fqo47.Computer.Networking.A.TopDown.Approach.6th.Edition

1. Lectures 3-4: Introduction
Dr. Ameer Gouda

2. Contents
1. Digital Subscriber Line (DSL)
2. Packet Switching
3. Circuit Switching
4. Delays

3. Digital Subscriber Line (DSL)

4. Digital Subscriber Line (DSL)
 A residence obtains DSL Internet access from the same local telephone
company (telco) that provides its wired local phone access.
 Each customer’s DSL modem uses the existing telephone line to
exchange data with a digital subscriber line access multiplexer
(DSLAM) located in the telco’s local central office (CO).
 The residential telephone line carries both data and traditional
telephone signals which are encoded at different frequencies:
 A high-speed downstream channel, in the 50 kHz to 1 MHz band
 A medium-speed upstream channel, in the 4 kHz to 50 kHz band
 An ordinary two-way telephone channel, in the 0 to 4 kHz band

5. Digital Subscriber Line (DSL)
 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.

6. 4-6
Two key network-core functions
forwarding: move packets
from router’s input to
appropriate router output
routing: determines
source-destination route
taken by packets
 routing algorithms
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
1
2
3
dest address in arriving
packet’s header

7.  mesh of interconnected
routers
 packet-switching: hosts
break application-layer
messages into packets
 forward packets from one
router to the next, across
links on path from source
to destination
 each packet transmitted at
full link capacity
The network core (packet-switching)
1-7

8. Packet-switching: store-and-forward
 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
one-hop numerical
example:
 L = 7.5 Mbits
 R = 1.5 Mbps
 one-hop transmission
delay = 5 sec
1-8
 end-end delay = 2L/R
(assuming zero propagation
delay)

9. Packet Switching: queueing delay, loss
1-9
queuing and loss:
 If arrival rate (in bits) to input link exceeds transmission
rate of output link for a period of time:
 packets will queue, wait to be transmitted on link
 packets can be dropped (lost) if memory (buffer) fills
up

10. Network Core: Circuit Switching
 A circuit-switched network is one that establishes a
dedicated circuit (or channel) between nodes and
terminals before the users may communicate.

11. Alternative core: circuit switching
end-end resources allocated to,
reserved for “call” between
source & dest:
 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

12. Multiplexing Circuit Switching
 Frequency Spectrum: the difference between the highest and lowest
frequencies available for network signals.
 Frequency Division Multiplexing (FDM): The frequency spectrum of a
link is shared among the connections established across the link.
 The link dedicates a frequency band to each connection for the duration of
the connection.
 Time Division Multiplexing (TDM): Time is divided into frames of
fixed duration and each frame is divided into a fixed number of time
slots.
 When the network establish a connection across a link, the network
dedicates one time slot in every frame to the connection.

13. Multiplexing Circuit Switching
FDM
frequency
time
TDM
frequency
time
4 users
Example:
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
Frame

14. Delay & Loss in Packet-Switched Networks
 What can happen to a packet as it travels from its
source to its destination.
 A packet starts in the source, passes through a series of
routers, and ends its journey in the destination.
 As a packet travels from one node to the subsequent
node along this path, the packet suffers from several
different types of delays at each node along the path.
 nodal processing delay, queuing delay, transmission
delay and propagation delay.
 Packet also suffer form packet loss

15. How do loss and delay occur?
 Packet arrival rate to link exceeds output link capacity
 Packets queue, wait for turn
 Arriving packets dropped (loss) if no free buffers

16. Delay in Packet Switched Networks
1. Nodal Processing:
– check bit errors
– determine output link
 2. Queuing
– time waiting at output link
for transmission
– depends on congestion
level of router
3. Transmission delay:
 R=link bandwidth (bps)
 L=packet length (bits)
 time to send bits into link =
L/R
4. Propagation delay:
 d = length of physical link
 s = propagation speed in
medium (~2x108 m/sec)
 propagation delay = d/s

17. Nodal Delay (Total trans. – End2End)
 dproc = processing delay
 typically a few microsecs or less
 dqueue = queuing delay
 depends on congestion
 dtrans = transmission delay
 = L/R, significant for low-speed links
 dprop = propagation delay
 a few microsecs to hundreds of msecs
prop
trans
queue
proc
nodal d
d
d
d
d 




18. Bandwidth vs. Throughput
 Network Bandwidth refers to the maximum amount of
data that can be transmitted over a network or
communication channel in a given period of time.
 Throughput refers to the actual amount of data that is
transmitted over a network or communication channel in a
given period of time