Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
10-1
Chapter 10
Circuits Switching and Packet Switching
10-2
Content
• Switched communication networks • Circuit switching networks • Circuit-switching concepts • Packet-switching principles • X.25 (mentioned but not covered) • Frame relay (mentioned but not covered)
10-3
Switching Networks
• Long distance transmission is typically done over a network of switched nodes
• Nodes not concerned with content of data • End devices are stations
– Computer, terminal, phone, etc.
• A collection of nodes and connections is a communications network
• Data routed by being switched from node to node
10-4
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
10-5
Simple Switched Network
10-6
Circuit Switching
• Dedicated communication path between two stations
• Three phases – Establish – Transfer – Disconnect
• Must have switching capacity and channel capacity to establish connection
• Must have intelligence to work out routing
10-7
Circuit Switching (II)
• Inefficient – Channel capacity dedicated for duration of
connection – If no data, capacity wasted
• Set up (connection) takes time • Once connected, transfer is transparent • Developed for voice traffic (phone)
10-8
Telecommunications Components • Subscriber
– Devices attached to network • Subscriber line
– Local Loop or subscriber loop – Connection to network – Few km up to few tens of km
• Exchange – Switching centers – End office - supports
subscribers • Trunks
– Branches between exchanges – Multiplexed
10-9
Circuit Establishment
10-10
Circuit Switching Concepts • Digital Switch
– Provide transparent signal path between devices
• Network Interface • Control Unit
– Establish connections • Generally on demand • Handle and acknowledge
requests • Determine if destination is
free • Construct path
– Maintain connection – Disconnect
10-11
Blocking or Non-blocking
• Blocking – A network is unable to connect stations because all
paths are in use – A blocking network allows this – Used on voice systems
• Short duration calls
• Non-blocking – Permits all stations to connect (in pairs) at once – Used for some data connections
10-12
Space Division Switching
• Developed for analog environment • Separate physical paths • Crossbar switch
– Number of crosspoints grows as square of number of stations
– Loss of crosspoint prevents connection – Inefficient use of crosspoints
• All stations connected, only a few crosspoints in use
– Non-blocking
10-13
Space Division Switch
10-14
Multistage Switch
• Reduced number of crosspoints
• More than one path through network – Increased reliability
• More complex control • May be blocking
10-15
Time Division Switching
• Modern digital systems rely on intelligent control of space and time division elements
• Use time division multiplexing to achieve switching.
• Two popular methods – Time slot interchange – TDM bus
10-16
Packet Switching Principles • Circuit switching designed for
voice – Resources dedicated to a
particular call – Much of the time a data
connection is idle – Data rate is fixed
• Both ends must operate at the same rate
• Data transmitted in small packets – Typically 1000 octets – Longer messages split into
series of packets – Each packet contains a portion
of user data plus some control information
• Control information – At least, routing (addressing)
information • Packets are received, stored
briefly (buffered) and past on to the next node – Store and forward
10-17
Use of Packets
10-18
Advantages
• Line efficiency – Single node to node link can be shared by many packets
over time – Packets queued and transmitted as fast as possible
• Data rate conversion – Each station connects to the local node at its own speed – Nodes buffer data if required to equalize rates
• Packets are accepted even when network is busy – Delivery may slow down
• Priorities can be used
10-19
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
10-20
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
10-21
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
What is the difference between the virtual circuit switching we
just saw and real circuit switching?
10-22
10-23
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
10-24
Circuit vs. Packet Switching
• Performance – Propagation delay – Transmission time – Node delay
10-25
Packet Size
10-26
Circuit v Packet Switching
• performance depends on various delays – propagation delay – transmission time – node delay
• range of other characteristics, including: – transparency – amount of overhead
10-27
Chapter 11
Asynchronous Transfer Mode (ATM)
10-28
ATM Cell
• ATM divides all data into small, fixed-size cells.
• Each cell contains exactly 53-octets. • 5 octets of header • 48 octets of data
10-29
An ATM Cell
10-30
ATM Logical Connections
• Connection oriented service • VCC (Virtual Channel Connection)
– Logical connection in ATM – A VCC is set up between two end users through the
network and a variable rate, full-duplex flow of fixed-size cells is exchanged over the connection.
• VPC (Virtual Path Connection) – A bundle of VCCs that have the same endpoints.
• VPI (VP Identifier) • VCI (VC Identifier)
10-31
TP: physical Transmission Path
TPs, VPs, and VCs
10-32
UNI: User Network Interface NNI: Network network Interface
Architecture of an ATM Network
10-33
Example of VPs and VCs
10-34
ATM switch changes the VPI/VCI in each cell it handles.
Routing with a VPC Switch
10-35
A Conceptual View of a VPC Switch
10-36
ATM Layers (ATM Adaptation Layer)
10-37
ATM Header
10-38
Header Fields
• GFC (General Flow Control) – for flow control
• PT (Payload Type) • CLP (Cell Loss Priority)
– 0: higher priority, should not be discarded … – 1: lower priority
• HEC (Header Error Control) – CRC: X8+X2+X+1
10-39
CBR: Constant Bit Rate VBR: Variable Bit Rate ABR: Available Bit Rate UBR: Unspecified Bit Rate
Service Classes
10-40
Service Classes and Capacity of Network
10-41
ATM Applications (I)
10-42
ATM Applications (II)
10-43
ATM Applications (III)