Details of data link layer

1. Data Link Layer

2. Service view
 Topology: the way in which loosely coupled computers are
interconnected.
 Synonym: configuration.
 Protocol: a set of rules and standards for communications
between computers.
 control sending, receiving of messages
 e.g., TCP, IP, HTTP, FTP, PPP
 Communication services provided to applications:
 Connectionless unreliable
 Connection-oriented reliable

3. Data link layer divided into two functionality
Network Layer
Data Lin k Layer
Physical Layer

4. Two functionality
Upper layer – responsible for data link control
• Called LLC – for flow and error control
Lower layer - responsible for resolving access the shared media
• Called MAC – for multiple access resolution

5. 802 Layers functions
 Physical
 Encoding/decoding
 Preamble generation/removal
 Bit transmission/reception
 Transmission medium and topology
 Logical Link Control
 Interface to higher levels
 Flow and error control
 Medium Access Control
 Data assembly and dismantle into frames
 Govern access to LAN transmission medium

6. Responsible for two sub layers:
• Logical Link Control (LLC). This sub layer is responsible for the data
transmission between computers or devices on a network.
• Media Access Control (MAC). On a network, the network interface
card (NIC) has an unique hardware address which identifies a computer
or device. The physical address is utilized for the MAC sub layer
addressing.

7. Logical Link Control (LLC)
 The LLC layer for LANs is concerned with the transmission of a
link-level protocol data unit (PDU) between two stations, without
the necessity of an intermediate switching node
 It has two characteristics:
 It must support the multi-access, shared medium nature of
the link
 It is relieved from some details of link access by the MAC
layer

8. Logical Link Control (LLC).
• The function of the Logical Link Control (LLC) is to manage and ensure
the integrity of data transmissions.
• The LLC provides Data Link Layer links to services for the Network
Layer protocols.
• This is accomplished by the LLC Service Access Points (SAPs) for the
services residing on network computers. Also, there is a LLC Control field
for delivery requests or services.
The Logical Link Control (LLC) has several service types:
•
•
•
Service type 1, is a connectionless service with no establishment of a
connection, and an unacknowledged delivery.
Service type 2, is a connection logical service with an acknowledgement
of delivery.
Service type 3, is a connectionless service with an acknowledgement of
delivery.

9. LLC Services
 Unacknowledged connectionless service
 Datagram-style
 Does not involve any flow and error control mechanisms
 Data delivery is not guaranteed
 Acknowledged Connection mode service
 A logical connection is set up between two stations
 Flow and error control are provided
 Acknowledged connectionless service
 A cross between the previous services
 Datagrams are to be acknowledged
 No prior logical connection is set up

10. 802.2 Logical Link Control
 It is used with the 802.3, 802.4, and 802.5 standards (lower DL
sub layers).
 Basically, 802.2 as the "translator" for the Data Link Layer.
 802.2 "specifies the general interface between the network layer
(IP, IPX, etc.) and the data link layer (Ethernet, Token Ring, etc).
 802.2 is concerned with managing traffic over the physical
network.

11. 802.2 Logical Link Control cont..
 The Data Link Layer wants to send some data over the network,
802.2 Logical Link Control helps make this possible. It also helps
by identifying the line protocol, like NetBIOS, or Netware.
 The LLC acts like a software bus allowing multiple higher layer
protocols to access one or more lower layer networks.
 For example, if you have a server with multiple network interface cards, the
LLC will forward packets from those upper layer protocols to the
appropriate network interface.

12. Media Access Control
 All LANs and MANs consist of a collection of devices that must
share the network’s transmission capacity.
 Function of a Medium Access Control (MAC) Protocol.
Controlling access is needed for efficient use of that capacity.
 The key parameters in any MAC technique are where and how.
 Where, refers to whether control info is exercised in a centralized or
distributed fashion.
 Centralized: a controller has the authority to grant access to the network
 Distributed: the stations collectively perform a MAC function to determine
dynamically the order in which stations transmit
 How, is constrained by the topology and is a trade-off among competing
factors, such as cost, performance and complexity

13. 13
Medium Access Control (MAC) Protocols
 Characteristics of the channels, data rate, voltage
levels, etc.
 Node access to the channel (medium access control
protocol)
Share data to its destination
Detect errors
Prevent multiple nodes
simultaneously (collision)
Ethernet and token ring
Implemented in hardware
from accessing the network

14. 14
LAN Protocols in Context

15. Multiple - Access Protocols

16. RANDOM ACCESS
In random access or contention methods, no station is
superior to another station and none is assigned the control
over another.
No station permits, or does not permit, another station to
send the data. At each instance, a station that has a data to
send, it uses a procedure defined by the protocol to make a
decision on whether or not to send.
The decision depends on the state of the medium (idle or
busy).

17. RANDOM ACCESS
• ALOHA
• Carrier Sense MultipleAccess
• Carrier Sense MultipleAccess with Collision Detection
• Carrier Sense MultipleAccess with CollisionAvoidance

18. Frames in a pure ALOHA network
PureALOHA:
• Each station sends a frame whenever is has a frame.
• One channel to share, possibility of collision between frames
from different stations

19. Procedure
for pure
ALOHA
protocol

20. Vulnerable time for pure ALOHA protocol
Vulnerable Time

21. Frames in a slotted ALOHA
SlottedALOHA:
• We divide the time into slots and force the station to send only at the beginning of the
time slot

22. Vulnerable time for slotted ALOHA protocol

23. Medium Access Control Methods
 The methods used for Medium Access Control
are:
 Carrier-sense multiple-access with collision detection
(CSMA/CD) for bus topologies
 Control token or Token Passing for bus and ring
topologies

24. CSMA/CD
 CSMA/CD is used only in bus type networks, where
a number of nodes share a common communication
channel (wire) known as the bus.
 CSMA/CD is used in traditional Ethernet
 Ethernet will be covered in detail in future lectures

25. CSMA/CD A B D
C
A B D
C
A B D
C
A B D
C
JAM JAM JAM JAM JAM JAM
Carrier
Sense
Multiple
Access
Collision
Collision
Detection
(Backoff
Algorithm)
Collision

26. Energy level during transmission, idleness, or
collision
•
•
•
Zero level – channel is idle
Normal level – successfully captured channel and sending frame
Abnormal level - collision and energy twice the normal level

27. Space / time model of the collision in CSMA
CSMA – each station first listen to the medium before sending the data
Principle : “sense before transmit” or “listen before talk”

28. Vulnerable time in CSMA

29. Behavior of three persistence methods
1-Persistent after station
finds the line idle, send its
frame
Non-persistent senses the
line; idle:
immediately; not
sends
idle:
waits random amount of
time and senses again
p-Persistent the channel
has time
duration
greater
slots with
equal to or
than max
propagation time

30. Flow diagram for three persistence methods

31. Collision of the first bit in CSMA/CD
CSMA/CD-Augments CSMAalgorithm to handle collision
Collision and abortion in CSMA/CD

32. Flow diagram for the CSMA/CD (Page 375)

33. Timing in CSMA/CA
Avoid collisions on wireless network because they cannot be detected
IFS- In CSMA/CA, the IFS can also be used to define the priority of a station
or a frame.
Contention window- In CSMA/CA, if the station finds the channel busy,
it does not restart the timer of the contention window;
it stops the timer and restarts it when the channel becomes idle.
Acknowledgment- Positive acknowledgment and time out timer guarantee
receiver has received the frame

34. Flow diagram for CSMA/CA

35. Controlled Access
In controlled access, the stations consult one another to
find which station has the right to send. A station cannot
send unless it has been authorized by other stations. We
discuss three popular controlled-access methods.
Topics discussed in this section:
• Reservation
• Polling
• Token Passing

36. Reservation access method
Reservation-station needs to make a reservation before sending data

37. Select and poll functions in polling access method
•
•
•
Polling – one device as primary station and the other device as secondary
station
Select – primary device wants to send data to secondary device, secondary
device gets ready to receive
Poll – primary device solicits (ask) transmissions from secondary devices

38. Logical ring and physical topology in token-passing
access method

39. • Token passing – stations in network organized in a logical ring –
predecessor and successor
Token – gives station right to access the channel; needs token
management
•
•
•
•
Physical ring – station sends the token to successor
Dual ring – uses second ring which operates in reverse direction
Bus ring (token bus) - stations are connected to single cable called bus,
but make logical ring
• Star ring - physical topology star, wiring inside hub makes the ring

40. IEEE 802
 Standards created for things like networking and it can be
compatible with one another.
 - Features standards for
 topology, and
 network cabling.

41. IEEE 802 Standard
LLC
Sub layer
Ethernet
IEEE 802.2
MAC
Sub layer
IEEE
802.3
(CSMA/CD)
IEEE
802.4
Token
bus
IEEE
802.5
Token
Ring
FDDI
Physic
al
Layer
Data Link
Layer
Physical
Layer
OSI Layers LAN Specification

42. IEEE 802 standard
 802.1: Management and Internetworking
 802.2: Logical Link Control (LLC)
 802.3: CSMA/CD (Ethernet)
 802.5: Token Ring
 802.11: Wireless LANs

43. IEEE 802.1 standard
• It handles the architecture, security, management and internetworking of
local area networks (LAN), metropolitan area networks (MAN) and wide
area networks (WAN) standardized by IEEE 802.
The following tasks:
• Provides services, including LAN/MAN management, media access
control (MAC) bridging, data encryption/encoding and network traffic
management
• IEEE 802.1 is comprised of four groups that focus on different standards
and policies in the following areas:




Internetworking
Audio/video (A/V) bridging
Data center bridging
Security
• The Internetworking group handles overall architecture, link aggregation,
protocol addressing, network path identification/calculation and other
technical practices and recommendations.

44. IEEE 802.3x Standard

45. Token passing network
•
•
Atoken always circulates around a ring net.
A user grabs a token to transmit data

46. Token Ring – IEEE 802.5
 A ring topology network developed in the late 1960s.
Supported mainly by IBM.
 Pushed into the background by Ethernet in the 1990s.
 a LAN protocol which resides at the data link layer (DLL)
of the OSI model.

47. Token Ring operation
 Whoever want to transmit the date can hold the token.
Token circulates in the ring.
 When a station needs to transmit data, it converts the
token into a data frame.
 When the sender receives its own data frame, it converts
the frame back into a token.
 If an error occurs and no token frame, or more than one,
is present, a special station (“Active Monitor”) detects
the problem and removes and/or reinserts tokens as
necessary.
 The Abort frame: used to abort transmission by the
sending station.

48. Token Ring operation
 If a station has a frame to transmit when it receives a
token, it sends the frame and then passes the token to the
next station; otherwise it simply passes the token to the
next station.
 Passing the token means receiving the token from the
preceding station and transmitting to the successor
station.
 The data flow is unidirectional in the direction of the
token passing.
 In order that tokens are not circulated infinitely, they are
removed from the network once their purpose is
completed.

49. Token Ring operation cont..
 Signal speed of this media is 1 Mbps or 4 Mbps.
 Differential 'Manchester' encoding scheme is used for
encoding the digital data.
 Logical Link Control (LLC): LLC is a sub layer of Data Link
Control that defines frame formats and protocols for the
transmission of connectionless or connection-oriented services.
 The information generated by the architectures referenced above
transmitted within the LLC Protocol Data Unit (PDD) frames.

50. Token bus – IEEE 802.4

51. Token bus – IEEE 802.4
• Token Bus (IEEE 802.4) is a standard for implementing token
ring over virtual ring in LANs.
The physical media has a bus or a tree topology and uses
coaxial cables.
A virtual ring is created with the nodes/stations and the token
is passed from one node to the next in a sequence along this
virtual ring.
Each node knows the address of its preceding station and its
succeeding station.
Astation can only transmit data when it has the token.
The working principle of token bus is similar to Token Ring.
•
•
•
•
•

52. Differences between Token Ring and Token
Bus
Token Ring Token Bus
The token is passed over the physical
ring formed by the stations and the
coaxial cable network.
The token is passed along the virtual ring
of stations connected to a LAN.
The stations are connected by ring
topology, or sometimes star topology.
The underlying topology that connects the
stations is either bus or tree topology.
It is defined by IEEE 802.5 standard. It is defined by IEEE 802.4 standard.
The maximum time for a token to reach
a station can be calculated here.
It is not feasible to calculate the time for
token transfer.

53. IEEE 802.3 Ethernet
 802.3 is the standard which Ethernet operates by its standard for
Multiple Access
CSMA/CD (Carrier Sense
Detection). This standard encompasses both
with Collision
the MAC and
Physical Layer standards.
 What, Ethernet will uses to control access the network medium
(network cable).
 If there is no data, any node may attempt to transmit,
 if the nodes detect a collision, both stop transmitting and wait a
random amount of time before retransmitting the data.
Four
•
step procedure
•
•
• If medium is idle, transmit
If medium is busy, listen until idle and then transmit
If collision is detected, cease transmitting
After a collision, wait a random amount of time before retransmitting

54. Scope of LAN protocols
•
•
•
Consider two stations that communicate via a shared medium LAN.
Higher layers (above LLC) provide end-to-end services between the stations
Below the LLC layer, the MAC provides the necessary logic for gaining
access to the network

55. IEEE 802.3 Ethernet cont..
 The original 802.3 standard is 10 Mbps (Megabits per second).
 802.3u defined the 100 Mbps (Fast Ethernet) standard,
 802.3z/802.3ab defined 1000 Mbps (Gigabit Ethernet), and
 802.3ae define 10 (Gigabit Ethernet.)
 Commonly, Ethernet networks transmit data in packets, or small
bits of information.
 A packet can be a minimum size of 72 bytes or a maximum of 1518 bytes.
 The most common topology for Ethernet is the star topology.

56. Ethernet Frame

57. ETHERNET
EVOLUTION
STANDARD
ETHERNET
10Mbps
FAST
ETHERNET
100Mbps
GIGABIT
ETHERNET
1Gbps
TEN GIGABIT
ETHERNET
10Gbps

58. Fast and Gigabit Ethernet
 Fast Ethernet (100Mbps) has technology very similar to
10Mbps Ethernet
 Uses different physical layer encoding (4B5B)
 Many NIC’s are 10/100 capable
 Can be used at either speed
 Gigabit Ethernet (1,000Mbps)
 Compatible with lower speeds
 Uses standard framing and CSMA/CD algorithm
 Distances are severely limited
 Typically used for backbones and inter-router connectivity
 Becoming cost competitive
 How much of this bandwidth is realizable?

59. Categories of Standard Ethernet

60. medium.
 The name 10BASE5 is derived from several characteristics of the physical
 The 10 refers to its transmission speed of 10 Mbit/s. The BASE is short for
baseband signaling as opposed to broadband, and the 5 stands for the
maximum segment length of 500 meters (1,600 ft.).
 It was the first Ethernet specification to use a bus topology with a
external transceiver connected via a tap to a thick coaxial cable.
10Base5:Thick Internet

61. 10base2:Thin Internet
Thesecond implementation is called
10Base2, thin Ethernet .
Thecable is thinner and more flexible.
Thetransceiver is apart of NIC, whichis installed inside the station.
The implementation is most cost effective than 10Base5as thin coaxial cable is
less expensive than thick coaxial cable and the tee connection are much cheaper
than taps.

62. 10Base-T:Twister Pair Ethernet
The third implementation is called 10Base- T or Twisted Pair Ethernet.
It uses star topology and the station are connected via two pairs of twisted
cable(one fro sending and one for receiving)between the station and the
hub.
The maximum length of the twisted cable here is defined as 100m,to
minimize the effect of attenuation in the twisted cable.

63. 10Base-F:Fiber Ethernet
Although there are several types of optical fiber 10Mbps Ethernet ,
the most common is called 10Base-F.
10Base-F uses a star topology to connect stations to a hub.
The stations are connected to a hub using two-optic cables.

64. It was designed to compete with LAN protocols such as FDDI or Fiber channel . IEEE created
Fast Ethernet under the name 802.3u.Fast Ethernet is backward-compatible with standard
Ethernet , but it can transmit data 10 times faster at rate of100Mbps.
Upgrade the data rate to100Mbps.
Make it compatible withstandard Ethernet.
Keepthe same 48 bit-address.
Keepthe same frame format.
Keepthe sameminimum and maximum framelengths.

65. COMMON FAST
ETHERNET
IMPLEMENTATION
100Base-TX
Two wires category
5UTP
100Base-FX
Two wires fiber Four wires category
3UTP
10Base-T4

66. •
•
•
•
•
Upgrade the data rate to 1Gbps.
Make it compatible with standard or fast Ethernet.
Use the same address ,frame format.
Keep the same minimum and maximum frame length.
To support auto negotiation as defined in Fast Ethernet.

67. GIGABITETHERNET
IMPLEMENTATION
1000Base-SX
Two wire
Shortwave fiber
1000Base-LX
Two-wire
Long –wave fiber
1000Base-CX
Two-wire
Copper(STP)
1000Base-T
Four-wire
UTP

68. Name Medium Specifieddistance
1000BASE-CX Shieldedbalancedcopper cable 25meters
1000BASE-KX Copperbackplane 1 meter
1000BASE-SX Multi-mode fiber
220 to 550 meters dependent on
fiberdiameter andbandwidth
1000BASE-LX Multi-mode fiber 550meters
1000BASE-LX Single-modefiber 5 km
1000BASE-LX10 Single-modefiber using1,310nm
wavelength
10 km
1000BASE-EX Single-modefiber at 1,310nm
wavelength
~40 km
1000BASE-ZX Single-modefiber at 1,550nm
wavelength
~70 km
1000BASE-BX10
Single-modefiber, over
single-strandfiber: 1,490nm
downstream1,310nm
upstream
10 km
1000BASE-T Twisted-pair cabling(Cat-5,Cat-5e,
Cat-6, Cat-7)
100meters
1000BASE-TX Twisted-pair cabling(Cat-6,Cat-7) 100meters
SUMMARY FOR GIGABIT ETHERNET IMPLEMENTATION

69. IEEE 802.11 Architecture

70. IEEE 802.11 Wireless Network Standards
 802.11 is the collection of standards setup for wireless
networking.
 You are probably familiar with the three popular standards:
802.11a, 802.11b, 802.11g and latest one is 802.11n. Each
standard uses a set of frequency to connect to the network
and has a defined upper limit for data transfer speeds.
 802.11a was one of the first wireless standards. It operates
in the 5Ghz radio band and can achieve a maximum of
54Mbps. Wasn't as popular as the 802.11b standard due to
higher prices and lower range.
 802.11b operates in the 2.4Ghz band and supports up to 11
Mbps. Range of up to several hundred feet in theory.

71. IEEE 802.11 Wireless Network Standards cont..
 802.11g is a standard in the 2.4Ghz band operating at
54Mbps. Since it operates in the same band as 802.11b,
802.11g is compatible with 802.11b equipment. 802.11a is
not directly compatible with 802.11b or 802.11g, since it
operates in a different band.
 Wireless LANs primarily use CSMA/CA - Carrier Sense
Multiple Access/Collision Avoidance. It has a "listen before
talk" method of minimizing collisions on the wireless
network. This results in less need for retransmitting data.

72. IEEE 802.11 Wireless Network Standards cont..

73. IEEE 802.6 - MAN
 The IEEE 802.6 standard describes a MAN (Metropolitan
Area Network) standard called DQDB (Distributed Queue
Dual Bus).
 The network is defined as a high-speed shared medium
access protocol for use over a dual, counter-flowing,
unidirectional bus networks.
 The use of paired bus provides a failure tolerant
configuration.
 DQDB is able to carry data, voice, and video transmissions,
with bandwidth being allocated using time slots on the bus.

74. IEEE 802.6 - DQDB Access Method
 IEEE 802.6 is a standard governed by the ANSI for Metropolitan
Area Networks (MAN). It used Fiber distributed data
interface (FDDI) network structure.
 failed due to its expensive implementation and lack of compatibility with
current LAN standards.
 This form supports 150 Mbit/s transfer rates.
 This standard has also failed, mostly for the same reasons that the
FDDI standard failed.
 MANs are traditionally designed using Synchronous Optical
Network (SONET), Synchronous Digital Hierarchy (SDH)
or Asynchronous Transfer Mode (ATM). Recent designs use
native Ethernetor MPLS.

75.  Thank you

76. IEEE 802 LAN/MAN
IEEE 802.1
Standards for LAN/MAN bridging and management and remote media
accesscontrol (MAC) bridging.
IEEE 802.2
Standards for Logical Link Control (LLC)standards for connectivity.
IEEE 802.3
Ethernet Standards for Carrier SenseMultiple Access with Collision
Detection (CSMA/CD).
IEEE 802.4 Standards for token passing bus access.
IEEE Standards for Logical Link Control (LLC)standards for connectivity.
802.24
IEEE 802.5
Standards for token ring accessand for communications between LANsand
MANs
IEEE 802.6 Standards for information exchange between systems.
IEEE 802.7 Standards for broadband LAN cabling.
IEEE 802.8 Fiber optic connection.
Notable IEEE Standards formats

77. IEEE 802.9 Standards forintegratedservices,likevoiceand data.
IEEE 802.10 Standardsfor LAN/MAN securityimplementations.
IEEE 802.11 Wireless Networking – "WiFi".
IEEE 802.12 Standardsfor demandpriority accessmethod.
IEEE 802.14 Standardsfor cabletelevision broadbandcommunications.
IEEE 802.15.1 Bluetooth
IEEE 802.15.4 Wireless Sensor/Control Networks –"ZigBee"
IEEE 802.15.6
Wireless BodyArea Network[3] (BAN) – (e.g. Bluetoothlow
energy)
IEEE 802.16 Wireless Networking – "WiMAX"

81. Hubs
 The active central element of the star
layout
 When a single station transmits, the hub
repeats the signal on the outgoing line to
each station
 Hubs can be cascaded in a hierarchical
configuration
 Ethernet hubs are physically a star but
logically a bus

82. Bridges
 Allow connections between LANs and to
WANs
 Used between similar networks
Read all frames from each network
Accept frames from sender on one network that
are addressed to a receiver on the other network
Retransmit frames from sender using MAC
protocol for receiver

83. Routers
 Similar to bridges but connect dissimilar
networks
 Convert format of the message to
correspond to the protocol of the other
network
 Network traffic is specifically addressed to
the router

84. 802.2 Logical Link Control
 Conversely, the LLC uses the services of the media access
control (MAC), which is dependent on the specific
transmission medium
 (Ethernet, Token Ring, FDDI, 802.11, etc.).
Ethernet,
Token Ring,
FDDI,
802.11,