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Cheat Sheet fir 1st Quiz
Network Fundamentals (University of Technology Sydney)
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Week 1
Network is a collection of computers and devices connected together to allow
sharing of resources between users. E.g. network file system, printer sharing.
Device types on the network:
- Host: another name for a computer
- Modem: converts between analog signals and digital signals in dialup access.
- NIC (network interface card): direct connection to LAN
- Hub: can connect more than 2 hosts. Strengthens the signal, not concerned
with the meaning of data, broadcasts the message to all of its ports.
- Switch: more intelligent than a hub, because a switch looks at the MAC
addresses (burned in physical address of the NIC) in the messages. Provides
direct physical connection between hosts when they want to communicate.
- Wireless access point: operates in the 2.4 or 5 GHz bandwidth. Several versions
– a, b, g, n. Increasing popularity.
- Router: forwards data packets between computer networks.
LAN is a network that connects computers and devices in a geographically
limited area.
Network Protocols govern all communication activities on the internet. They
define format, order of messages sent and received among network entities, and
actions taken on message transmission, receipt. Essentially rules you have to
follow, such as road traffic protocols.
Network edge: mobile, home and institutional
networks
Network core: regional ISP and global ISP
(interconnected routers and network of network)
Routing: determines source-destination route taken by packets.
The planning of path through internet core, plan fastest route for
packets.
Forwarding: move packets from router’s input to appropriate
router output.
Packet switching vs circuit switching
Packet switching allows more users to use network.
For example: on a 2MBPS link, with each user using 1MBPS when active, and
active 20% of time.
Circuit Switching: only 2 users, link needs to be reserved for each user, Other
users have to wait….
Packet switching: with 3 users, probability of >2 active at same time is less
than .008
Congestion unlikely to occur
3C3 * p^3 * (1-p)^3-3 = 0.2^3 = 0.008
Packet Switching
- great for bursty data
- resource sharing
- simple, no call setup
- HOWEVER, excessive congestion possible: packet delay and loss
as a result. Protocols needed for reliable data transfer and
congestion control.
Network of Networks
- ISP peering connects multiple ISPs
- IXP interconnects many different ISPs
Access Networks
Week 3
DSL: dedicated using phone line
Cable Network (including HFC): shared,
FTTH: bandwidth not shared
WIFI (802.11)
Network apps are apps that use networks, i.e. not offline
Client Server Architecture
- Server: always-on host, permanent IP address, data centres for scaling
- Clients: communicate with server, may be intermittently connected, may
have dynamic IP addresses, do not communicate directly with each other.
P2P Architecture
- No always-on server, peer host can be both client and server, peers
request service from other peers, provide service in return to other peers
(self scalability)
- copyright issues and difficult to maintain.
Process Communicating
Physical Media
bit: propagates between
transmitter/receiver pairs
physical link: what lies between transmitter & receiver
guided media: signals propagate in solid media; copper,
fiber, coax
unguided media: signals propagate freely, e.g., radio
Twisted Pair: 2 insulated copper wires – for ethernet
Coaxial Cable: 2 concentric copper conductors,
bidirectional, broadband (hfc, multiple channels on cable)
Fiber Optic Cable: glass fiber carrying light pulses, each
pulse a bit. High speed operation: high speed point-topoint transmission. Very expensive as you have to convert
electronic singla to light an then back again. Low error rate
Radio signal carried in electromagnetic spectrum, no
physical “wire”, bidirectional. Includes:
Terrestrial microwave
LAN
Wide area
Satellite
Process: program running within a host.
IPC: within same host, two processes communicate using this (defined by OS)
Client Process: process that initiates communication
Server Process: process that waits to be contacted
Process sends/receives messages to/from its socket (analagous to post box)
- Sending process relies on transport infrastructure on the other side of door to
deliver message to socket at receiving process. Essentially where a process plugs
into the network
Packet Switching: hosts break application layer messages into packets.
Forward packets from one router to the next, across links on a path
from source to destination
Each packet transmitted at full link capacity
In seconds, we convert to
milliseconds
To receive messages, process must have identifier. Host device has unique 32 bit IP
address. Not enough to identify process as many processes can be running on same
host. However need port numbers (application ID) associated with process on host
in order to identify process, e.g. 80 for HTTP and 25 for mail server.
App Transport Services
Data integrity/reliability:
- Some apps (e.g. file transfer) require 100% reliable data transfer
- Other apps can tolerate some loss (e.g. audio)
Timing:
- Some apps (e.g. interactive games) require low delay to be “effective”
Throughput:
- Some apps (e.g. multimedia) require minimum amount of throughput to be
“effective”
- Other apps (“elastic apps”) make use of whatever throughput to be effective
Internet Transport Protocols Services
TCP:
- reliable transport between sending and receiving process
- flow control: sender won’t overwhelm receiver
- congestion control: throttle sender when netowkr overloaded
- does not provide: timing, security, minimum throughput guarantee
UDP:
- unreliable data transfer between sending and receiving process
- does not provide: reliability, flow control, congestion control, timing, throughput
guarantee, security, or connection setup.
- OK for loss tolerating connections and low-latency
Delay and Loss: is arrival (in bits) to
link exceeds transmission rate of link
for a period of time:
- Packets will queue, wait to be
transmitted on link
TCP required for non loss tolerating connections
- Packets can be dropped (lost) if
memory (buffer) fills up
- when the packet arrival rate
(temporarily) exceeds output link
capacity
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Week 02
Layers: each layer implements a service
- via its own internal-layer actions
- relying on services provided by layer below
- such that online system can get message from one end to another
Internet protocol stack
application: supporting network applications
- SMTP(email), HTTP(web), Apps (…)
transport: process-process data transfer
- TCP, UDP
network: routing of datagrams from source to destination
- IP, routing protocols
data link: data transfer between neighbouring network elements
- Ethernet, 802.11(WiFi)
physical: bits “on the wire”
Modular approach makes it easy to deal with complex systems, for system
identification and design. Makes updating to particularly layers easier without
affecting other layers.
4 Sources of Packet Delay
Layers (extended)
- The header in each packet includes the IP address of the
destination (end system B). The packet switch uses the
destination IP address in the packet to determine the outgoing
link.
Application-layer message: data which an application wants to
send and passed onto the transport layer;
transport-layer segment: generated by the transport layer and
encapsulates application-layer message with transport layer
header;
network-layer datagram: encapsulates transport-layer segment
with a network-layer header;
link-layer frame: encapsulates network-layer datagram with a
link-layer header.
Throughput
Throughput: rate (bits/time unit) at which bits transferred
between the sender /receiver (end-to-end). Value of lowest R
= throughput
- Instantaneous: rate at any given point in time
- Average: rate over longer period
Overall throughput is Rc
(weaker link)
Overall throughput is Rs
(smaller link)
dproc: nodal processing
- check bit errors
- determine output link
- typically msec
dqueue: queuing delay
- time waiting at output link
for transmission
- depends on congestion level
of router
dtrans: transmission delay
- Time it takes to put bit on
the link
- L: packet length (bits)
- R: link bandwidth (bps)
- dtrans = L/R
dprop: propagation delay
- d: length of physiscal link
- s: propagation speed (~2x108
m/sec)
- dprop = d/s
Dtrans is time it takes to load entire batch
- At time dtrans, the last bit of the packet is just leaving host A
Dprop is time it takes to “propagate” to the other end.
- If dprop > dtrans (takes longer to get to the end than to load everything
on), then the first bit of the packet will be somewhere along the
link and not yet reached host B
- If dprop < dtrans (takes longer to load everything than to get to the
end) then the first bit of the packet has reached host B.
Queuing Delay (revisited)
- R: link bandwidth (bps)
- L: packet length (bits)
- a: average packet arrival rate (packet / seconds)
- Arrival Rate: La (bps)
- La/R ~ 0: avg. queueing
delay small
- La/R  1: avg. queueing
delay large
- La/R > 1: more “work”
arriving than can be serviced,
average delay infinite!
Packet Loss
- queue (aka buffer) preceding link in buffer has finite capacity
- packet arriving to full queue dropped (aka lost)
- lost packet may be retransmitted by previous node, by source end system,
or not at all
Bottleneck link: link on endto-end throughput
= min(RC, Rs, R/10)
Web and HTTP
HTTP is the web’s application layer protocol
- Client: browser that requests, receives and displays web objects
- Server: sends objects in response to request
- uses TCP, port 80
- “stateless”, maintains no info about past client requests
Persistent (keep-alive) HTTP
is default. Multiple objects
can be sent over single TCP
connection between client,
server.
Non-Persistent (close) HTTP:
at most one object sent over
TCP connection. Connection
then closed. Downloading
multiple objects requires
multiple connections
Total transmission time = 2RTT +
Tx(html file) + RTT + Tx(Obj 1) +Tx(Obj 2)
+ …..
Total transmission time =
2RTT+Tx(html file)+
2RTT+Tx(object1)+
2RTT+Tx(object2)+
2RTT+Tx(object3)+
…
Non persistent can be faster with parallel TCP connections. Requests for
objects can occur simultaneously. Total transmission time
= 2RTT + Tx(html file) + 2RTT + Tx(al objects in parallel)
This combination can also create unfairly shared backbone bottleneck
link to your advantage, as more bandwidth can be grabbed. Design apps
with this in mind. However, there can be a limit of number of parallel
connections
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