Computer Networks (CN)
EE-353
Huma Ghafoor
Lectures 5 and 6 (Chap. 1)
Huma Ghafoor NUST-SEECS Fall-2023
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Overview:
• What is the Internet?
• The Network Edge
• The Network Core
• Delay, loss, and throughput in
Packet-Switched Networks
• Protocol Layers and their
Service Models
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2. The Network Edge:
• Why are they called as end systems or hosts?
• Because they sit at the edge of the internet and they host
(i.e.,) run application program.
• Hosts are sometimes further divided into two categories:
• clients
• servers
• Informally, clients tend to be desktop and mobile PCs,
smartphones, and so on, whereas servers tend to be
more powerful machines that store and distribute Web
pages, stream video, relay e-mail, and so on.
• Today, most of the servers from which we receive search
results, e-mail, Web pages, and videos reside in large data
centers.
• For example, as of 2020, Google has 19 data centers
on four continents, collectively containing several
million servers.
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2. The Network Edge:
mobile network
Network edge:
national or global ISP
• hosts: clients and servers
• servers often in data centers
local or
regional ISP
home network
content
provider
network
datacenter
network
enterprise
network
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Source: J.F Kurose and K.W. Ross
2.1. Access Networks:
• The network that physically connects an
end system to the first router (also
known as the “edge router”) on a path
from the end system to any other distant
end system.
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2. The Network Edge:
mobile network
Network edge:
national or global ISP
• hosts: clients and servers
• servers often in data centers
local or
regional ISP
Access networks, physical media:
• wired, wireless communication links
home network
content
provider
network
datacenter
network
enterprise
network
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Source: J.F Kurose and K.W. Ross
2. The Network Edge:
Q: How to connect end systems to edge
router?
mobile network
national or global ISP
• Home Access: DSL, Cable, FTTH, and 5G Fixed
wireless
• Access in the Enterprise (and the Home):
Ethernet and WiFi (will study in Chap. 6 and 7)
• Mobile access networks (Wide-Area Wireless
Access: 3G and LTE 4G and 5G) (Chap. 7)
local or
regional ISP
home network
content
provider
network
datacenter
network
enterprise
network
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Source: J.F Kurose and K.W. Ross
2.1.1. Home Access: DSL, Cable, FTTH, and 5G Fixed
Wireless:
• The two most prevalent types of broadband residential access
are
• Digital subscriber line (DSL) and Cable Internet Access
• residence typically obtains DSL Internet access
from the same local telephone company (telco)
that provides its wired local phone access.
• when DSL is used, a customer’s telco is also its
ISP
• The home’s DSL modem takes digital data and
translates it to high frequency tones for
transmission over telephone wires to the local
CO (Central Office);
• the analog signals from many such houses (100s
or even 1000s) are translated back into digital
format at the DSLAM (Digital subscriber line
Huma Ghafoor NUST-SEECS
access multiplexer).
Fall-2023
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2.1.1. Home Access: DSL, Cable, FTTH, and 5G Fixed
Wireless:
central office
DSL splitter
modem
voice, data transmitted
at different frequencies over
dedicated line to central office
telephone
network
DSLAM
ISP
DSL access
multiplexer
▪ 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
Huma Ghafoor NUST-SEECS Fall-2023
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Source: J.F Kurose and K.W. Ross
2.1.1. Home Access: DSL, Cable, FTTH, and 5G Fixed
Wireless:
• On the customer side, a splitter separates the data
and telephone signals arriving to the home and
forwards the data signal to the DSL modem.
• On the telco side, in the CO, the DSLAM separates the
data and phone signals and sends the data into the
Internet.
• Because the downstream and upstream rates are
different, the access is said to be asymmetric.
• Engineers have expressly designed DSL for short
distances between the home and the CO; generally, if
the residence is not located within 5 to 10 miles of
the CO, the residence must resort to an alternative
form of Internet access.
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2.1.1. Home Access: DSL, Cable, FTTH, and 5G Fixed
Wireless:
cable headend
…
cable splitter
modem
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Channels
frequency division multiplexing (FDM): different channels transmitted in
different frequency bands
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Source: J.F Kurose and K.W. Ross
2.1.1. Home Access: DSL, Cable, FTTH, and 5G Fixed
Wireless:
cable headend
…
cable splitter
modem
CMTS
data, TV transmitted at different
frequencies over shared cable
distribution network
cable modem
termination system
ISP
▪ HFC: hybrid fiber coax
• asymmetric: up to 40 Mbps – 1.2 Gbps downstream transmission rate, 30-100 Mbps
upstream transmission rate
▪ network of cable, fiber attaches homes to ISP router
• homes share access network to cable headend
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Source: J.F Kurose and K.W. Ross
2.1.1. Home Access: DSL, Cable, FTTH, and 5G Fixed
Wireless:
• It makes use of the cable television company’s existing
cable television infrastructure.
• Fiber optics connect the cable head end to neighborhoodlevel junctions, from which traditional coaxial cable is then
used to reach individual houses and apartments.
• Each neighborhood junction typically supports 500 to
5,000 homes.
• Because both fiber and coaxial cable are employed in this
system, it is often referred to as hybrid fiber coax (HFC).
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2.1.1. Home Access: DSL, Cable, FTTH, and 5G Fixed
Wireless:
• Cable internet access requires special modems, called
cable modems. As with a DSL modem, the cable modem
is typically an external device and connects to the home
PC through an Ethernet port (will discuss in Chap. 6).
• At the cable head end, the cable modem termination
system (CMTS) serves a similar function as the DSL
network’s DSLAM.
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2.1.1. Home Access: DSL, Cable,
FTTH, and 5G Fixed Wireless :
• One important characteristic of cable Internet access is that it is a
shared broadcast medium.
• In particular, every packet sent by the head end travels
downstream on every link to every home and every packet sent by
a home travels on the upstream channel to the head end.
• For this reason, if several users are simultaneously downloading a
video file on the downstream channel, the actual rate at which
each user receives its video file will be significantly lower than
the aggregate cable downstream rate.
• On the other hand, if there are only a few active users and they
are all Web surfing, then each of the users may actually receive
Web pages at the full cable downstream rate, because the users
will rarely request a Web page at exactly the same time.
• Because the upstream channel is also shared, a distributed multiple
access protocol is needed to coordinate transmissions and avoid
collisions.
Huma Ghafoor NUST-SEECS Fall-2023
Source: Internet
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2.1.1. Home Access: DSL, Cable, FTTH, and 5G Fixed
Wireless:
Wireless and wired
devices
to/from headend or
central office
often combined
in single box
cable or DSL modem
WiFi wireless access
point (54, 450 Mbps)
router, firewall, NAT
wired Ethernet (1 Gbps)
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Source: J.F Kurose and K.W. Ross
2.1.1. Home Access: DSL, Cable, FTTH, and 5G
Fixed Wireless:
• An up-and-coming technology that promises even higher speeds is the deployment of fiber to the home (FTTH) [Fiber broadband
2020].
• FTTH concept is simple— provide an optical fiber path from the CO directly to the home (provides Internet access rates in Gbps
range).
• There are two competing optical-distribution network architectures that perform this splitting:
• active optical networks (AONs)
• essentially switched Ethernet (Chap. 6)
• passive optical networks (PONs)
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2.1.1. Home Access: DSL, Cable, FTTH, and 5G Fixed
Wireless:
• Passive optical networks (PONs)
• Each home has an optical network terminator
(ONT), which is connected by dedicated optical
fiber to a neighborhood splitter.
• The splitter combines a number of homes
(typically less than 100) onto a single, shared
optical fiber, which connects to an optical line
terminator (OLT) in the telco’s CO.
• The OLT, providing conversion between optical
and electrical signals, connects to the Internet via
a telco router.
• In the home, users connect a home router
(typically a wireless router) to the ONT and
access the Internet via this home router.
• In the PON architecture, all packets sent from
OLT to the splitter are replicated at the splitter
(similar to a cable head end).
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2.1.1. Home Access: DSL, Cable, FTTH, 5G
Fixed Wireless:
• 5G fixed wireless not only promises highspeed residential access, but will do so
without installing costly and failure-prone
cabling from the telco’s CO to the home.
• With 5G fixed wireless, using beam-forming
technology, data is sent wirelessly from a
provider’s base station to a modem in the
home.
• A WiFi wireless router is connected to the
modem (possibly bundled together), similar to
how a WiFi wireless router is connected to a
cable or DSL modem.
Huma Ghafoor NUST-SEECS Fall-2023
Source: Internet
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Wireless access networks
Shared wireless access network connects end system to router
▪ via base station aka “access point”
Wireless local area networks
(WLANs)
▪ typically within or around
building (~100 ft)
▪ 802.11b/g/n (WiFi): 11, 54, 450
Mbps transmission rate
Wide-area cellular access networks
▪ provided by mobile, cellular network
operator (10’s km)
▪ 10’s Mbps
▪ 4G/5G cellular networks
to Internet
to Internet
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Source: J.F Kurose and K.W. Ross
Access networks: enterprise networks
Enterprise link to
ISP (Internet)
institutional router
Ethernet
switch
institutional mail,
web servers
▪ 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
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Source: J.F Kurose and K.W. Ross
Access networks: data center networks
▪ high-bandwidth links (10s to 100s
Gbps) connect hundreds to thousands
of servers together, and to Internet
mobile network
national or global ISP
local or
regional ISP
home network
Courtesy: Massachusetts Green High Performance Computing
Center (mghpcc.org)
Huma Ghafoor NUST-SEECS Fall-2023
content
provider
network
datacenter
network
enterprise
network
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Source: J.F Kurose and K.W. Ross