Edge & Core Networking Flashcards
1
Q
Network edge
A
- Hosts (end-systems) : clients and servers
- Servers often in data centers
2
Q
Access networks, physical media
A
Wired, wireless communication links
3
Q
Network core:
A
- Interconnected routers
- Network of networks
4
Q
How to connect end systems to edge router?
A
- Residential access nets
- Institutional access networks (school,company)
- Mobile access networks (WiFi, 4G/5G)
5
Q
Access networks: Digital subscriber line (DSL)
A
- 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
6
Q
Frequency division multiplexing
A
Frequency division multiplexing (FDM): different channels transmitted in different frequency bands
7
Q
Access networks: cable-based access
A
- (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
8
Q
Fiber To The Home (FH) and 5G Wireless
A
- 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.
9
Q
Access networks: home networks
A
10
Q
Access networks: enterprise networks
A
- 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
11
Q
Accessn networks: data center networks
A
- high-bandwidth links (10s to 100s Gbps) connect hundreds to thousands of servers together, and to Internet
12
Q
Links: physical media
(bit)
A
Propagates between transmitter/receiver pairs
13
Q
Links: physical media (physical link)
A
What lies between transmitter & receiver
14
Q
Links: physical media (guided media)
A
Signals propagate in solid media: copper, fiber, coax
15
Q
Links: physical media (unguided media)
A
Signals propagate freely, e.g., radio
16
Q
Links: physical media (Twisted pair (TP))
A
- two insulated copper wires
- Category 5: 100 Mbps, 1 Gbps
Ethernet - Category 6: 10Gbps Ethernet
- Category 5: 100 Mbps, 1 Gbps
17
Q
Links: physical media (Coaxial cable)
A
- two concentric copper conductors
- bidirectional
- broadband:
- multiple frequency channels on cable
- 100’s Mbps per channel
- multiple frequency channels on cable
18
Q
Links: physical media (Fiber optic cable)
A
- 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
19
Q
Wireless access networks
A
- Shared wireless access network, connects end system to router
- via base station aka “access point”
20
Q
Wireless access networks (Wireless local area network (WLANs))
A
- Typically, within or around building (~100 ft)
- 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate
21
Q
Wireless access networks (Wide-area celullar access networks)
A
- provided by mobile, cellular network operator (10’s km)
- 10’s Mbps
- 4G cellular networks (5G being deployed)
22
Q
Links: physical media (Wireless radio)
A
- 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
23
Q
Links: Physical media (Radio link types)
A
- 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
- Geostationary satellites
24
Q
Host: sends packets of data
A
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
25
The network core
- 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
26
Two key network-core functions
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
27
Routing analogy
28
Forwarding
29
Packet-switching: store-and-forward
- 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
30
Packet-switching: queueing
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
31
Alternative to packet switching: circuit switching
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
32
Circuit switching: FDM
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
33
Circuit switching: TDM
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)
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?
- 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)
35
Packet switching versus circuit switching
Is packet switching a “slam dunk winner”?
- 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
36
How to provide circuit-like behavior with packet-switching?
* “It’s complicated.” We’ll study various techniques that try to make packet switching as “circuit-like” as possible.
37
Internet structure a "network of networks"
- 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
38
Internet structure: a “network of networks”
Question: given thousands of stub networks, how to connect them together?
39
Internet structure: a “network of networks”
Option: connect each access ISP to one global transit ISP?
Customer and provider ISPs have economic agreement.
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
41
Internet structure: a “network of networks”
...and regional networks may arise to connect access nets to ISPs
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
43
Internet structure: a “network of networks”
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
44
Tier-1 ISP Network map: Sprint
