Department of Engineering Science
ES465/CES 440, Intro. to Networking & Network Management
Local Area Networks: Packets, Frames, &
Topologies
http://www.sonoma.edu/users/k/kujoory
References
• “Douglas Comer, Computer Networks & Internet,” 6th ed, Pearson, 2014, Ch13.
• “Computer Networks,” A. Tanenbaum, 4th ed., Prentice Hall, 2002, ISBN: 0130661023.
• “Behrouz A. Forouzan, Data Communications Networking,” 4th ed, Mc-Graw Hill, 2007
• “Data & Computer Communications,” W. Stallings, Prentice Hall, 7th Ed., 2004.
• “Computer Networks: A Systems Approach," L. Peterson, B. Davie, 4th Ed., Morgan
Kaufmann 2007.
• References cited in the slides
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Topics Covered
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13.1 Introduction
13.2 Circuit Switching
13.3 Packet Switching
13.4 Local & Wide Area Packet Networks
13.5 Standards for Packet Format & Identification
13.6 IEEE 802 Model & Standards
13.7 Point-to-Point & Multi-Access Networks
13.8 LAN Topologies
13.9 Packet Identification, Demultiplexing, MAC Addresses
13.10 Unicast, Broadcast, & Multicast Addresses
13.11 Broadcast, Multicast, & Efficient Multi-Point Delivery
13.12 Frames & Framing
13.13 Byte & Bit Stuffing
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13.1 Introduction
• This chapter
– begins the part of the text that examines packet
switching & computer network technologies
– explains the IEEE standards model
– concentrates on the concepts of hardware addressing
& frame identification
• Later chapters
– exp & the discussion by considering the use of packets
in Wide Area Networks
– cover a variety of wired & wireless networking
technologies that accept & deliver packets
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13.2 Circuit Switching
• Circuit switching refers to a
communication mechanism
that establishes a path
between a sender & receiver
– with guaranteed isolation
from paths used by other
pairs of senders & receivers
• Circuit switching is usually
associated with legacy
telephone technology
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– Circuit switching networks
use electronic devices to
establish circuits
– Instead of having each circuit
correspond to a physical path
• multiple circuits are
multiplexed over shared media
– because a telephone system
provides a dedicated
connection between two
telephones
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• Fig. 13.1 illustrates the
concept
• & the result is known as a
virtual circuit
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13.2 Circuit Switching
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13.2 Circuit Switching
• Three general properties
define a circuit switched
paradigm:
– Point-to-point communication
• means that a circuit is formed
between exactly two endpoints
– Separate steps for circuit
creation, use, & termination
• distinguishes circuits that are
switched (i.e., established
when needed) from circuits
that are permanent
– Performance equivalent to an
isolated physical path
• circuit switching must provide
the illusion of an isolated path
for each pair of communicating
entities
• Switched circuits use a threestep process analogous to
placing a phone call
– a circuit is established
between two parties
– the two parties use the circuit
to communicate
– the two parties terminate use
• communication between two
parties is not affected in any
way by communication among
other parties
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13.3 Packet Switching
• A packet switching system uses statistical multiplexing
– multiple sources compete for the use of shared media
• Fig. 13.2 illustrates the concept
Figure 13.2 A packet-switched network sending
one packet at a time across a shared medium.
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13.3 Packet Switching
• A packet switching system requires
a sender to divide each message
into blocks of data that are known
as packets
– the size of a packet varies
2. No set-up required before
communication begins
– system remains ready to deliver a
packet to any destination at any time
• a sender does not need to perform
initialization before communicating
– each packet switching technology
defines a maximum packet size
• Three general properties define a
packet switching
1. Arbitrary, asynchronous
communication
– allows a sender to communicate with
one recipient or multiple recipients
• a given recipient can receive messages
from one sender or multiple senders
– system does not need to notify the
underlying system when
communication terminates
3. Performance varies due to
statistical multiplexing among
packets
– multiplexing occurs among packets
rather than among bits or bytes
– communication can occur at any time
• & a sender can delay arbitrarily long
between successive communications
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13.3 Packet Switching
• One of the chief advantages of packet switching is the
lower cost that arises from sharing resources
• To provide communication among N computers
– With a circuit-switched network must have
• a connection for each computer plus at least N/2 independent paths
– With packet switching, a network must have
• a connection for each computer, but only requires one path that is
shared
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13.4 Local & Wide Area Packet Networks
• Packet switching technologies are commonly classified according to
the distance they span
– The least expensive networks use technologies that span a short distance
(e.g., inside a single building)
– The most expensive span long distances (e.g., across several cities)
• Fig. 13.3 summarizes the terminology used
Figure 13.3 The three categories of packet switched networks.
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13.5 Standards for Packet Format & Identification
• Each packet sent across
such a network must
contain the identification
of the intended recipient
• All senders must agree on
the exact details of how to
identify a recipient &
where to place the
identification in a packet
• Standards organizations
create protocol
documents that specify all
details
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• Standards have been
created by various
organizations
– Institute for Electrical &
Electronic Engineers (IEEE)
is most famous
• In 1980, IEEE organized
the Project 802 LAN/MAN
Standards Committee to
produce standards for
networking
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13.5 Standards for Packet Format & Identification
• IEEE is mostly composed
of engineers who focus on
the lower two layers of
the protocol
• In fact, if one reads the
IEEE documents
– it may seem that all other
aspects of networking are
unimportant
– each emphasizes particular
layers of the stack
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– Institute for Electrical &
Electronic Engineers (IEEE)
– World Wide Web Consortium
(W3C)
– Internet Engineering Task
Force (IETF)
• However, other standards
organizations exist &
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• Fig. 13.4 gives a humorous
illustration of a protocol as
viewed by various standards
organizations, such as
• Each standards organization
focuses on particular layers
of the protocol stack
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13.5 Standards for Packet Format & Identification
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13.6 IEEE 802 Model & Standards
• IEEE divides Layer 2 of the protocol stack into two
conceptual sub-layers, as Fig. 13.5 illustrates
– The Logical Link Control (LLC)
• Link establishment/termination, frame acknowledgment, frame
sequencing, frame traffic control & service access points
– The Media Access Control (MAC)
• sublayer specifies how multiple computers share underlying medium
• Frame delimiting, error checking, media access management
Figure 13.5 The conceptual
division of Layer 2 into sublayers
according to the IEEE model.
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13.6 IEEE 802 Model & Standards
• IEEE assigns a multi-part
identifier of the form
XXX.YYY.ZZZ
– XXX denotes the category of
the standard
– YYY denotes a subcategory
– If a subcategory is large
enough, a third level can be
added
• Fig. 13.6 lists examples of
IEEE assignments
• IEEE has created many
working groups that are
each intended to standardize
one type of network
technology
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– Each working group consists
of representatives from the
industrial & academic
communities; meets regularly
to discuss approaches &
devise standards
– IEEE allows a working group
to remain active provided the
group makes progress & the
technology is still deemed
important
– If a working group decides
that the technology under
investigation is no longer
relevant, the group can
decide to disband
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Fig. 13.6
Examples of the
identifiers IEEE
has assigned to
various LAN
standards
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13.7 Point-to-Point & Multi-Access Networks
• LAN technologies allow
multiple computers to
share
• Professionals say that
LANs connect computers
– a medium in such a way that
any computer on the LAN can
communicate with any other
– the term multi-access is
used to describe this &
– with the understanding that a
device such as a printer can
also connect to a multiaccess LAN
• we say that a LAN is a multiaccess network
• LAN technologies
generally provide direct
connection among
communicating entities
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13.8 LAN Topologies
• Many LAN technologies have been invented
– How specific technologies are similar & how they differ?
• Each network is classified into a category according to its
topology or general shape
• This section describes four basic topologies that are used
to construct
–
–
–
–
13.8.1
13.8.2
13.8.3
13.8.4
Bus Topology
Ring Topology
Mesh Topology
Star Topology
• Fig. 13.7 illustrates the topologies
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13.8 LAN Topologies
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13.8.1 Bus Topology
• Bus topology usually
consists of a single cable
to which computers attach
• The ends of a bus network
must be terminated
– to prevent electrical signals
from reflecting back along
the bus
• Any computer attached to
a bus can send on the
cable &
• Because all computers
attach directly to the cable
– any computer can send data
to any other computer
• The computers attached
to a bus network must
coordinate
– to ensure that only one
computer sends a signal at
any time
– all computers receive the
signal
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13.8.2 Ring Topology
• Ring topology arranges for
computers to be connected
in a closed loop
– a cable connects the first
computer to a second
computer, another cable
connects the second
computer to a third, & so on,
until a cable connects the
final computer back to the
first
– name ring arises because
one can imagine the
computers & the cables
connecting them arranged in
a circle as Fig. 13.7 illustrates
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• In practice, the cables in a ring
network do not form a circle
• Instead, they run along
hallways or rise vertically from
one floor of a building to
another
• Ring topology requires a
computer to connect to a
small device that forms the
ring
– this is needed for the ring to
continue operation even if
some of the computers are
disconnected
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13.8.3 Mesh Topology
• A network that uses a mesh topology provides a direct
connection between each pair of computers
• The chief disadvantage of a mesh arises from the cost:
– -n)/2)
– The important point is that the number of connections needed for
a mesh network grows faster than the number of computers
• Because connections are expensive
– few LANs employ a mesh topology in special circumstances
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13.8.4 Star Topologies
• In star topology, all computers attach to a central point
• The center of a star network is often called a hub
• A typical hub consists of an electronic device
– that accepts data from a sending computer & delivers it to the
appropriate destination
• In practice, star networks seldom have a symmetric
shape (hub is located an equal distance from all
computers)
– Instead, a hub often resides in a location separate from the
computers attached to it
– E.g., computers can reside in individual offices, while the hub
resides in a location accessible to an organization's networking
staff
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13.8.5 The Reason for Multiple Topologies
• Each topology has
advantages & disadvantages
• A ring makes it easy for
computers to coordinate
access & to detect whether
the network is operating
correctly
– However, an entire ring
network is disabled if one of
the cables is cut
• A star helps protect the
network from damage to a
single cable because each
cable connects only one
machine
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• A bus requires fewer wires
than a star, but has the
same disadvantage as a ring
– a network is disabled if
someone accidentally cuts
the main cable
• Later chapters that describe
specific network
technologies
• Compare the topologies considering
cost, point of failure, construction,
efficiency, etc., .
Topologies
Advantages
Disadvantages
Bus
Ring
Star
Mesh
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13.9 Packet Identification, Demultiplexing, MAC Addresses
• IEEE has created a standard for addressing
• Consider packets traversing a shared medium as Fig. 13.2 illustrates
Figure 13.2 A packet-switched network sending
one packet at a time across a shared medium.
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13.9 Packet Identification, Demultiplexing, MAC Addresses
• Each packet that travels
across the shared medium is
intended for a specific
recipient &
– only the intended recipient
should process the packet
• Demultiplexing uses an
identifier known as an
address
• Each computer is assigned a
unique address &
– each packet contains the
address of the intended
recipient
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• In the IEEE addressing
scheme, each address
consists of 48 bits; IEEE
uses the term Media
Access Control address (or
simply MAC address)
– networking professionals
often use the term Ethernet
address
• IEEE allocates a unique
address for each piece of
interface
– Each Network Interface Card
(NIC) contains a unique IEEE
address assigned when the
device was manufactured
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13.9 Packet Identification, Demultiplexing, MAC Addresses
• IEEE assigns a block of addresses to each vendor &
– allows the vendor to assign a unique value to each device
– there is a 3-byte Organizationally Unique ID (OUI)
• OUI identifies the equipment vendor
• a 3-byte block that identifies a particular NIC
• Fig. 13.8 illustrates the division
Figure 13.8 The division of a 48-bit IEEE MAC address.
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13.10 Unicast, Broadcast, & Multicast Addresses
• The IEEE addressing supports three types of addresses
– that correspond to three types of packet delivery
• Fig. 13.9 provides a summary
Figure 13.9 The three types of MAC addresses &
the corresponding meanings.
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13.10 Unicast, Broadcast, & Multicast Addresses
• It may seem odd that the IEEE address format reserves a
bit to distinguish between unicast & multicast
– but does not provide a way to designate a broadcast address
• The standard specifies that a broadcast address consists
of 48 bits that are all 1s
– Thus, a broadcast address has the multicast bit set
• Broadcast can be viewed as a special form of multicast
– Each multicast address corresponds to a group of computers
– Broadcast address corresponds to a group that includes all
computers on the network
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13.11 Broadcast, Multicast, & Efficient Multi-Point Delivery
• Broadcast & multicast addresses are useful in LANs
– because they permit efficient delivery to many computers
• To understand the efficiency
– recall that a LAN transmits packets over a shared medium
• In a typical LAN
–
–
–
–
each computer on the LAN monitors the shared medium
extracts a copy of each packet &
then examines the address in the packet
determine whether the packet should be processed or ignored
• Algorithm 13.1 gives the algorithm a computer uses to
process packets
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13.11 Broadcast, Multicast, & Efficient Multi-Point Delivery
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13.12 Frames & Framing
• Chapter 9 introduces the
concept of framing
– In synchronous
communication systems it is
used as a mechanism that
allows a receiver to know
where a message begins &
ends
• In more general terms,
framing refers to the
structure added to a
sequence of bits or bytes
that allows a sender &
receiver to agree on the
exact format of the
message
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• In a packet-switched
network, each frame
corresponds to a packet
• A frame consists of two
conceptual parts:
– Header that contains
metadata, such as an
address
• Contains more information
used to process the frame
– Payload that contains the
message being sent &
• is usually much larger than the
frame header
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13.12 Frames & Framing
• A message is opaque
– in the sense that the network only examines the frame header
– the payload can contain an arbitrary sequence of bytes that are
only meaningful to the sender & receiver
• Some technologies delineate each frame by sending a
short prelude before the frame & a short postlude after it
• Fig. 13.10 illustrates the concept
Figure 13.10 Typical structure of a frame in a packet-switched network.
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13.12 Frames & Framing
• Assume that a packet header consists of 6 bytes
– the payload consists of an arbitrary number of bytes
• We can use ASCII character set
– Start Of Header (SOH) character marks the beginning of a frame,
&
– End Of Transmission (EOT) character marks the end
• Fig. 13.11 illustrates an example of the format
IEEE 802.3 & Ethernet Frame Header contain S & D MAC addresses each 6 octets),
IEEE802.1 (4 octets, optional), Length or Ether Type 2 octets.
SOH contains 7 octets of 1010101010 plus an octet of 1010,1011. Octets are transmitted LSB first.
https://en.wikipedia.org/wiki/Ethernet_frame
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13.13 Byte & Bit Stuffing
• In the ASCII character set
– SOH has hexadecimal value 0x01
– EOT has the hexadecimal value 0x04
• An important question arises
– what happens if the payload of a frame includes one or more bytes
with value 0x01 or 0x04?
• The answer lies in a technique known as byte stuffing
– that allows transmission of arbitrary data without confusion
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13.13 Byte & Bit Stuffing
• To distinguish between
data & control information,
such as frame delimiters
– The Sender changes a data
to replace each control byte
with a sequence
– The receiver replaces the
sequence with the original
one
– As a result, a frame can
transfer arbitrary data &
• the underlying system never
confuses data with control
information
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• The technique is known
as byte stuffing
– the terms data stuffing &
character stuffing are
sometimes used
• A related technique used
with systems that transfer
a bit stream is known as
bit stuffing
• As an example of byte
stuffing, consider a frame
as illustrated in Fig. 13.12
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13.13 Byte & Bit Stuffing
• Because SOH & EOT are used to delimit the frame
– those two bytes must not appear in the payload
• Byte stuffing reserves a third character to mark occurrences of
reserved characters in the data
– E.g., suppose the ASCII character ESC (0x1B) has been selected as the
third character
– When any of the three special characters occur in the data
• the sender replaces the character with a two-character sequence
• Fig. 13.12 lists one possible mapping
EOT = 0x04
ESC = 0x1B
SOH = 0x01
A = 0x41
B = 0x42
C = 0x43
Figure 13.12 A example of byte stuffing that maps each
special character into a 2-character sequence.
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13.13 Byte & Bit Stuffing
• As the figure specifies, the sender replaces
– each occurrence of SOH by the two characters ESC + A
– each occurrence of EOT by the characters ESC + B, &
– each occurrence of ESC by the two characters ESC + C
• A receiver reverses the mapping
– by looking for ESC followed by one of A, B, or C & replacing the 2character combination with the appropriate single character
• Fig. 13.13 shows an example payload & the same
payload after byte stuffing has occurred
– Note that once byte stuffing has been performed, neither SOH nor
EOT appears anywhere in the payload
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13.13 Byte & Bit Stuffing
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Appendix
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Circuit Switching vs Packet Switching
Intercity
Trunk
Subscriber
Loops
Physical copper
connection established
when call is made.
Circuit
Switching
End
Office
Connecting
Trunk
End
Office
Long-Distance
Office
Subscriber
Loops
Long-Distance
Office
Data packet
(Datagrams)
Receiver
Sender
Packet
Switching
Switches
Internet
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What are the Sources of Delay
The delay may come from several sources.
• Voice/image digitizing/encoding
• Transmission
– Voice/image/data
compression/decompression
– Time to transmit the packets from the
computer & send them over the links
along the path
• Accumulation
– To collect several voice/image
samples in a packet before encoding.
• Processing
– Time to process the packets
– The lower the port / link capacity, the
larger the transmission delay.
– The larger the packet size, the larger
the transmission delay.
• Propagation
– Negligible for a fast processor
– Propagation of voice/data packets
across the media - air, fiber, wire
• Queuing
– Buffering time in the gateways and
routers along the path
– The longer the distance, the larger
the propagation delay.
– Can vary
– Depends on the medium – air, fiber,
wire
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Propagation and Transmission Time
Channel
Transmitter
Receiver
SSU
NYU
“Message”
Point of reception
Point of launch
x
Transmission Time
Bits in transit
from transmitter
to receiver
x
x
X=0
Propagation Time
X=L
All bits have
been received
Transmission
Time
Transmission time – the time to output the bit sequence of the message.
Propagation time – the time for a bit to travel from transmitter to receiver.
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Propagation and Transmission Time
Channel Distance (meter)
Propagation Time 
Propagatio n speed (meter/sec )
Message size (bits)
Transmissi on Time 
Transmissi on speed (bits/sec)
Total number of bits in message (bits)

bit rate (bits/sec)
Definition by approximation:
Latency ≈ Propagation Delay + Transmission Delay
More generally (because other factors may be present):
Latency = Propagation Delay + Transmission Delay
+ Queuing Delay + Processing Delay
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Propagation and Transmission Speed
Propagation speed is the speed a bit travels over the channel medium of
length L (i.e., the distance from source to destination),
Distance L (meter)
Propagation speed 
Travel time (sec)
Transmission speed is the rate (units of bps) at which message bits arrive at
destination,
Number of bits transmitted (bits)
Transmission speed 
Transmission time (sec)
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