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
.