Layer 2 LAN Technologies
& Media Access Methods
(II)
Minimum Ethernet Frame Size
•To ensure that no node may completely receive a frame before
the transmitting node has finished sending it, Ethernet defines a
minimum frame size (i.e. no frame may have less than 46 bytes
of payload).
• The minimum frame size is related to the distance which the
network spans, the type of media being used and the number of
repeaters which the signal may have to pass through to reach
the furthest part of the LAN.
• Together these define a value known as the Ethernet Slot
Time, corresponding to 512 bit times at 10 Mbps.
Minimum Ethernet Frame Size
•The longest time between starting to transmit a frame and
receiving the first bit of a jam sequence is twice the
propagation delay from one end of the cable to the other.
•This means that a frame must have enough bits to last twice the
propagation delay.
•The 802.3 CSMA/CD Bus LAN transmits data at the standard
rate of r = 10Mbps.
•The speed of signal propagation is about v = 2108m/s.
A
(a)
A
Packet starts at
time 0
B
A
Packet at time tp-
B
(b)
Collision occurs
at time tp
(c)
B
A
Jam sequence gets
back to A at 2tp
(d)

Jam sequence

Jam sequence
B
IEEE 802.3: Minimum Frame Length
In order to calculate the minimum frame length, we must first
work out the propagation delay from one end of the cable to the
other.
IEEE 802.3: Minimum Frame Length
Example #1: Cable = 400m, transm. speed = 10 Mbit/sec,
propagation speed = 2*108 m/sec
Propagation delay time:
d
400
6
t prop 


2

10
 2  sec
8
V
2  10
The round-trip propagation delay is, of course, twice this. Thus the
round trip delay is 2  t prop  4 sec
With a data rate of
R  10Mbps each bit has
1
1
tb 

 0.1 sec duration
R
10,000,000
IEEE 802.3: Minimum Frame Length
Example #1 – cont.
The number of bits we can fit into a round-trip propagation
delay is
nb  2 
tp
tb
4

 40bits
0.1
The minimum frame length is thus 40 bits (5 bytes).
A margin of error is usually added to this (often to make it a
power of 2) so we might use 64 bits (8 bytes).
IEEE 802.3: Minimum Frame Length
Example # 2
Two nodes are communicating using CSMA/CD protocol.
Speed transmission is 100 Mbits/sec and frame size is 1500 bytes. The propagation
speed is 3*10**8 m/sec.
Calculate the distance between the nodes such that the
time to transmit the frame = time to recognize that the collision have occurred.
t frame 
Tround
L
1500  8
4


1
.
2

10
R
100  10 6
_ trip
t prop 
t prop
d

V

 t frame  2  t prop
t frame
2
1.2  104

 6  105
2
 

d  t prop V  6 105  3108  18103  18 km
IEEE 802.3: Minimum Frame Length
• The standard frame length is at least 512 bits (64 bytes)
long, which is much longer than our minimum requirement of
64 bits (8 bytes).
– We only have to start worrying when the LAN reaches lengths
of more than 2.5km.
• 802.3 CSMA/CD bus LANs longer than 500m are usually
composed of multiple segments joined by in-line passive
repeaters, which output on one cable the signals received on
another cable.
– When we work out the minimum frame length for these longer
LANs, we also have to take the delays caused by the passive
repeaters (about 2.5sec each) into account as well.
Shortest Ethernet Frame
Why specify a shortest frame of 64byte?
64 bytes sent at 10Mbps  51.2sec
500m/segment, 4 repeaters between nodes 2500m 25 sec
propagation delay
The frame should be longer enough for sender to detect the
collision(2x25 or about 50 sec )
Node A
R1
R2
R3
R4
500m 25 sec propagation delay
Node B
IEEE 802.3: Non-Deterministic
• The 802.3 CSMA/CD bus LAN is said to be a nondeterministic network. This means that no host is
guaranteed to be able to send its frame within a reasonable
time (just a good probability of doing so).
– When the network is busy, the number of collisions rises
dramatically and it may become very difficult for any hosts to
transmit their frames.
• A real-time computing application (such as an assembly
line) will demand that data is transmitted within a specified
time period.
– Since the 802.3 bus LAN cannot guarantee this, its use for
real-time applications may not only be undesirable but
potentially dangerous in some situations.
Ethernet evolution through four
generations
11
100 Mbps IEEE Standards
• The most widely accepted Ethernet standard today is
100BaseT, which is also called fast Ethernet
– The current IEEE standard for 100BaseT is 802.3u
• Subcategories:
– 100BaseTX: Two-pair Category 5 or higher UTP
– 100BaseT4: Four-pair Category 3 or higher UTP
– 100BaseFX: Two-strand fiber-optic cable
– Because of its widespread use, the cable and equipment
in fast Ethernet are inexpensive
– Architecture of choice for all but heavily used servers
and multimedia applications
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100BaseTX
• 100BaseTX is the standard that’s usually in mind when
discussing 100 Mbps Ethernet
• Requires two of the four pairs bundled in a Category 5
twisted-pair cable
• Although three cable types are available for 100BaseT,
100BaseTX is the most widely accepted
– Generally called fast Ethernet
13
100BaseT4
• 100BaseT4 Ethernet uses all four pairs of wires
bundled in a UTP cable
• Advantage: capability to run over Category 3 cable
– One of the biggest expenses of building a network is
cable installation, so many organizations with
Category 3 cabling chose to get the higher speed
with the existing cable plant by using 100BaseT4
instead of 100BaseTX
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100BaseFX
• 100BaseFX uses two strands of fiber-optic cable
– Advantages:
• Impervious to electrical noise and electronic
eavesdropping
• Can span much greater distances between devices
– Disadvantage: far more expensive than twisted-pair
– Rarely used as a complete 100BaseTX replacement
• Used as backbone cabling between hubs or switches and
to connect wiring closets between floors or buildings
• Connect client or server computers to the network when
immunity to noise and eavesdropping is required
15
100BaseT Design Considerations
16
100BaseT Design Considerations
17
10 Mbps IEEE Standards
• Four major implementations of 10 Mbps Ethernet
–
–
–
–
10Base5: Ethernet using thicknet coaxial cable
10Base2: Ethernet using thinnet coaxial cable
10BaseT: Ethernet over UTP cable
10BaseF: Ethernet over fiber-optic cable
• Of these 10 Mbps standards, only 10BaseT and
10BaseF are seen today
• 10Base2 and 10Base5 are essentially obsolete
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10BaseT
19
10BaseF
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Gigabit Ethernet: IEEE 802.3ab and
802.3z Standards
• Gigabit Ethernet implementations
– 802.3z-1998 covers 1000BaseX specifications,
including the L (long wavelength laser/fiber-optic), S
(short wavelength laser/fiber-optic), and C (copper
jumper cables)
– 802.3ab-1999 covers 1000BaseT specifications, which
require four pairs of 100 ohm Category 5 or higher
cable
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1000BaseT
22
1000BaseLX
23
1000BaseSX
24
1000BaseCX
25
10 Gigabit Ethernet: 10 Gbps IEEE
802.3ae Standard
• Defined to run only on fiber-optic cabling, both SMF
and MMF, on a maximum distance of 40 km
– Provides bandwidth that can transform how WAN
speeds are thought of
• Runs in full-duplex mode only
– CSMA/CD is not necessary
• Primary use: as network backbone
– It also has its place in storage area networks (SANs)
– Will be the interface for enterprise-level servers
26
10 Gigabit Ethernet: 10 Gbps IEEE
802.3ae Standard
• Standards
– 10GBASE-SR: Runs over short lengths (between 26
and 82 meters) over MMF
• For high-speed servers, SANs, etc.
– 10GBASE-LR: Runs up to 10 km on SMF
• For campus backbones and MANs
– 10GBASE-ER: Runs up to 40 km over SMF
• Primary applications are for MANs
– 10GBASE-SW: Uses MMF for distances up to 300 m
– 10GBASE-LW: Uses SMF for distances up to 10 km
– 10GBASE-EW: Uses SMF for distances up to 40 km
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Wireless Ethernet: IEEE 802.11b, a,
and g
• AP serves as the center of a star topology network
• Stations can’t send and receive at the same time
– CSMA/CA is used instead of CSMA/CD
• 802.11b/a/g use handshaking before transmission
– Station sends AP an RTS and it responds with CTS
• Standards define a maximum transmission rate, but
speeds might be dropped to increase reliability
• No fixed segment length
– Maximum of 300 feet without obstructions
• Can be extended with large, high-quality antennas
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Broadband Technologies
• Baseband systems use a digital encoding scheme
at a single fixed frequency
• Broadband systems use analog techniques to
encode information across a continuous range of
values
– Signals move across the medium in the form of
continuous electromagnetic or optical waves
– Data flows one way only, so two channels are
necessary for computers to send and receive data
– E.g., cable TV
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Cable Modem Technology
30
Digital Subscriber Line (DSL)
• Competes with cable modem for Internet access
– Broadband technology that uses existing phone lines
to carry voice and data simultaneously
– Most prominent variation for home Internet access is
Asymmetric DSL (ADSL)
• Splits phone line in two ranges: Frequencies below 4
KHz are used for voice transmission, and frequencies
above 4 KHz are used to transmit data
• Typical connection speeds for downloading data
range from 256 Kbps to 8 Mbps; upload speeds are in
the range of 16 Kbps to 640 Kbps
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