Chapter 11
Ethernet Evolution:
Fast and Gigabit Ethernet
Revolution:
From a LAN to a bridged LAN
From a bridged LAN to a switched LAN
From a switched LAN to a full-duplex switched LAN
From a 10 Mbps Ethernet to a Fast Ethernet (100 Mbps)
From a Fast Ethernet to a Gigabit Ethernet (1000 Mbps)
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Figure 11-1
Sharing Bandwidth
In an unbridged network, the total capacity (10Mbps) is shared among
all stations with a frame to send.
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Figure 11-2
11.1 BRIDGED ETHERNET
A Network with and without a Bridge
A bridge divides the network into two or more segments.
Bandwidth-wise, each segment is independent.
Each station is theoretically offered 10/12 Mbps in a network with a heavy load.
Each station is now offered 6/12 Mbps if the traffic is not going through the bridge.
This is the first advantage of using a bridge.
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Figure 11-3
Collision Domains in a Non-bridged and Bridged Network
Second advantage of a bridge: separation of collision domain
12 stations contend for access to the medium
(4-port bridge)
Only 3 stations contend for access to the medium
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Figure 11-4
Switched Ethernet (Sec. 11.2)
A layer-2 switch is an N-port bridge with additional sophistication that
allows faster handling of the packets.
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Figure 11-5
11.3 FULL-DUPLEX ETHERNET
Full-Duplex Switched Ethernet
No need for carrier sensing or collision detection. No need for CSMA/CD access method.
The CS and CD functionality of the MAC sublayer can be turned off.
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11.4 MAC CONTROL
Traditional Ethernet :
Connectionless at the MAC sublayer, using CSMA/CD.
There is no explicit flow control or error control to inform the sender.
Receiver does not send any positive or negative acknowledgement.
In traditional Ethernet, there are two sources of errors:
1. The frame is corrupted during transmission.
Probability of bit error rate (BER) is very low: 10 ^ (-10)
If it happens, the receiver discards the frame by checking FCS and
the upper layer (LLC) notifies the sender for retransmission
2. The frame is lost due to collision.
Sender can detect this situation (using CD protocols) and resend the frame.
In full-duplex switched Ethernet, there are also two sources of errors:
1. The frame is corrupted during transmission.
This is the same situation as in traditional Ethernet.
2. The frame is lost because the switch buffer is full
The switch discards the frame.
But, there is no CSMA/CD mechanism to inform the sender that the packet
is lost.  There is a need for an explicit ACK in full-duplex switched Ethernet.
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Figure 11-6
MAC Control Layer
(optional)
Why MAC control sublayer: for flow and error control.
How:
by inserting special control packets between data packets coming from the upper layers.
The MAC control packet is encapsulated in a MAC frame in the same way as
the data packets.
For efficiency, a frame carrying a MAC control packet should be as smallas possible
 the size of a MAC control packet <= 46 bytes
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Figure 11-7
Encapsulation of a MAC Control
Packet in a MAC Frame
DA: device (station or switch) at the other end of the link, not the final destination of
the data frame. A special multicast address 01-08-C2-00-00-01 for three reasons:
1. The sender does not need to know the address of the device at the other end.
2. This address is blocked by all bridges and switches.
3. This address is ignored by all stations that do not use MAC control option.
SA: address of the device that sends the MAC control packets
Type/length: indicates the type of the frame, not the length which is of fixed size.
FCS: CRC error detection field
Code: MAC control packet identifier, 0001 (based 16) for PAUSE packets.
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Figure 11-8
Interleaving of MAC Control Frames
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Figure 11-9
Format of the PAUSE Packet
PAUSE packet:
Currently, this is the only packet defined by the MAC control sublayer.
Purpose: used to temporarily slow down the flow of frames between two
device (switch or station) connected at the ends of a full-duplex link.
It provides a very simple flow control (called stop-start)
Code
parameters
Code: a 2-byte field with the value 0001 (base 16)
Parameters: the only parameter defined is the parameter p for pause-time
actual pause-time = p * slot time, where
slot-time = the duration of 512 bits
Rule: current PAUSE packet overwrites previous PAUSE packet
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Figure 11-10
11.5 FAST ETHERNET
Layers in Fast Ethernet
LLC is the same as
discussed in Chap. 9
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11.6 FAST ETHERNET MAC SUBLAYER
Main idea: keep the MAC sublayer for 10 Mbps Ethernet untouched.
Frame format, min. and max. frame length, and addressing are the same for
both 10- and 100-Mbps Ethernet.
Access method: CSMA/CD (no need for full-duplex Fast Ethernet)
The implementation keeps CSMA/CD for backward compatibility
Slot time: duration of 512 bits = 51.2 microseconds for 10-Mbps Ethernet
= 5.12 microseconds for 100-Mbps Ethernet
Slot time and max. network length:
Max. length = propagation speed * (slot time / 2)
= 5120 m for 10-Mbps Ethernet (= (2*10^8)(51.2*10^(-6) / 2))
(2500 m specified)
= 512 m for 100-Mbps Ethernet (= (2*10^8)(5.12*10^(-6) / 2))
(250 m specified)
Auto negotiation (a new feature added):
1. to allow two devices to negotiate the mode (half- or full-duplex) or
data rate of operation (10- or 100-Mbps)
2. to allow one device to have multiple capabilities
3. to allow a station to check a hub’s capabilities
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Figure 11-11
Example of Auto Negotiation
Notes on negotiation:
1. Covers only the link, not the whole network (btw. a station & a hub or btw. two hubs)
2. Can only occur during link initialization, based on common capabilities
3. Each device at the end of the link advertises its capabilities to the other
3. A hierarchy of common capabilities is defined to facilitate the decision
4. Use a separate frame format and signaling system
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Figure 11-12
Auto Negotiation Message Format
Selector field: defines the type of the LAN technology (=10000 for Ethernet)
Ability field: for advising the sender’s capabilities, when set
A0: 10Base-T
A1: 10Base-T full duplex
A2: 100Base-TX
A3: 100Base-T dull-duplex
A4: 100base-T4
A5: Pause operation
A6: Reserved
A7: Reserved
Fault bit: announces that a fault has occurred when set
Ack bit: announces that a message was successfully received when set
Next page bit: another message coming (called next page) when set
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Figure 11-13
11.7 Fast Ethernet Physical Layer
passing data in 4-bit
format (nibble) to MII
MII is needed only for
external transceiver
Can be used with 10– and 100-Mbps data rate
Transceiver
(encoding/decoding)
medium dependent
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Figure 11-14
MII
MII operates at both 10– and 100-Mbps
Backward compatible
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Figure 11-15
Signals in MII
4-bits parallel data path (TX data)
4-bits parallel data path (RX data)
Inside NIC
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Figure 11-16
MII Connector
40-pin D-connector
20 twisted-pair cables
for synchronization btw PHY
& Reconciliation sublayer
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Figure 11-17
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MII Cable
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Figure 11-18
Fast Ethernet Implementations
2-wire implementation
4-wire implementation
uses 2 pairs of C-5 UTP
or STP
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uses 2 strands of fiberoptical cables
uses 4 pairs of C-3 UTP
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Figure 11-19
100Base-TX
Implementation
2 pairs: one for TX, one for RV
can operate in full-duplex mode
NIC with internal transceiver
In full-duplex mode, frames are buffered
and switched internally within the hub
(like a switch)
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Figure 11-20
Encoding and Decoding in 100Base-TX
3-levels, multiline
Transmission, see
next slide
4 parallel bits
from NIC
Inside the transceiver
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Figure 11-21
MLT-3 Signal
Similar to NRZ-I, but uses three levels
Transition at the beginning of a 1 bit
No transition at the beginning of a 0 bit
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Figure 11-22
100Base-FX
Implementation
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Figure 11-23
Encoding and Decoding in 100Base-FX
4 parallel bits
from NIC
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Figure 11-24
NRZ-I Encoding
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Figure 11-25
100Base-T4 Implementation
4 pairs of C-3 UTP
Encoding/decoding is more complex here
Uses 8B/6T (8 binary bits  6 ternary signals)
See Appendix K
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Figure 11-26
Example of 8B/6T Encoding
-
8 binary bits
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+
0
+
-
0
6 ternary signals as a single unit
(each unit represents 8 bits)
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Figure 11-27
Using Four Wires in 100Base-T4
Mbaud
33.3 Mbps
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Figure 11-28
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Layers in Gigabit Ethernet
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Figure 11-29
Two Approaches in Gigabit Ethernet
Medium Access
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Figure 11-30
A Frame Using Carrier Extension Method
=
=
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Figure 11-31
Frame Bursting Approach
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Figure 11-32
Gigabit Ethernet Physical Layer
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Figure 11-33
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Gigabit Ethernet Implementations
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Figure 11-34
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1000Base-X Implementation
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Figure 11-35
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Encoding in 1000Base-X
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Figure 11-36
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1000Base-T Implementation
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Figure 11-37
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Encoding in 1000Base-T
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