Edge & Core Networking

Question 1

Network edge

Answer 1

• Hosts (end-systems) : clients and servers
• Servers often in data centers

Question 2

Access networks, physical media

Answer 2

Wired, wireless communication links

Question 3

Network core:

Answer 3

• Interconnected routers
• Network of networks

Question 4

How to connect end systems to edge router?

Answer 4

• Residential access nets
• Institutional access networks (school,company)
• Mobile access networks (WiFi, 4G/5G)

Question 5

Access networks: Digital subscriber line (DSL)

Answer 5

• Use existing telephone line to central office DSLAM
• data over DSL phone line goes to Internet
• voice over DSL phone line goes to telephone net
• 24-52 Mbps dedicated downstream transmission rate
• 3.5-16 Mbps dedicated upstream transmission rate

Question 6

Frequency division multiplexing

Answer 6

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

Question 7

Access networks: cable-based access

Answer 7

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

Question 8

Fiber To The Home (FH) and 5G Wireless

Answer 8

• Optical fiber path directly to home
• Can potentially provide very high Internet access rates
• 5G fixed wireless is beginning to be deployed
  
  • No cabling from the telco’s CO to the home.
  • Data is sent wirelessly from a provider’s base station to the a modem in the home.

Question 9

Access networks: home networks

Answer 9

Question 10

Access networks: enterprise networks

Answer 10

• Companies, universities, etc.
• 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

Question 11

Accessn networks: data center networks

Answer 11

• high-bandwidth links (10s to 100s Gbps) connect hundreds to thousands of servers together, and to Internet

Question 12

Links: physical media
(bit)

Answer 12

Propagates between transmitter/receiver pairs

Question 13

Links: physical media (physical link)

Answer 13

What lies between transmitter & receiver

Question 14

Links: physical media (guided media)

Answer 14

Signals propagate in solid media: copper, fiber, coax

Question 15

Links: physical media (unguided media)

Answer 15

Signals propagate freely, e.g., radio

Question 16

Links: physical media (Twisted pair (TP))

Answer 16

• two insulated copper wires
  
  • Category 5: 100 Mbps, 1 Gbps
    Ethernet
  • Category 6: 10Gbps Ethernet

Question 17

Links: physical media (Coaxial cable)

Answer 17

• two concentric copper conductors
• bidirectional
• broadband:
  
  • multiple frequency channels on cable
    
    • 100’s Mbps per channel

Question 18

Links: physical media (Fiber optic cable)

Answer 18

• glass fiber carrying light pulses, each pulse a bit
• high-speed operation:
  
  • high-speed point-to-point transmission (10’s-100’s Gbps)
• low error rate:
  
  • repeaters spaced far apart
  • immune to electromagnetic noise

Question 19

Wireless access networks

Answer 19

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

Question 20

Wireless access networks (Wireless local area network (WLANs))

Answer 20

• Typically, within or around building (~100 ft)
• 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate

Question 21

Wireless access networks (Wide-area celullar access networks)

Answer 21

• provided by mobile, cellular network operator (10’s km)
• 10’s Mbps
• 4G cellular networks (5G being deployed)

Question 22

Links: physical media (Wireless radio)

Answer 22

• signal carried in various “bands” in electromagnetic spectrum
• no physical “wire”
• broadcast, “half-duplex” (sender to receiver)
• propagation environment effects:
  
  • reflection
  • obstruction by objects
  • Interference/noise

Question 23

Links: Physical media (Radio link types)

Answer 23

• Wireless LAN (WiFi)
  
  • 10-100’s Mbps; 10’s of meters
• wide-area (e.g., 4G cellular)
  
  • 10’s Mbps over ~10 Km
• Bluetooth: cable replacement
  
  • short distances, limited rates
• terrestrial microwave
  
  • point-to-point; 45 Mbps channels
• Satellite
  
  • Geostationary satellites
    
    • At 36,000 km above ground
    • Remain above the same spot on Earth
    • up to 45 Mbps per channel
    • 270 msec end-end delay
  • Low-Earth Orbiting Satellites
    
    • E.g., Starlink
    • Much closer to Earth
    • Satellites rotate around the Earth

Question 24

Host: sends packets of data

Answer 24

host sending function:
- 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

Question 25

The network core

Answer 25

- Mesh of interconnected routers - packet-switching: hosts break application-layer messages into packets - network forwards packets from one router to the next, across links on path from source to destination

Question 26

Two key network-core functions

Answer 26

1. Forwarding : - aka “switching” - local action:move arriving packets from router’s input link to appropriate router output link 2. Routing: - global action: determine source destination paths taken by packets - Defining routing algorithms

Question 27

Routing analogy

Question 28

Forwarding

Question 29

Packet-switching: store-and-forward

Answer 29

- packet 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

Question 30

Packet-switching: queueing

Answer 30

Queueing occurs when work arrives faster than it can be serviced Packet queuing and loss: if arrival rate (in bps) to 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

Question 31

Alternative to packet switching: circuit switching

Answer 31

end-end resources allocated to, reserved for “call” between source and destination - in diagram, each link has four circuits. - call gets 2nd circuit in top link and 1st circuit in right link. - dedicated resources: no sharing - circuit-like (guaranteed) performance - circuit segment idle if not used by call (no sharing) - commonly used in traditional telephone networks

Question 32

Circuit switching: FDM

Answer 32

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

Question 33

Circuit switching: TDM

Answer 33

Time Division Multiplexing (TDM) - time divided into slots - each call allocated periodic slot(s), can transmit at maximum rate of (wider) frequency band (only) during its time slot(s)

Question 34

Packet switching versus circuit switching example: - 1 Gb/s link - each user: - 100 Mb/s when “active” - active 10% of time Q: how many users can use this network under circuit-switching and packet switching?

Answer 34

- circuit-switching: 10 users - packet switching: with 35 users, probability > 10 active at same time is less than .0004 * Q: how did we get value 0.0004? A: HW problem (for those with course in probability only)

Question 35

Packet switching versus circuit switching Is packet switching a “slam dunk winner”?

Answer 35

- 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

Question 36

How to provide circuit-like behavior with packet-switching?

Answer 36

* “It’s complicated.” We’ll study various techniques that try to make packet switching as “circuit-like” as possible.

Question 37

Internet structure a "network of networks"

Answer 37

- hosts connect to Internet via access Internet Service Providers (ISPs) - access ISPs in turn must be interconnected - so that any two hosts (anywhere!) can send packets to each other - resulting network of networks is very complex - evolution driven by economics, national policies

Question 38

Internet structure: a “network of networks” Question: given thousands of stub networks, how to connect them together?

Answer 38

Question 39

Internet structure: a “network of networks” Option: connect each access ISP to one global transit ISP? Customer and provider ISPs have economic agreement.

Answer 39

Question 40

Internet structure: a “network of networks” But if one global ISP is viable business, there will be competitors …. who will want to be connected

Answer 40

Question 41

Internet structure: a “network of networks” ...and regional networks may arise to connect access nets to ISPs

Answer 41

Question 42

Internet structure: a “network of networks” … and content provider networks (e.g., Google, Microsoft, Akamai) may run their own network, to bring services, content close to end users

Answer 42

Question 43

Internet structure: a “network of networks”

Answer 43

At “center”: small # of well-connected large networks - “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage - content provider networks (e.g., Google, Facebook): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs

Question 44

Tier-1 ISP Network map: Sprint

Answer 44