Transcript Circuit switching
Computer Communication & Networks
Lecture # 03
Circuit Switching, Packet Switching
Nadeem Majeed Choudhary
Communication Network
Communication networks Switched networks
End nodes send to one (or more) end nodes
Broadcast networks
End nodes share a common channel (TV, radio…)
Circuit switching
Dedicated circuit per call (telephone, ISDN) (physical)
Packet switching
Data sent in discrete portions (the Internet)
Switching Networks
Long distance transmission is typically done over a network of switched nodes A collection of nodes and connections is a communications network Nodes not concerned with content of data End devices are stations Computer, terminal, phone, etc.
Data routed by being switched from node to node
Nodes
Nodes may connect to other nodes only, or to stations and other nodes Node to node links usually multiplexed Network is usually partially connected Some redundant connections are desirable for reliability Two different switching technologies Circuit switching Packet switching
Simple Switched Network
Switching Activities
Some 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 multiplexed Multiple paths enhance reliability
Circuit Switched Networks
A circuit-switched network consists of a set of switches connected by physical links.
A connection between two stations is a dedicated path made of one or more links.
However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.
Circuit switching (cnt’d)
Three phases involved in the communication process: 1.
2.
3.
Establish the circuit Transmit data Terminate the circuit If circuit not available: busy signal (congestion)
8.9
Note
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.
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- Applications
Developed for voice traffic (phone) Inefficient Channel capacity dedicated for duration of connection If no data, capacity wasted Set up (connection) takes time Once connected, transfer is transparent
Telecom Components
Subscriber Devices attached to network Subscriber line Link between subscriber and network Also called Local Loop or Subscriber Loop Range from Few km up to tens of km Exchange Switching center in the network End office specific switching center that supports subscribers Trunks Branches between exchanges Multiplexed
Circuit Establishment
Time diagram of circuit switching
switch
node 1 node 2 host 1 host 2 circuit establishment data transmission time DATA Delay host 1- node 1 Processing delay node 1 Delay host 2- host 1
Circuit Switching Concepts
Digital Switch Provide transparent signal path between devices Typically allows full duplex transmission Network Interface Control Unit Establish connections - Generally on demand, Handle and acknowledge requests, Determine if destination is free,construct path Maintain connection Disconnect
Blocking or Non-blocking Circuit Switching
Blocking A network may not be able to connect stations because all paths are in use (more stations than path) Used on voice systems Short duration calls Non-blocking Permits all stations to connect (in pairs) at once (at least as many paths as stations) Used for some data connections
Circuit Switching: FDM and TDM
FDM Example: 4 users frequency time TDM frequency time
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.
Solution
We shift (modulate) each of the three voice channels to a different bandwidth. 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 (contd.)
Example
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?
Solution
For five channels, we need at least four guard bands. This means that the required bandwidth is at least
5 × 100 + 4 × 10 = 540 kHz
Applications
AM Radio Band 530-1700KHz Each AM Station needs 10KHz FM Radio Band 88-108MHz Each FM Station needs 200KHz TV Each Channel needs 6MHz
Switching Technique
Station breaks long message into packets Packets sent one at a time to the network Packets handled in two ways Datagram Virtual circuit
Packet Switching
Each end-end data stream divided into
packets
user A, B packets
share
network resources each packet uses full link bandwidth resources used
as needed
Bandwidth division into “pieces” Dedicated allocation Resource reservation 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
Packet switching
-
Why not message switching?
node 1 node 2 host 1 host 2 message
propagation delay
host 1 – node1 message
processing & set-up delay
of a message at node 1 time Store-and-Forward message
Use of Packets
Datagram
Each packet treated independently Packets can take any practical route Packets may arrive out of order Packets may go missing Up to receiver to re-order packets and recover from missing packets
Datagram Diagram
Virtual Circuit
Preplanned route established before any packets sent Call request and call accept packets establish connection (handshake) Each packet contains a virtual circuit identifier instead of destination address No routing decisions required for each packet Clear request to drop circuit Not a dedicated path
Virtual Circuit Diagram
Source-to-destination data transfer in a virtual-circuit network
Virtual Circuits vs Datagram
Virtual circuits Network can provide sequencing and error control Packets are forwarded more quickly No routing decisions to make Less reliable Loss of a node loses all circuits through that node Datagram No call setup phase Better if few packets More flexible Routing can be used to avoid congested parts of the network
Circuit vs. Packet Switching
Circuit Switched
Bandwidth guaranteed Circuit capacity not reduced by other network traffic Circuit costs independent of amount of data transmitted, resulting in wasted bandwidth
Packet Switched Bandwidth 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 capacity packets queue, wait for turn packet being transmitted (delay) A B packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers
1. Store and forward delay
store-and-forward
forward delay packet switches introduced store and 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 Delay
Within 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
Assignment # 01
Q1)
2) Solve the following exercise problems. (Chapter # 15, 17, 20, 24
Q2)
Solve the following exercise problems. (Chapter # 8) 13, 17
Readings
Chapter 8 (B. A Forouzan) Section 8.1, 8.2, 8.3
37
References
Chapter 8
(Data & computer Communication by Behroz A. Forozun)
Chapter 10
( Computer Communication by William Stallings)
Chapter 1
(Computer Networking by James K. Kurose)