lecture # 03 switching course instructor: engr. sana ziafat
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Lecture # 03Switching
Course Instructor:Engr. Sana Ziafat
Communication NetworkCommunication networks
Broadcast networksEnd nodes share a common channel
(TV, radio…)
Switched networks End nodes send to one (or more) end nodes
Packet switchingData sent in discrete portions
(the Internet)
Circuit switchingDedicated circuit per call
(telephone, ISDN)
(physical)
Figure 8.2 Taxonomy of switched networks
Switching NetworksLong distance transmission is typically done
over a network of switched nodesA collection of nodes and connections is a
communications networkNodes not concerned with content of dataEnd devices are stations
Computer, terminal, phone, etc.Data routed by being switched from node to
node
NodesNodes may connect to other nodes only, or to
stations and other nodesNode to node links usually multiplexedNetwork is usually partially connected
Some redundant connections are desirable for reliability
Two different switching technologiesCircuit switchingPacket switching
Simple Switched Network
Switching ActivitiesSome nodes connect only to other nodes
(intermediary nodes). Sole purpose is to switch data
Some nodes have one or more stations attached. They accept from and deliver data to the attached station.
Node-to-node links are usually multiplexedMultiple paths enhance reliability
Circuit Switched NetworksA circuit-switched network consists of a A circuit-switched network consists of a
set of switches connected by physical set of switches connected by physical links. A connection between two stations links. A connection between two stations is a is a dedicated pathdedicated path made of one or more made of one or more links. However, each connection uses links. However, each connection uses only one dedicated channel on each link. only one dedicated channel on each link. Each link is normally divided into n Each link is normally divided into n channels by using FDM or TDM.channels by using FDM or TDM.
Circuit switching (cnt’d)Three phases involved in the communication process:
1. Establish the circuit
2. Transmit data
3. Terminate the circuit
If circuit not available: busy signal (congestion)
8.10
In circuit switching, the resources need to be reserved during the setup phase;the resources remain dedicated for the entire duration of data transfer until the
circuit is terminated.
Note
Circuit switching
A dedicated communication path (sequence of links-circuit) is established between the two end nodes through the nodes of the network
Bandwidth: A circuit occupies a fixed capacity of each link for the entire lifetime of the connection. Capacity unused by the circuit cannot be used by other circuits.
Latency: Data is not delayed at switches
Circuit Switching- ApplicationsDeveloped for voice traffic (phone)Inefficient
Channel capacity dedicated for duration of connection
If no data, capacity wastedSet up (connection) takes timeOnce connected, transfer is transparent
Telecom ComponentsSubscriber
Devices attached to networkSubscriber line
Link between subscriber and network Also called Local Loop or Subscriber Loop
Almost all Local Loops are TPWRange from Few km up to tens of km
ExchangeSwitching center in the networkEnd office specific switching center that supports
subscribersTrunks
Branches between exchangesMultiplexed
Circuit Establishment
Time diagram of circuit switching
circuit establishment
data transmission
host 1 node 1 node 2 host 2
Delay host 1- node 1
time
Processing delay node 1
DATA
Delay host 2- host 1
switch
Circuit Switching: FDM and TDMFDM
frequency
time
TDM
frequency
time
4 users
Example:
Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands.
SolutionWe shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure on next Slide. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them.
Example
Example (contd.)
Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 kHz between the channels to prevent interference?
SolutionFor five channels, we need at least four guard bands. This means that the required bandwidth is at least
5 × 100 + 4 × 10 = 540 kHz
Example
ApplicationsAM Radio
Band 530-1700KHzEach AM Station needs 10KHz
FM RadioBand 88-108MHzEach FM Station needs 200KHz
TVEach Channel needs 6MHz
Switching TechniqueStation breaks long message into packetsPackets sent one at a time to the networkPackets handled in two ways
DatagramVirtual circuit
Figure 8.7 A datagram network with four switches (routers)
8.23
Figure 8.8 Routing table in a datagram network
8.24
A switch in a datagram network uses a routing table that is based on the destination address.
Note
Packet Switchingeach end-end data stream
divided into packetsuser A, B packets share
network resources each packet uses full link
bandwidth resources used as needed
resource contention: aggregate resource
demand can exceed amount available
congestion: packets queue, wait for link use
store and forward: packets move one hop at a time Node receives complete
packet before forwarding
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
Packet switching- Why not message switching?-
Store-and-Forward
host 1 node 1 node 2 host 2
propagation delay host 1 – node1
processing & set-up delay of a message at node 1
time
message
message
message
Use of Packets
DatagramEach packet treated independentlyPackets can take any practical routePackets may arrive out of orderPackets may go missingUp to receiver to re-order packets and
recover from missing packets
DatagramDiagram
Virtual CircuitPreplanned route established before any
packets sentCall request and call accept packets establish
connection (handshake)Each packet contains a virtual circuit
identifier instead of destination addressNo routing decisions required for each
packetClear request to drop circuitNot a dedicated path
VirtualCircuitDiagram
Source-to-destination data transfer in a virtual-circuit network
Virtual Circuits vs DatagramVirtual circuits
Network can provide sequencing and error controlPackets are forwarded more quickly
No routing decisions to makeLess reliable
Loss of a node loses all circuits through that nodeDatagram
No call setup phase Better if few packets
More flexible Routing can be used to avoid congested parts of the
network
Circuit vs. Packet SwitchingCircuit Switched
Bandwidth guaranteed
Circuit capacity not reduced by other network traffic
Circuit costs independent of amount of data transmitted, resulting in wasted bandwidth
Packet SwitchedBandwidth
dynamically allocated on as-needed basis
May have concurrent transmissions over physical channel
May have delays and congestion
More cost-effective, offer better performance
How do loss and delay occur?packets queue in router buffers packet arrival rate to link exceeds output link
capacitypackets queue, wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers: arriving packets dropped (loss) if no free buffers
1. Store and forward delaystore-and-forward packet switches introduced store
and forward delay
delay is proportional to the packet's length in bits.
If a packet consists of L bits, and the packet is to be forwarded onto an outbound link of R bps, then the store-and-forward delay at the switch is L/R seconds.
2. Queuing DelayWithin each router there are multiple buffers (also called
queues), with each link having an input buffer (to store packets that have just arrived to that link) and an output buffer.
If packet has to wait in output buffer packets suffer output buffer queuing delays
These delays are variable and depend on the level of congestion in the network.
Since the amount of buffer space is finite, an arriving packet may find that the buffer is completely filled with other packets waiting for transmission packet loss will occur
38
ReadingsChapter 8 (B. A Forouzan)
Section 8.1, 8.2, 8.3
ReferencesChapter 8 (Data & computer Communication by
Behroz A. Forozun)Chapter 10 ( Computer Communication by William
Stallings)Chapter 1 (Computer Networking by James K.
Kurose)
Q & A
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