1. Department of Engineering Science ES465/CES 440, Intro
Department of Engineering Science ES465/CES 440, Intro. to Networking & Network Management
Local Area Networks: Packets, Frames, & Topologies
References
“Douglas Comer, Computer Networks & Internet,” 6th ed, Pearson, 2014, Ch13.
“Computer Networks,” A. Tanenbaum, 4th ed., Prentice Hall, 2002, ISBN:
“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

2. Topics Covered 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
Unicast, Broadcast, & Multicast Addresses
Broadcast, Multicast, & Efficient Multi-Point Delivery
Frames & Framing
Byte & Bit Stuffing

3. 13.1 Introduction This chapter Later chapters
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

4. 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
because a telephone system provides a dedicated connection between two telephones
Fig illustrates the concept
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
& the result is known as a virtual circuit

5. 13.2 Circuit Switching

6. 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
communication between two parties is not affected in any way by communication among other parties
circuit switching must provide the illusion of an isolated path for each pair of communicating entities
Switched circuits use a three- step 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

7. 13.3 Packet Switching
A packet switching system uses statistical multiplexing
multiple sources compete for the use of shared media
Fig illustrates the concept
Figure A packet-switched network sending
one packet at a time across a shared medium.

8. 13.3 Packet Switching 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
system does not need to notify the underlying system when communication terminates
Performance varies due to statistical multiplexing among packets
multiplexing occurs among packets rather than among bits or bytes
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
each packet switching technology defines a maximum packet size
Three general properties define a packet switching
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
communication can occur at any time
& a sender can delay arbitrarily long between successive communications

9. 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

10. 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 summarizes the terminology used
Figure The three categories of packet switched networks.

11. 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
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

12. 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
However, other standards organizations exist &
each emphasizes particular layers of the stack
Fig gives a humorous illustration of a protocol as viewed by various standards organizations, such as
Institute for Electrical & Electronic Engineers (IEEE)
World Wide Web Consortium (W3C)
Internet Engineering Task Force (IETF)
Each standards organization focuses on particular layers of the protocol stack

13. 13.5 Standards for Packet Format & Identification

14. 13.6 IEEE 802 Model & Standards
IEEE divides Layer 2 of the protocol stack into two conceptual sub-layers, as Fig 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 The conceptual division of Layer 2 into sublayers
according to the IEEE model.
After deciding that more detail was needed at the data-link layer, the 802 standards committee divided the data-link layer into two sublayers:
Logical Link Control (LLC) Establishing and terminating links, controlling frame traffic, sequencing frames, and acknowledging frames
The LLC sublayer manages data-link communication and defines the use of logical interface points called service access points (SAP). Other computers can refer to and use SAPs to transfer information from the LLC sublayer to the upper OSI layers. Category defines these standards.
Media Access Control (MAC) Managing media access, delimiting frames, checking frame errors, and recognizing frame addresses
The MAC sublayer is the lower of the two sublayers, providing shared access to the physical layer for the computers' NICs. The MAC layer communicates directly with the NIC and is responsible for delivering error-free data between two computers on the network.

15. 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 lists examples of IEEE assignments
IEEE has created many working groups that are each intended to standardize one type of network technology
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

16. Examples of the identifiers IEEE has assigned to various LAN
Fig. 13.6
Examples of the identifiers IEEE has assigned to various LAN
standards

17. 13.7 Point-to-Point & Multi-Access Networks
LAN technologies allow multiple computers to share
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 &
we say that a LAN is a multi- access network 
LAN technologies generally provide direct connection among communicating entities
Professionals say that LANs connect computers
with the understanding that a device such as a printer can also connect to a multi- access LAN

18. 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
Bus Topology
Ring Topology
Mesh Topology
Star Topology
Fig illustrates the topologies

19. 13.8 LAN Topologies

20. 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 &
all computers receive the signal
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

21. 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 illustrates
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

22. 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

23. 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

24. 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
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

25. 13.9 Packet Identification, Demultiplexing, MAC Addresses
IEEE has created a standard for addressing
Consider packets traversing a shared medium as Fig illustrates
Figure A packet-switched network sending
one packet at a time across a shared medium.

26. 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 
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

27. 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 illustrates the division
Figure The division of a 48-bit IEEE MAC address.

28. 13.10 Unicast, Broadcast, & Multicast Addresses
The IEEE addressing supports three types of addresses
that correspond to three types of packet delivery
Fig provides a summary
Figure The three types of MAC addresses &
the corresponding meanings.

29. 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

30. 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

31. 13.11 Broadcast, Multicast, & Efficient Multi-Point Delivery

32. 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
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
Metadata = data providing more information

33. 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 illustrates the concept
Figure Typical structure of a frame in a packet-switched network.

34. 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 illustrates an example of the format
IEEE & 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 plus an octet of 1010,1011. Octets are transmitted LSB first.

35. 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

36. 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
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

37. 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 lists one possible mapping
Figure A example of byte stuffing that maps each
special character into a 2-character sequence.
EOT = 0x04
ESC = 0x1B
SOH = 0x01
A = 0x41
B = 0x42
C = 0x43

38. 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 2- character combination with the appropriate single character
Fig 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

39. Byte & Bit Stuffing

40. Appendix

41. Circuit Switching vs Packet Switching
Physical copper
connection established
when call is made.
Subscriber
Loops
End Office
Connecting Trunk
Long-Distance Office
Intercity Trunk
Circuit Switching
Packet Switching
Sender
Receiver
Internet
Switches
Data packet
(Datagrams)

42. What are the Sources of Delay
The delay may come from several sources.
Voice/image digitizing/encoding
Voice/image/data compression/decompression
Accumulation
To collect several voice/image samples in a packet before encoding.
Processing
Time to process the packets
Negligible for a fast processor
Queuing
Buffering time in the gateways and routers along the path
Can vary
Transmission
Time to transmit the packets from the computer & send them over the links along the path
The lower the port / link capacity, the larger the transmission delay.
The larger the packet size, the larger the transmission delay.
Propagation
Propagation of voice/data packets across the media - air, fiber, wire
The longer the distance, the larger the propagation delay.
Depends on the medium – air, fiber, wire

43. Propagation and Transmission Time
Transmitter
“Message”
Transmission Time
Propagation Time
Channel
SSU
NYU
Receiver
Transmission
Time
Bits in transit
from transmitter
to receiver x
Point of launch
Point of reception
All bits have
been received
X = 0
X = L
Transmission time – the time for the message bits to get out of the transmitter port.
So far as the data is available at transmit port, this time depends on the port speed and the message size.
E.g., the computer may send a message out of its Ethernet port at over 10 Mbps or out of the telephone port at 64 kbps.
Propagation time – the time it takes for a bit to travel from transmitter to receiver.
This time depends upon the propagation distance and medium - fiber, copper, or air
E.g., a message my propagate close to speed of light in the air, while it propagates at about 2/3 of speed of light in copper wire.
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.

44. Propagation and Transmission Time
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

45. 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), Transmission speed is the rate (units of bps) at which message bits arrive at destination,