protocol layers, service models
• History

The Internet: A "Nuts and Bolts" View

• Billions of connected computing devices exist, commonly referred to as hosts or end systems.
• Hosts run network applications at the Internet's "edge."
• Packet switches forward data in chunks.
• Routers and switches, are types of packet switches.
• Communication links include fiber, copper, radio, and satellite.
• Transmission rate is referred to as bandwidth.
• Networks consist of devices, routers, and links managed by an organization.

The Internet: "Network of Networks"

• The Internet is described as a "network of networks" where interconnected ISPs operate.
• Protocols oversee sending and receiving messages including HTTP (Web), streaming video, Skype, TCP, IP, WiFi, 4G, and Ethernet.
• Internet standards include RFC (Request for Comments) and IETF (Internet Engineering Task Force).

Human vs. Network Protocols

• Human protocols dictate specific messages sent as well as the specific actions taken when receiving a message
• Network protocols do the same for computers.
• Network protocols govern all communication activity on the Internet.

Network Edge

• Includes hosts as clients and servers, often located in data centers.

Access Networks and Physical Media

• Consider various ways to connect end systems to the edge router.
• Options include residential access nets, institutional networks (school, company), and mobile access networks (WiFi, 4G/5G).
• Digital Subscriber Line (DSL) uses the existing telephone line to connect to the central office DSLAM.
• DSL has dedicated downstream transmission rates of 24-52 Mbps and upstream rates of 3.5-16 Mbps.
• Cable-based access relies on frequency division multiplexing (FDM) to transmit different channels in different bands.
• Hybrid fiber coax (HFC) is asymmetric, offering shared access networks to cable headends for homes.
• EuroDOCSIS 3.0 offers 50Mbps downstream and 27Mbps upstream per channel, and EuroDOCSIS 3.1 offers >1.5Gbps downstream and >0.8Gbps upstream.
• Gigabit Passive Optical Network (GPON) is asymmetric.
• GPON offers transmission rates of up to 2.488 Gbps downstream and 1.244 Gbps upstream.
• Wireless LANs (WLANs) typically operate within buildings (~100m) using 802.11b/g/n/ac/ax (WiFi).
• 802.11b/g/n/ac/ax has transmission rates of 11, 54, 450, 600 Mbps, or more (6.77Gbps).
• Wide-area cellular access networks, such as 3G, 4G (LTE), and 5G, provide services from mobile, cellular network operators over larger areas (10's km).
• Wide-area cellular access networks typically offer speeds between 1 Mbps and 1 Gbps.
• Ethernet provides wired access at 100Mbps, 1Gbps, and 10Gbps.
• WiFi wireless access points at 11, 54, and 450 Mbps.
• High-bandwidth links connect hundreds to thousands of servers together and to the Internet, at transfer rates of 10s to 100s of Gbps.

Links: Physical Media

• A bit propagates between transmitter/receiver pairs along a physical link.
• Guided media propagates signals in solid media such as copper, fiber or coax.
• Unguided media propagate signals freely, such as radio.
• Twisted pair (TP) cables consist of two insulated copper wires, with categories specifying different speeds.
• Category 5 offers 100 Mbps to 1 Gbps Ethernet.
• Category 6 offers 10Gbps.
• Category 8 offers 40Gbps.
• Coaxial cable contains two concentric copper conductors, supports bidirectional communication, and is broadband.
• Fiber optic cable uses glass fiber carrying light pulses, offering high-speed point-to-point transmission (10's-100's Gbps).
• Wireless radio, using various bands, has no physical "wire" and supports broadcast, "half-duplex" communication.
• Wireless LAN (WiFi) such as 802.11ad offers 6.77 Gbps, whereas 802.11ac offers 866 Mbps.
• Bluetooth replaces cables with short distances and limited rates.
• Terrestrial microwave offers point-to-point communication at 45 Mbps channels.
• The Internet's structure includes interconnected routers that forward packets.
• Packet-switching breaks application-layer messages into packets.
• Networks forward packets from one router to the next.
• Each packer transmits at the full link capacity.
• In packet-switching, hosts break application-layer messages into packets.
• Networks forward these packets from one router to the next until they reach their destination.
• Routers use forwarding tables together with destination addresses to send data.
• The link has four circuits.
• Each all gets the second circuit in the top link and the first circuit in the right link.
• Circuit segment is idle is no call is using it.
• Circuit switching is commonly used in traditional telephone networks.
• FDM divides electromagnetic frequencies into narrow bands, with each call assigned its own.
• TDM divides time into slots, allocating periodic slots to each call.

Packet Switching vs. Circuit Switching

• Packet switching is generally better for "bursty" data with resource sharing and simpler setup.
• One Gb/s links are generally used for data.
• Packet switching is considered for 35 users.
• Excessive Congestion is possible.
• Packet delay and loss are due to buffer overflow.
• Circuit-like behavior with packet-switching can be achieved but is complicated.

Internet Structure

• Hosts connect to the Internet via access Internet Service Providers (ISPs).
• Access ISPs interconnect so any two hosts can send packets to each other.
• The resulting network of networks is complex.
• The approach to connect millions of access ISPs involves a global transit ISP.
• Customer and provider ISPs have a economic agreement.
• There will be ISPs competing with each other.
• Internet exchange points (IXP) enable competiting ISPs to connect to each other.
• Regional networks often connect access nets to ISPs.
• Content provider networks like Google may run their own network connecting to end users.
• The Internet's well-connected, large networks can be classified as Tier 1 commercial ISPs and content provider networks.
• Tier 1 ISPs have national and international coverage.
• Content provider networks have a private network that connects data centers to the Internet.

Packet Delay

• Packets queue in router buffers while waiting for transmission.
• Queue length grows when the arrival rate to the link exceeds the output link capacity which impacts packet queuing time.
• Packet loss occurs when the memory holding queued packets fills.
• Queuing occurs when work arrive faster than it can be serviced.
• Packets that are transmitted on the output link are delayed.

Packet Delay Sources

• Nodal processing involves checking for bit errors and determining the output link, and is usually tiny (microseconds)
• Queueing is the time spent waiting at the output link for transmission and depends on the congestion level of the router.
• Transmission delay depends on link bandwidth and packet length.
• Propagation delay depends on the length of the physical link and the propagation speed.

A Caravan Analogy

• Toll booths in this analogy take 12 sec / car of bit transmission time.
• A caravan of 10 cars would see a transmission time of 120 seconds to pass their toll booth.
• Cars now supposed to propagate at 1000 km/hr.
• Suppose that the tolls in place now rake 1 min / car to service car.

Packet Queuing Delay

• Average packet arrival rate
• La/R > more "work" being arrived with long delay because the work is infinite.

Real World Internet

• Traceroute performs delay measurements.
• Three packets sent to router.
• Measures the time between the transmission and the reply.

Packet Loss

• Occurs when queues preceding link in buffer have finished capacity.
• The buffer is a waiting area.
• Lost packets may be retransmitted, by source, or by not at all.

Throughput

• The rate (bits/time unit) at which bits are sent.
• Depends on given point or instantaneous time.
• Depends on average rate over time.
• Throughput rate is constricted by bottleneck link

Protocol Layers

Explicit structure allows identification of complex systems.

• The layered reference model allows discussion of complicated systems.
• Modularization allows maintenance/updating without disruption
• The change in the layer is transparent.
• Layered architecture means the change in the gate does not affect the system,

Layered Internet Protocol Stack Includes

• Application: network applications with HTTP, IMAP, SMTP, DNS
• Transport: TCP, UDP Link: Data transfer with Ethernet, 802.11 (WiFi), PPP
• Physical: Bits on the wire

OSI Model Layers Include

• Two layers not found in Internet protocol stack.
• Presentation
• Session

Layered Encapsulation

• Encapsulation and layering are methods of organizing and sending services

Internet Origin

• 1961: shows effectiveness theory.
• 1964: packet-s witching in military nets
• 1967: Arpanet created
• 1969: First node operation
• Aloha net
• Cerf and Kahn architecture
• Ethernet at Xerox Pace
• Arpanet has 200 nodes.
• New deployment protocols and proliferation.
• National Networks.
• 100,000 hosts
• Arpanet decommissionend
• Restrictions
• Weba
• The world wide web boom
• More filler apps
• Back bone links at gbps speed

Internet Today

• High speed deployments.
• Software defined work
• service providers
• Rise of smartphones
• Billions.