Download - W3Professors

Contributed by: Anubhooti Malhotra
Himani Sharma
BCA-2(A)
 The Institution of Electrical and Electronics Engineers (IEEE) has developed several
standards for LANs. These standards are collectively known as IEEE 802 or Project
802.
 The IEEE project 802 divides data link layer into two sub layers:
Logical link control (LLC) and Medium access control (MAC).
 LLC provides one single data link control protocol for all IEEE LANs. This single LLC
protocol can provide interconnectivity between different LANs as it makes the MAC
sub layer transparent. On the other hand MAC provides different protocols for the
different LANs.
802.1 Network management and Internetworking .
802.2 Logical Link Control (LLC).
802.3 Ethernet or CSMA/CD.
802.4 Token Bus.
802.5 Token Ring.
802.6 Metropolitan Area Networks (MAN) or
Distributed Queue Dual Bus (DQDB).
802.7 Band pass Technical Advisory Group.
802.8 Fibre Optic Technical Advisory Group.
802.9 Integrated Data and Voice Network.
802.10 Security Working Group.
802.11 Wireless LAN.
802.12 Demand priority
802.13
802.14 Cable Modems
802.15 Wireless PAN
802.15.1 Bluetooth certification
802.15.4 ZigBee certification
802.16 Broadband Wireless Access(WiMax
certification)
802.16e (Mobile) Broadband Wireless Access
802.16.1 Local Multipoint Distribution Service
802.17 Resilient Package Ring
802.18 Radio Regulatory TAG
802.19 Coexistence TAG
802.20 Mobile Broadband Wireless Access
802.21Media Independent Handoff
802.22 Wireless Regional Area Network
802.23 Wireless ISDN System
IEEE 802.3 supports a LAN standard originally developed by Xerox
and later extended by a joint venture between Digital Equipment
Corporation, Intel Corporation, and Xerox. This was called Ethernet.
IEEE 802.3 defines two categories: baseband and broadband. The
word base specifies a digital signal.
The word broad specifies an analog signal. IEEE divides the
baseband category into five different standards: 10Base5, 10Base2,
10Base-T, 1Base5, and 100Base-T. The first num (10, 1, 100)
indicates the data rate in Mbps.
The last number or letter (5, 2, 1, or T) indicates the maximum
cable length or the type of cable.
Carrier Sense Multiple Access with Collision Detection
Whenever multiple users have unregulated access to a
single line, there is a danger of signals overlapping and
destroying each other.
 Such overlaps, which turns the signals into unusable
noise, are called collisions.
 As traffic increases on a multiple access link, so do
collisions. A LAN therefore needs a mechanism to
coordinate traffic, minimize the number of collisions that
occur, and maximize the number of frames that are
delivered successfully.
In CSMA system, any workstation wishing to transmit must first
listen for existing traffic on the line.
A device listens by checking for a voltage.
 If no voltage is detected, the line is considered idle and the
transmission is initiated.
CSMA cuts down on the number of collisions but does not
eliminate them.
Collisions can still occur.
 If another station has transmitted too recently for its signal to
have reached the listening station, the listener assumes the line
is idle and introduces its own signal onto the line.
The final step is the addition of collision detection (CD).
 In CSMA/CD the station wishing to transmit first listens to
make certain the link is free, then transmits its data, then
listens again.
 During the data transmission, the station checks the line
for the extremely high voltages that indicate a collision.
If a collision is detected, the station quits the current
transmission and waits a predetermined amount of time for
the line to clear, then sends its data again.
Each station on an Ethernet
network
(such
as
a
PC,
workstation, or printer) has its own
network interface card (NIC). The
NIC usually fits inside the station
and provides the station with a six
byte physical address. The number
on the NIC is unique.
The baseband system use
Manchester digital encoding.
There is one broadband
system, 10Broad36.
Ethernet LANs can support data
rates between 1 and 100 Mbps.
Preamble: The first field of the 802.3 frame, the preamble,
contains seven bytes of alternating 0s and 1s that alert the
receiving system to the coming frame and enable it to synchronize
its input timing.
Start frame delimiter (SFD): The second field of the 802.3 frame
signals the beginning of the frame. The SFD tells the receiver
that everything that follows is data, starting with the addresses.
Destination Address (DA): The destination address (DA) field is
allotted six bytes and contains the physical address of the
packet’s next destination.
Source Address (SA): The source address (SA) field is
also allotted six bytes and contains the physical address of
the last device to forward the packet.
Length/type of PDU: These next two bytes indicate the
number of bytes in the coming PDU.
802.2 frame (PDU): This field of the 802.3 frame contains
the entire 802.2 frame as modular, removable unit.
CRC: The last field in the 802.3 frame contains the error
detection information in this case a CRC-32.
The first of the physical standards defined in the
IEEE 802.3 model is called 10Base5, thick Ethernet,
or Thicknet.
 The nickname derives from the size of the cable,
which is roughly the size of garden hose and too
stiff to bend with your hands.
10Base5 is a bus topology LAN that uses
baseband signalling and has maximum segment
length of 500 meters.
The second Ethernet implementation defined by the IEEE
802 series is called 10Base2 or thin Ethernet.
 Thin Ethernet provides an inexpensive alternative to
10Base5 Ethernet, with the same data rate.
The advantages of thin Ethernet are reduced cost and ease
of installation. It also uses a bus topology.
o The most popular standard defined in the
IEEE 802.3 series is 10Base-T, a star
topology LAN using unshielded twisted pair
(UTP) cable instead of coaxial cable.
o It supports a data rate of 10 Mbps and has
a maximum length of 100 meters.
 Star LAN is an AT & T product used infrequently today
because of its slow speed. At only 1 Mbps, it is 10 times
slower than the three standards discussed above.
 What is interesting about Star LAN is its range, which can
be increased by a mechanism called Daisy Chaining. Like
10Base-T, star LAN uses twisted pair cable to connect
stations to a central intelligent hub.
 Local area networks have a direct application in
a factory automation and process control, where
the nodes are computers controlling the
manufacturing process.
 In this type of application, real time processing
with minimum delay is needed. Processing must
occur at the speed as the objects moving along the
assembly line.
Furthur…..
 Ethernet Is not suitable protocol for this purpose because
the number of collisions is not predictable and the delay in
sending data from the control centre to the computers along
the assembly line is not a fixed value.
 Token Bus combines the features of Ethernet and Token
Ring.
It combines the physical configuration of Ethernet and
collision free feature of Token Ring.
Token Bus is a physical bus that operates as a logical ring
using tokens.
Stations are logically organized into a ring.
A token is passed among stations. If a station wants to send data, it
must wait and capture the token. However, like Ethernet, stations
communicate via a common bus.
Token Bus is limited to factory automation and process control and
has no commercial application in data communication. Also, the details
of the operation are very involved.
When a station passes the token, it sends a token to its logical
neighbour irrespective of where that station is physically located on the
cable.
MAC SUB LAYER
When the ring is initialized, stations are inserted into it in order
of station address, from highest to lowest. Token passing is
done from high to low address.
Whenever a station acquires the token, it can transmit frames
for a specific amount of time.
If a station has no data, it passes the token immediately upon
receiving it.
The Token Bus defines four priority classes, 0, 2, 4, and 6 for
traffic, with 0 the lowest and 6 the highest.
Each station is internally divided into four substations, one at
each priority level i.e. 0, 2, 4 and 6.
ring.
As input comes into the MAC sub layer from above, the
data are checked for priority and routed to one of the four
substations.
Thus each station maintains its own queue of frames to
be transmitted.
When a token comes into the station over the cable, it is
passed internally to the priority 6 substation, which can
begin transmitting its frames, if it has any.
When it is done or when its time expires, the token is
passed to the priority 4 substation, which can then
transmit frames until its timer expires. After this the token
is then passed internally to priority 2 substation.
This process continues until either the priority 0
substation has sent all its frames or its time expires.
After this the token is passed to the next station in the
ring.
FRAME FORMAT
The various fields present in the frame format are:
Preamble: This field is 1 byte long. It is used for
synchronization.
Start Delimiter: This one byte field marks the beginning of
frame.
Frame Control: This one byte field specifies the type of frame. It
distinguishes data frame from control frames. For data frames it
carries frame’s priority.
Destination Address: It specifies 2 to 6 bytes destination
address.
Source Address: It specifies 2 to 6 bytes source address.
Data: This field may be up to 8182 bytes long when 2 bytes
address are used & up to 8174 bytes long when 6 bytes address
is used.
Checksum: This 4 byte field detects transmission errors.
End Delimiter: This one byte field marks the end of frame.
Token Ring resolves this uncertainty by requiring
that stations take turns sending data. Each station
may transmit only during its turn and may send only
one frame during each turn. The mechanism that
coordinates this rotation is called Token passing. A
token is a simple placeholder frame that is passed
from station to station around the ring.
Token Ring resolves this uncertainty by
requiring that stations take turns sending
data. Each station may transmit only
during its turn and may send only one
frame during each turn. The mechanism
that coordinates this rotation is called
Token passing. A token is a simple
placeholder frame that is passed from
station to station around the ring.
ADDRESSING
Token ring uses a 6-byte address which is
imprinted on the NIC card similar to Ethernet
addresses.
ELECTRICAL SPECIFICATION
Signaling: Token ring uses differential
Manchestar encoding.
Data rate: token ring supports data rates of
upto 16 Mbps.
References
Castelli, Matthew (2002). Network Consultants Handbook.
Cisco Press. ISBN 1-58705-039-0.
Gallo, Michael; Hancock, William M. (2001). Networking
Explained. Digital Press. ISBN 1-55558-252-4.
External links
IEEE 802.5 Web Site
Get the IEEE 802.5 standard
Troubleshooting Cisco Router Token Ring Interfaces
Futureobservatory.org discussion of IBM's failure in token
ring technology
802 Committee website
IEEE 802 Standards