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CS3640
Overview (2): Network Edge & CoreProf. Supreeth Shastri Computer Science The University of Iowa
In-depth look into the structure and functioning of the Internet
▪ Network Edge
▪ Network Core
▪ Packet Switching
Lecture goals
Chapter 1.2 - 1.3
A closer look at Internet structure
Network edge ▪ hosts: clients and servers ▪ servers often in data centers
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
A closer look at Internet structure
Network edge ▪ hosts: clients and servers ▪ servers located in data centers
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
Access networks, physical media ▪ wired, wireless communication links
A closer look at Internet structure
Network edge ▪ hosts: clients and servers ▪ servers often in data centers
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
Access networks, physical media ▪ wired, wireless communication links
Network core ▪ interconnected routers ▪ network of networks
Access networksmobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
Q: How are edge devices connected to the network core? ▪ cable based access ▪ telephone line based access ▪ wireless or cellular access
Access network characteristics ▪ Transmission rate of access networks ▪ Dedicated or shared access among hosts
Access technology: cable-based
cable modem
splitter
…cable headend
Channels
V I D E O
V I D E O
V I D E O
V I D E O
V I D E O
V I D E O
D A T A
D A T A
C O N T R O L
1 2 3 4 5 6 7 8 9
Frequency Division Multiplexing (FDM)different channels transmitted in different frequency bands
ISP
Coaxial cable asymmetric transmission rates
40 Mbps -1.2 Gbps downstream and 30-100 Mbps upstream
shared access
ISP
Access technology: digital subscriber line (DSL)
central office telephone network
DSL AccessMux
DSL modem
splitter
dedicated access
Telephone line Transmission rates could be
symmetric or asymmetric Typical range: 1 - 52 Mbps
Access technology: wireless
Wireless local area networks (WLANs) ▪ 802.11b/g/n (WiFi) ▪ Within or around building (~100 ft) ▪ 11 - 450 Mbps transmission rate
to Internet to Internet
Wide-area cellular networks ▪ Provided by cellular operators ▪ 3G and 4G cellular networks ▪ Range of 10’s miles ▪ 10’s Mbps transmission rate
shared access
Access networks: residential
to/from headend or central office
cable or DSL modem
router, firewall, NAT
wired Ethernet (1 Gbps)
WiFi wireless access point (54, 450 Mbps)
Wireless and wired devices
often combined in single box
Access networks: enterprise
▪ Companies, universities, government agencies etc., ▪ Mix of wired (e.g., Ethernet) and wireless (e.g., WiFi) technologies ▪ Connected with a hierarchy of switches and routers
Ethernet switch
enterprise web and mail servers
enterprise router
link to ISP (Internet)
Access networks: data center networks
▪ Hundreds to thousands of servers connected to each other and to Internet
▪ High-bandwidth wired links (1 to 100s Gbps)
▪ Switches and routers organized in intricate topologies to maximize bandwidth and minimize latency
➡ Spine - leaf topology ➡ Three layer topology
Photo courtesy: Cisco Systems
Host perspective: sending packets of data
Packet 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 (aka bandwidth)
R :transmission rate of the link
host
12
two packets, L bits each
packet transmission
delay
time needed to transmit L-bit
packet into link
L (bits) R (bits/sec)
= =
Network core and switching
The network coremobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
▪ it is a mesh of interconnected routers
▪ it performs 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
Two key network-core functions
1
23
0111
destination address in arriving packet’s header
routing algorithm
header value output link0100 0101 0111 1001
3 2 2 1
local forwarding tableForwarding ▪ aka “switching” ▪ local action: move
arriving packets from router’s input link to appropriate router output link
local forwarding table
Routing ▪ global action: determine
source-destination paths taken by packets
▪ routing algorithms
routing algorithm
Routing vs. forwarding
routing
Routing vs. forwarding
forwarding
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
sourceR bps
destination123
L bits per packet
R bps
One-hop example ▪ L = 10 Kbits ▪ R = 100 Mbps ▪ one-hop transmission delay =
0.1 msec
Packet-switching: queueing
Queueing occurs when work arrives faster than it can be serviced
S1
S2
R1100 Mb/s
1.5 Mb/sR2
100 Mb/s
Packet-switching: queueing
S1
S2
R1100 Mb/s
1.5 Mb/sR2
Packet queuing and loss
if arrival rate to link exceeds its transmission 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
100 Mb/s10 Kb/s
An alternative: circuit switching
End-end resources are allocated and reserved for the “call” between source and destination ▪ each link has four circuits ▪ our call is allocated 2nd circuit in top link and
1st circuit in right link ▪ dedicated resources: no sharing ▪ guaranteed performance ▪ circuit segments remain idle if not used by call ▪ commonly used in traditional telephone
networks
Circuit switching: FDM and TDM
freq
uenc
y
time
freq
uenc
y
time
4 usersFrequency Division Multiplexing (FDM) ▪ optical, electromagnetic frequencies divided
into frequency bands ▪ each call allocated its own band, can
transmit at max rate of that narrow band
Time Division Multiplexing (TDM) ▪ time divided into slots ▪ each call allocated periodic slots, can
transmit at maximum rate of (wider) frequency band during its time slots
Packet switching vs. circuit switching
N users, each of them is ▪ active 10% of time ▪ use 100 Mb/s when “active”
How many users can use this network under circuit-switching and packet switching?
N users 1 Gbps link
…..
10Circuit switching
35With users
are active at the same time is>10
< 0.0004
probability that>10Packet switching
Packet switching vs. circuit switching
▪ Great for “bursty” data • efficient resource sharing via statistical multiplexing • simpler call setup with no a priori reservations
▪ Congestion could occur • packets could be delayed or lost due to buffer overflow • protocols are needed for reliable data transfer, congestion control
Is packet switching a “slam dunk winner”?
Is there a way to get circuit-like behavior with packet switching?
▪ Possible but complicated (we have three dedicated lectures studying TCP)
Internet structure: a network of networks
▪ hosts connect to Internet via access Internet Service Providers (ISPs)
▪ these access ISPs in turn must be interconnected ➡ to allow that any pair of hosts (anywhere!) to
send packets to each other
▪ resulting network of networks is complex ➡ evolution driven by economics, national policies
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
datacenter network
content provider network
Internet structure: a network of networksHow to connect millions of access ISPs together?
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access netaccess
net
access net
…
…
……
……
…
…
…
……
Internet structure: a network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access netaccess
net
access net
…
…
……
……
connecting each access ISP to each other directly doesn’t scale:
O(N2) connections.
How to connect millions of access ISPs together?
Internet structure: a network of networksOption: connect each access ISP to one global transit ISP
Global ISP
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access netaccess
net
access net
…
…
……
……
ISP A
ISP C
ISP B
Internet structure: a network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access netaccess
net
access net
…
…
……
……
But if one global ISP is viable business, there will be competitors….
ISP A
ISP C
ISP B
Internet structure: a network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access netaccess
net
access net
…
…
……
……
IXP
peering link
IXP
Internet exchange point
But if one global ISP is viable business, there will be competitors… who will want to be connected
Internet structure: a network of networks… and regional networks will arise to connect access nets to ISPs
ISP A
ISP C
ISP B
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access netaccess
net
access net
…
…
……
……
IXP
IXP
regional ISP
Internet structure: a network of networks
ISP A
ISP C
ISP B
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access netaccess
net
access net
…
…
……
……
IXP
IXP
regional ISP
… and content providers (e.g., Google, Akamai, Amazon) may run their own network
Content provider network
Internet structure: a network of networks
access ISP
access ISP
access ISP
access ISP
access ISP
access ISP
access ISP
access ISP
At the center: a small number of well-connected large networks ▪ “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T) for national & international coverage ▪ content provider networks (e.g., Google, Akamai, Amazon): private networks that connect data
centers to Internet, often bypassing tier-1 and regional ISPs
Regional ISP Regional ISP
Tier 1 ISP Tier 1 ISP
IXP
Content providers
IXPIXP
Tier-1 ISP network: Sprint (circa 2019)
links to/from Sprint customer networks
links to peering networks
to/from other Sprint PoPS…
…
… … …
POP: point-of-presence
Content provider network: Google (circa 2017)
Image courtesy: Google cloud
Continuing our in-depth exploration into the structure and functioning of the Internet
▪ Network performance
▪ Protocol architecture
Next Lecture
Chapter 1.4 - 1.5
Spot Quiz (ICON)
