IEEE 802.3
Brief background of IEEE802 standards
for LAN and MAN
While connecting computers through networks we need to
have set of rules/standards for the data to travel from
one computer to other computer. The right example for this
can be road traffic rules. It's self understood, why we
need traffic rules while driving, in same sense for the
data packets to travel from one computer terminal to other
terminal they should also follow set of rules and regulations.
One such set of rules for the networking traffic to follow
is IEEE802 standards. Its developed by IEEE (Institute of
Electrical and Electronics Engineers, Inc.) The IEEE is
the world's leading professional association for the advancement
of technology. It's a non- profit organization offering
its members immense benefits.
The standards such as IEEE 802 helps industry provide advantages
such as, interoperability, low product cost, and easy to
manage standards.
IEEE standards deal with only Local Area Networks (LAN)
and Metropolitan Area Networks (MAN). See in the figure
below, to know where exactly the IEEE802 standards are used
in a OSI layer.
The IEEE 802 standards are further divided into many parts.
They are,
IEEE 802.1 Bridging (networking) and Network Management
IEEE 802.2 Logical link control (upper part of data link
layer)
IEEE 802.3 Ethernet (CSMA/CD)
IEEE 802.4 Token bus (disbanded)
IEEE 802.5 Defines the MAC layer for a Token Ring (inactive)
IEEE 802.6 Metropolitan Area Networks (disbanded)
IEEE 802.7 Broadband LAN using Coaxial Cable (disbanded)
IEEE 802.8 Fiber Optic TAG (disbanded)
IEEE 802.9 Integrated Services LAN (disbanded)
IEEE 802.10 Interoperable LAN Security (disbanded)
IEEE 802.11 Wireless LAN & Mesh (Wi-Fi certification)
IEEE 802.12 demand priority (disbanded)
IEEE 802.13 Not Used
IEEE 802.14 Cable modems (disbanded)
IEEE 802.15 Wireless PAN
IEEE 802.15.1 (Bluetooth certification)
IEEE 802.15.4 (ZigBee certification)
IEEE 802.16 Broadband Wireless Access (WiMAX certification)
IEEE 802.16e (Mobile) Broadband Wireless Access
IEEE 802.17 Resilient packet ring
IEEE 802.18 Radio Regulatory TAG
IEEE 802.19 Coexistence TAG
IEEE 802.20 Mobile Broadband Wireless Access
IEEE 802.21 Media Independent Handoff
IEEE 802.22 Wireless Regional Area Network
Here we discuss most popular and key parts of above list
IEEE 802.3 Ethernet (CSMA/CD)
A method called Carrier Sense Multiple Access with Collision
Detection (CSMA/CD) was used to send data over shared single
co-axial cable connected to all computers on a network.
In this method, the computer terminals (also called as stations)
transmits the data over cable whenever the cable is idle,
If more than one station transmit at same time and if they
collide, the transmission will be stopped by such stations.
They will wait for some random time and restart transmission.
The concept of sharing single cable or wire between multiple
stations was used for first time in Hawaiian Islands. It
was called ALOHA systems; built to allow radio communication
between machines located at different places in Hawaiian
Islands. Later Xerox PARC built a 2.94 mbps CSMA/CD system
to connect multiple personal computers on a single cable.
It was named as Ethernet.
Ethernet or IEEE802.3 standards only define MAC (Data link)
and Physical layer of standard OSI model.
Don't confuse TCP/IP with Ethernet. TCP/IP defines Transport
and network layers.
Wiring and cabling standards of 802.3
There are four cabling standards as per 802.3, each one
has evolved over the time for their special advantages.
The four types of cables are,
1. 10Base5
2. 10Base2
3. 10Base-T
4. 10Base-F
The table below compares all four types of cables
| Technical Name |
Cable/Wire type |
Max. Segment/wire Length |
Maximum number of Nodes/Segment |
Advantages |
| 10Base5 |
Thick coaxial |
500 meters |
100 |
Long cable length |
| 10Base2 |
RG58 (thin)
coaxial |
185 meters |
30 |
Low cost |
| 10BaseT |
Twisted pair (like telephone
wire) |
100 meters |
1024 |
Easy to maintain |
| 10BaseF |
Fiber-optic |
2,000 meters |
1024 |
No noise interference |
The 10 in the technical name refer to data speed of 10Mbits/sec.
"Link Integrity" and "Auto-partition"
are part of the 10BaseT specification. This means that all
network equipment claiming compliance with 10BaseT must
support Link Integrity and Auto-partitioning.
10Base5
10 Base5 is also called as ThickNet or thick Ethernet. It
uses RG-8 thick coaxial trunk cable, which looks like orange
colored garden hose. The cable is tapered with taps called
vampire taps in which a pin is carefully forced halfway
into the cable's core. The connection can be made to the
desired computer network interface card (NIC) from these
vampire taps. ThickNet can travel 500 meters per segment,
and it can have a maximum of 100 taps per segment. Each
tap requires a minimum distance of 2.5 meters before the
next tap and has a maximum drop distance of 50 meters. The
cable must be terminated with a 50-ohm terminator resistor.
Due to its complex and slow nature 10Base5 is no more preferred.
The severe drawback is entire line will fail for any single
failure on the trunk. This cable can be termed as obsolete/outdated
technology.
The one plus point of ThickNet is that, once it's up and
running, it will continue to do so until you tell it otherwise.
Although it is slow and unwieldy, 10Base5 technology is
very reliable.
Here is the figure showing how the cables are connected
to Network Interface Cards inside the computer using 10base5.
10Base2
10Base2 is not very different from 10 Base5. The most notable
physical difference between 10Base2 and 10Base5 is the size
of the co-axial cable. 10Base2 is thinner than the 10Base5
and so is called as ThinNet or thin Ethernet. Another difference
is that 10Base2 is set up in a daisy chain. Daisy chain
is a wiring scheme in which, for example, device A is wired
to device B, device B is wired to device C, device C is
wired to device D, et cetera.
10Base2 uses BNC connectors attached to a thin coaxial
cable. The maximum segment length of 10Base2 is 185 meters,
and the maximum number of devices per segment is 30.
10Base is also outdated/obsolete technology. In rare cases
it could be deployed as a backbone for a network.
Here is the figure showing how the cables are connected
to Network Interface Cards inside the computer using 10base5.
10Base-T
10Base-T is the most popular cabling method. Its also called
Standard Ethernet, or twisted pair, 10Base-T works on a
star topology connecting all computers to a hub. It is best
used with Category 5 cable (so it can be upgraded to Fast
Ethernet) and can have a maximum of three hubs daisy-chained
together.
Since it is simple and cheap to implement it is most opted
one. The specifications of Standard Ethernet include the
following:
It uses RJ45 connectors on unshielded twisted-pair (UTP)
cable.
The maximum cable length is 100 meters (before a repeater
is needed).
The maximum number of devices per segment is 1,024 (although
performance will become quite poor before that number is
ever reached).
The 10Base-T standard is best employed within a LAN where
cost is a factor-and speed and distance are not.
Link Integrity is concerned with the condition of the cable
between the network adapter and the hub. If the cable is
broken, the hub will automatically disconnect that port.
Auto partitioning occurs when an Ethernet hub port experiences
more than 31 collisions in a row. When this happens, the
hub will turn off that port, essentially isolating the problem.
10Base-F
In 10BaseF the twisted copper wires are replaced by a optical
fiber. 10Base-F uses a higher quality cabling technology,
multimode (or single-mode) fiber-optic cable, to transport
data. The particular technology has two subdivisions that
must be addressed: the newer 10Base-FL and 10BaseFOIRL.
Because it is older, the 10BaseFOIRL (Fiber-optic Inter-repeater
Link) technology doesn't have quite the capabilities of
the newer 10Base-FL. With 10BaseFOIRL, you have the following
specs:
It's based on IEEE 802.3.
The segment length is 1,000 meters.
There are three sizes of duplex multimode fiber: 50-, 62.5-,
or 100-micron. Of these three, 62.5-micron is the most common.
ST or SMA 905 connectors are used by 10BaseFOIRL.
It must be used in a star configuration.
AUI connectors have to be connected to fiber transceivers.
The much-improved 10Base-FL technology offers a different
set of specs:
It's based on the 10Base-F IEEE 802.3 spec.
It's able to interoperate with FOIRL and is designed to
replace the FOIRL specification.
The segment length is 2,000 meters (if exclusively using
10Base-FL).
The maximum number of devices per segment is two; one is
the station and the other is the hub.
The maximum number of repeaters that may be used between
devices is two.
NICs with standard AUI ports must use a fiber-optic transceiver.
The benefits of optical fiber are,
No radio or magnetic interference.
Transmissions are safe from electronic bugging,
Cable is extremely lightweight,
10Base-FL fiber-optic technologies are best implemented
in long runs where reliability and security are critical.
Different types of cable topologies:
The four types of cable topologies are, linear, spine,
tree, segmented.
Linear: The linear topology is like a single cable
running in all portions of building. The stations are connected
to the cable through tapping.
Spine: It looks like our back one spinal cord, where
multiple numbers of horizontal cables are connected to a
vertical line through special amplifiers or repeaters.
Tree: This is most general topology because a network
with two paths between some pairs of stations would suffer
from interference between the signals.
Segmented: Since each version of 802.3 has maximum
cable length per segment, to allow larger networks, repeaters
can connect multiple cables.
Manchester Encoding: The normal binary logics of
one and zero are no more used to send data from one station
to other station. The reason of not using plain binary signal
is they cause ambiguities resulting in false interpretation
of sent data. The major culprit is zero, where even no data
is sent the receiver can assume it as zero.
So to clear out the ambiguity or to ensure proper interpretation
of data, a coding technique called Manchester coding is
employed in IEEE802.3 standards.
There are two of Manchester coding, they are simple Manchester
coding and differential Manchester coding.
Summary
Each bit is transmitted in a fixed time (the "period").
A 0 is expressed by a low-to-high transition, a 1 by high-to-low
transition (according to G.E. Thomas' convention -- in the
IEEE 802.3 convention, the reverse is true).
The transitions which signify 0 or 1 occur at the midpoint
of a period.
Transitions at the start of a period are overhead and don't
signify data.
Manchester code always has a transition at the middle of
each bit period and may (depending on the information to
be transmitted) have a transition at the start of the period
also. The direction of the mid-bit transition indicates
the data. Transitions at the period boundaries do not carry
information. They exist only to place the signal in the
correct state to allow the mid-bit transition. Although
this allows the signal to be self-clocking, it doubles the
bandwidth requirement compared to NRZ coding schemes (or
see also NRZI).
In the Thomas convention, the result is that the first
half of a bit period matches the information bit and the
second half is its complement.
If a Manchester encoded signal is inverted in communication,
it is transformed from one convention to the other. This
ambiguity can be overcome by using differential Manchester
encoding.
Differential Manchester Encoding Shown in above figure
is a variation of basic Manchester encoding.
A '1' bit is indicated by making the first half of the signal
equal to the last half of the previous bit's signal i.e.
no transition at the start of the bit-time. A '0' bit is
indicated by making the first half of the signal opposite
to the last half of the previous bit's signal i.e. a zero
bit is indicated by a transition at the beginning of the
bit-time. In the middle of the bit-time there is always
a transition, whether from high to low, or low to high.
A reversed scheme is possible, and no advantage is given
by using either scheme.
All 802.3 baseband systems use Manchester encoding due to
its simplicity. The high signal is +0.85 Volts and low signal
is -0.85 V giving a DC value of 0 volts.
The 802.3 MAC sub layer protocol
The IEEE802.3 based Ethernet frame consists of preamble
of 56 bit-size, start of the frame delimiter of 8bit size,
destination address of 48 bit-size, sources address of
48 bit-size, type field to identify higher layer protocol
of 16 bit-size, data field of variable bit-size, and frame
check sequence field of 32 bit size.
The figure below explains better.
802.3 Ethernet MAC sub layer Protocol Minimum Frame Size
Longest segment = 500 meters
At most 4 repeaters
Maximum LAN length is 2500 m
Maximum round-trip time is 50µsec
10 Mbps implies 100 nsec / bit, 500 bits takes 50 µsec
802.3 uses 512 bits (64 bytes) as minimum frame size
The binary Exponential Backoff Algorithm
Exponential backoff is an algorithm that uses feedback
to multiplicatively decrease the rate of some process,
in order to gradually find an acceptable rate. It is often
used in network congestion avoidance to help determine
the correct sending rate. For example, a sender might
send a message, set a timer to wait 0.25 seconds for an
acknowledgment, and if none arrives, retransmit the message
and wait 0.5 seconds for an acknowledgment. It will continue
to retry until it receives an acknowledgement and will
wait, 1s, 2s, 4s, 8s, etc. each time before retrying.
Time slots are defined to be 51.2µsec during contention
period. After i collisions, backoff random number of intervals
between 0 and 2i -1. i is bounded at 10. After 16 attempts,
the sender quits
Basic Intuition used in the algorithm is,
----- Assume
that number of contending stations is small until proven
otherwise
----- If i were
fixed at 1023, lots of unnecessary waiting
-----If i were
fixed at 1, potential for unbounded waiting
The performance of Ethernet (802.3)
Here we evaluate the perfromance of 802.3 under the conditions
of full load and constant load.
Metcalfe and Boggs - ignore binary exponential backoff
and assume constant probability, p, of retransmission
in each slot probability that one station acquires a slot,
A, is
where
k = number of stations ready to transmit
p = probability that a station will retransmit
A is maximized when p is 1/k
When p is 1/k, A --> 1/e as k
--> infinity
is
the probability that the contention window is j slots
Mean number of slots per contention is:
Each slot is bounded by 2t, so the mean window size is
bounded by
Assuming optimal p (p = 1/k), A= 1/e and
Let P be the mean transmission time / frame
Let
F = frame length
B = bandwidth
L = cable length
c = speed of light
P = F/B
and
As BL increases, efficiency decreases
Here is the chart showing channel effeciency V/S Number
of stations trying to send
.
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