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.
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
