Transcript Ch. 8 Circuit Switching

Ch. 10 Circuit Switching and
Packet Switching
10.1 Switched Communication Networks
• Fig. 10.1 Simple switching network.
– End stations are attached to the "cloud".
– Inside the cloud are communication network nodes
interconnected with transmission lines.
– The transmission lines often use multiplexing.
– The network is generally not fully connected, but
alternate paths exist.
• Two technologies for WANs
– Circuit Switching
– Packet Switching
10.2 Circuit-Switching Networks
• The three phases of a circuit switched
connection are
– Circuit establishment
– Data transfer
– Circuit disconnect
10.2 Circuit-Switching Networks (p.2)
• Four generic architectural components of the
public telecommunications network:
–
–
–
–
Subscribers
Subscriber line (or local loop)
Exchanges
Trunks
• Fig. 10.2 illustrates the public switched
telephone network (PSTN).
• Fig. 10.3 illustrates two possible connections
over the PSTN.
10.3 Circuit-Switching Concepts
• Fig.10.4 Elements of a Circuit-Switch Node
– Digital Switch
• Provides a transparent signal path between any pair of
attached devices.
– Control Unit
• Establishes connections.
• Maintains connections.
• Tears down connections.
– Network Interface
• Functions and hardware needed to connect digital and
analog terminals and trunk lines.
10.3 Circuit-Switching Concepts (p.2)
• Blocking vs. Nonblocking
– Relates to the capability of making connections.
– A blocking network is one in which blocking is
possible.
– A nonblocking network permits all stations to
be connected (in pairs) as long as the stations
are not in use.
10.3 Circuit-Switching Concepts (p.2)
• Space-Division Switching
– Defn: A circuit-switching technique in which
each connection through the switch takes a
physically separate and dedicated path.
– Basic building block--a metallic crosspoint or
semiconductor gate.
– "Crossbar" Matrix (Fig. 10.5)
– Multi-stage space-division switches reduces the
total number of crosspoints required, but increases
complexity and introduces the possibility of
blocking.(Fig. 10.6)
10.3 Circuit-Switching Concepts (p.3)
• Time-Division Switching
– Defn: A circuit-switching technique in which time
slots in a time-multiplexed stream of data are
manipulated to pass data from an input to an output.
– All modern circuit switches use digital time division
techniques or some combination of space division
switching and time division switching.
10.4 Softswitch Architecture
• Specialized software is run on a computer that
turns it into a smart phone switch (Fig.10.10).
– Performs traditional circuit-switching functions.
– Can convert a stream of digitized voice into packets
(VoIP).
• Media Gateway (MG) performs the physical
switching function.
• Media Gateway Controller (MGC) performs
call processing.
• RFC 3015--communications between the two.
10.5 Packet-Switching Principles
• Definition: A method of transmitting
messages through a communication
network, in which long messages are
subdivided into short packets. The packets
are then sent through the network to the
destination node. (See Fig. 10-8)
10.5 Packet-Switching Principles (p.2)
• Two Techniques
– Datagram (Fig. 10.9)
•
Each packet contains addressing
information and is routed separately.
– Virtual Circuits (Fig. 10.10)
• A logical connection is established
before any packets are sent; packets
follow the same route.
10.5 Packet-Switching Principles (p.3)
• Packet Size
– Each packet has overhead.
– With a larger packet size
• Fewer packets are required (less overhead.)
• But longer queuing delays exist at each packet
switch.
– Figure 10.11 illustrates this issue.
10.5 Packet-Switching Principles (p.4)
• Delay in Switching Networks
– Setup Time--connection oriented networks
(removed from chapter but not problems)
– Transmission Time
– Propagation Delay
– Nodal Delay--processing time at nodes.
• Fig. 10.13 and Table 10.1 compare the
performance of circuit switching, datagram
packet switching, and virtual-circuit packet
switching.
10.6 Packet-Switching Principles (p.5)
• Delay in Circuit Switched Networks
– Call setup time.
– Message transmission time--occurs once at the
source.
– Propagation delay--sum of all links.
– Very little node delay.
10.6 Packet-Switching Principles (p.6)
• Delay in Packet Switching
– Connection Setup Time
• Required for virtual circuit.
• None for datagram.
– Packet transmission time and propagation
delay occurs on each link.
– Processing delay occurs at every node.
• Datagram networks may require more than virtual
circuit networks.
Problem 10.4
• Consider the delay across a network.
–
–
–
–
–
–
Let B= data rate on every link.
Let N= the number of links.
Let L= the length of the source message.
Let D= the average delay on a link.
Let S= setup time (when required.)
Let P= packet size for packet switched
networks--fixed length packets.
– Let H=the number of bits of overhead in each
packet header, for packet switched networks.
Problem 10.4 (p.2)
• Circuit Switching Delay
– Let t0 be the time that the first bit is transmitted at
the source node and t1 be the time that the last bit
is received at the destination node.
– Then let T= t1-t0 be the "end-to-end" delay.
– Follow the last bit across the network.
– No network layer overhead and little nodal delay.
– Ignore any data link protocol delay (U=1).
– T = S + L/B + N x D
Problem 10.4 (p.3)
• Datagram Packet Switch Delay
– Let NoPa= Number of Packets= L/(P-H)
rounded up (ceiling).
– Assume no link level related overhead (U=1.)
– The last packet waits at the source and then is
transmitted over every link in a store and
forward fashion.
– T= (NoPa-1)P/B + N(P/B + D)
• Virtual-Circuit Packet Switch Delay
– T= S + (NoPa-1)P/B + N(P/B + D)
X.25 (no longer in text)
• First approved in 1976 and revised in 1980,
1984, 1988, 1992, and 1993.
• Specifies an interface between a host system and
a packet-switched networks.
• Almost universally used and is employed for
packet-switching in ISDN.
• Virtual circuits are used in an X.25 network.
X.25 (p.2)
• Three Layers are defined
– X.21 is the physical layer interface (often
EIA-232 is substituted)
– LAP-B is the link-level logical interface-it is a subset of HDLC.
– Layer 3 has a multi-channel interface-sequence numbers are used to
acknowledge packets on each virtual
circuit.