Gigabit Ethernet supports data rates up to 1000 Mbps. It uses the same CSMA/CD protocol as previous Ethernet standards but with some modifications like carrier extension to support higher speeds. The GMII interface connects the MAC layer to different physical media layers and supports auto-negotiation to select operating modes. Gigabit Ethernet provides higher bandwidth and performance compared to previous standards while maintaining compatibility. However, it is not widely used yet and works best for data network traffic.

1. Gigabit Ethernet
Manasa.K
CWB0912002, FT-2012
M. Sc. (Engg.) in Electronic System Design
Engineering
Module leader: Narasimha Murthy K.R
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2. Overview
• Introduction
• Gigabit Ethernet Layers
• Media Access Control (MAC)
• Gigabit Media-Independent Interface (GMII)
• Auto Negotiation and Physical Media
• Advantages
• Disadvantages
• Conclusion
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3. Introduction
• Ethernet is a physical and data link layer technology for LAN. Higher level
network protocols like IP/TCP use Ethernet as their transmission medium.
• It was developed by Robert Metcalfe in 1980’s.
• The first generation Ethernet supported a maximum data rate of 10 Mbps.
Later, the second generation Fast Ethernet standards increased this
maximum data rate to 100 Mbps.
• Now the latest third generation, Gigabit Ethernet provides a data rate of
1000 Mbps and was developed in June 1998
• It is compatible with the existing Ethernets, as it uses the same CSMA/CD
and MAC protocols.
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4. Gigabit Ethernet Layers
•Gigabit Ethernet (IEEE802.3z) addresses the two lowest layers of the model:
•Layer 2, the Data-link layer, which describes how data are organized into frames and
sent over the network, and
•Layer 1, the Physical layer, which describes the network medium and
signalling specifications.
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5. Medium Access Control (MAC)
• The MAC layer of Gigabit Ethernet uses same CSMA/CD protocol as Ethernet.
• Gigabit Ethernet maintains maximum and minimum frame size of Ethernet, as it is
10 times faster than the Fast Ethernet, to maintain the slot size, the cable length should
be reduced to 10meters, which is not useful.
• Instead Gigabit Ethernet uses a bigger slot size of 512bytes.
*To maintain compatibility with Ethernet, the minimum frame size is not
increased instead the carrier event is extended.
*If the frame is shorter than 512 bytes, then it is padded with extension
symbols.
Carrier Extension:
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6. MAC
• Carrier extension helps in detecting the collision. It appends a set of special
symbols to the end of short MAC frames so that the resulting block is at least
512bytes.
• The extension symbols are removed before the FCS is checked by the receiver. So
the next layer is not even aware of the carrier extension.
Frame bursting:
• If the source host has many small frames to send, it can send them in a burst after
a MAC arbitration.
• Except for the first frame, other frames sent in a burst need not be carrierextended.
• If the first frame is larger than 512 bytes, it need not be carrier-extended.
• The burst can be as long as 8192 bytes.
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7. Gigabit Media-Independent Interface
• The GMII is the interface between the MAC
layer and the Physical layer. It allows any
physical layer to be used with the MAC layer.
• It is an extension of the MII ( Media
Independent Interface ) used in Fast Ethernet.
It uses the same management interface as MII.
It supports 10, 100 and 1000 Mbps data rates.
• It provides separate 8-bit wide receive and
transmit data paths, so it can support both fullduplex as well as half-duplex operation.
• The GMII provides 2 media status signals : one indicates presence of the carrier, and
the other indicates absence of collision. The Reconciliation Sublayer (RS) maps these
signals to Physical Signalling which is understood by the existing MAC sublayer.
• With the GMII, it is possible to connect various media types such as shielded and
unshielded twisted pair, and single-mode and multi mode optical fibre, while using the
same MAC controller.
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8. GMII
The GMII is divided into three sub layers : PCS, PMA and PMD.
PCS (Physical Coding Sub layer)
• It provides a uniform interface to the Reconciliation layer for all physical media. Carrier
Sense and Collision Detect indications are generated by this sub layer. It also manages the
auto-negotiation process by which the NIC (Network Interface) communicates with the
network to determine the network speed (10,100 or 1000 Mbps) and mode of operation (halfduplex or full-duplex).
PMA (Physical Medium Attachment)
• This layer serializes code groups for transmission and de-serializes bits received from the
medium into code groups.
PMD (Physical Medium Dependent)
• This sub layer maps the physical medium to the PCS. This layer defines the physical layer
signalling used for various media. The MDI ( Medium Dependent Interface), which is a part
of PMD is the actual physical layer interface. This layer defines the actual physical
attachment, such as connectors, for different media types.
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9. Auto Negotiation and Physical Media
• Auto Negotiation was defined in Fast Ethernet to automatically select operational
speeds between 10 and 100 Mbps. It was adapted to Gigabit Ethernet primarily to
select between duplex mode and the use of link-level flow control.
• On a 1000Base-X link, the configuration information exchanged through Auto
Negotiation is encoded into a special sequence of 8B/10B codes that transfers 16 bits
of configuration information at a time.
• On a typical 1000Base-X link, the Auto Negotiation exchange will be completed
roughly 40 milliseconds after the cables are plugged in or the equipment is turned on.
Physical Media:
Unshielded Twisted Pair (UTP-5): 4-pairs
Shielded Twisted Pair (STP)
Multimode Fiber: 50 mm and 62.5 mm
Single-Mode Fiber
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10. Advantages
•
Increased bandwidth for higher performance.
•
Full-duplex capacity, allowing data to be transmitted and received at the
time so that the effective bandwidth is virtually doubled.
•
Provides full compatibility with the large installed base of Ethernets
and Fast Ethernet nodes.
•
Low cost of acquisition and ownership.
•
Transfer large amount of data across the network quickly.
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11. Disadvantages
• Not originally designed to support real-time voice and video traffic.
works best when working solely with network traffic comprised solely of
data.
•
Cannot prioritize traffic to deliver effective Class of Service (CoS).
•
Gigabit Ethernet is still not in use.
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12. Summary
• Gigabit Ethernet supports data rate of 1000Mbps.
• CSMA/CD protocol are same as that of Ethernet and Fast Ethernet. It helps in
reducing the collision.
• Carrier extension is extended with special symbols so that the block is 512 bytes.
Frame bursting is used to achieve higher throughput.
• GMII is an interface between MAC and Physical layer, supports half and full
duplex modes of operation and also connects various media.
• Auto negotiation selects the duplex modes, transfers 16 bit of information data at a
time.
• Gigabit Ethernet uses higher bandwidth and provides higher performance.
• It is still not in use. And it only works with network traffics.
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13. References
• Howard Frazier Gigabit Ethernet: From 100 to1,000 Mb Cisco Systems Inc (25th
May 2013)
• T. C. Tan (August 2000) Gigabit Ethernet and structured cabling Electronics &
Communication Engineering Journal (25th May 2013)
• Vinay Moorthy Gigabit Ethernet Version 2 CSE IIT Kharagpur ( 26th May 2013)
• Gigabit Ethernet available
from<http://www.cisco.com/web/about/ac123/ac147/ac174/ac199/about_cisco_ipj_
archive_article09186a00800c85a6.html >The Internet Protocol Journal - Volume 2,
No. 3 (26th May 2013)
• Ethernet Introduction to Gigabit Ethernet available from
<http://www.cisco.com/en/US/tech/tk389/tk214/tech_brief09186a0080091a8a.html
> ( 26th May 2013)
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14. Thank You
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