ECE 6160: Advanced Computer Networks

ECE 6160: Advanced Computer Networks

Gigabit Ethernet

Instructor: Dr. Xubin (Ben) He

Email: Hexb@tntech.edu

Tel: 931-372-3462

Course web: http://www.ece.tntech.edu/hexb/616f05

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Prev…

• HIPPI

• FC

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GBE: borrows from FC and Ethernet

• Ethernet:– Frame format

– CSMA/CD

– Link layer technology

• FC:– Physical specifications, fibre

optics

– 8b/10b data encoding

– Ordered sets for link commands and delimiters

Preamble SOF DA SA Type Data CRC EXT

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GBE objectives

• 1000 Mb/s MAC

• 802.3 Ethernet Frame format

• Meet all 802 requirements except possibly Hamming distance

• Preserve min and max frame size of 802.3

• Full and half-duplex operation

• Support star-wired topologies

• Use CSMA/CD with at least 1 repeater

• Support Fiber and, if possible, copper

• At least 500 m over multimode fiber, At least 25 m over copper

Wiring-closet or data center backbone 100 m desirable

• At least 2 km on single mode fiber

• Collision domain diameter of 200 m

• Accommodate 802.3x flow control

• Cost effective

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GBE(802.3z)/Ethernet(802.3) and FC

802.2 LLC

802.2 LLC

CSMA/CD or full full duplex MAC

8b/10b

SerDes

Connector

Media

802.3CSMA/CD

802.3 Physical

FC-1

FC-0

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GBE Layers

Data Link Layer MAC client sublayer

MAC control (optional)

MAC

Physical Layer Reconciliation

Gigabit media independent interface

Media dependent PHY group

Medium-dependent interface

Medium

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MAC issue

• Carrier Extension

• Frame Bursting

• Buffered Distributor

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Carrier extension

• Slot time (or slot size):A signal propagates from one end to the other. Minimum time to detect a collision.

• Carrier Extension is a way of maintaining 802.3 minimum and maximum frame sizes with meaningful cabling distances.

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Slot-Time

• The MAC Layer of Gigabit Ethernet uses the same CSMA/CD protocol as Ethernet. The maximum length of a cable segment used to connect stations is limited by the CSMA/CD protocol. If two stations simultaneously detect an idle medium and start transmitting, a collision occurs.

• Ethernet has a minimum frame size of 64 bytes.

• The reason for having a minimum size frame is to prevent a station from completing the transmission of a frame before the first bit has reached the far end of the cable, where it may collide with another frame. Therefore, the minimum time to detect a collision is the time it takes for the signal to propagate from one end of the cable to the other.

• This minimum time is called the Slot Time. ( A more useful metric is Slot Size, the number of bytes that can be transmitted in one Slot Time. In Ethernet, the slot size is 64 bytes, the minimum frame length.)

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Gigabit Ethernet Slot Time

• The maximum cable length permitted in Ethernet is 2.5km (allowing up to 4 repeaters in a single network path). As the bit rate increases, the sender transmits the frame faster. As a result, if the same frames sizes and cable lengths are maintained, then a station may transmit a frame too fast and not detect a collision at the other end of the cable. So, one of two things has to be done :

(i) Keep the maximum cable length and increase the slot time ( and therefore, minimum frame size) OR

(ii) keep the slot time same and decrease the maximum cable length

OR both. In Fast Ethernet, the maximum cable length is reduced to only 100 meters, leaving the minimum frame size and slot time intact.

• Gigabit Ethernet maintains the minimum and maximum frame sizes of Ethernet. Since, Gigabit Ethernet is 10 times faster than Fast Ethernet, to maintain the same slot size, maximum cable length would have to be reduced to about 10 meters, which is not very useful. Instead, Gigabit Ethernet uses a bigger slot size of 512 bytes.

• To maintain compatibility with Ethernet, the minimum frame size is not increased, but the "carrier event" is extended. If the frame is shorter than 512 bytes, then it is padded with extension symbols. These are special symbols, which cannot occur in the payload. This process is called Carrier Extension.

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Frame bursting

• Carrier Extension wastes bandwidth. Up to 448 padding bytes may be sent for small packets. This results in low throughput. In fact, for a large number of small packets, the throughput is only marginally better than Fast Ethernet.

• Packet Bursting is "Carrier Extension plus a burst of packets". When a station has a number of packets to transmit, the first packet is padded to the slot time if necessary using carrier extension. Subsequent packets are transmitted back to back, with the minimum Inter-packet gap (IPG) until a burst timer (of 1500 bytes) expires. Packet Bursting substantially increases the throughput.

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Buffered distributor (Full Duplex Repeater or Buffered Repeater)

• A Buffered Distributor is a multi-port repeater with full-duplex links. It provides hub functionality with full duplex mode of operation. Each port has an input FIFO queue and an output FIFO queue. A frame arriving to an input queue is forwarded to all output queues, except the one on the incoming port.

• It provides full duplex connectivity, just like a switch, yet it is not so expensive, because it is just an extension of a repeater.

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Full Duplex Operation

• Half Duplex Ethernet & Fast Ethernets is based on CSMA/CD

• IEEE ratified 802.3x in 1995 = Full Duplex = can send send and receive frames simultaneously

• Requires point-to-point environment, stations connected to HUBS or ROUTERS will work at Half Duplex

• Stations connected back to back or to Layer-2 Switches will work at Full Duplex

• Full Duplex devices are NOT interoperable with Half Duplex devices – mismatch errors

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GMII

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Physical media

• Copper– Unshielded Twisted Pair (UTP-5): 4-pairs

– Shielded Twisted Pair (STP)

• Fibre optics– Multimode Fiber: 50 m and 62.5 m

– Single-Mode Fiber: 10 m

• Bit Error Rate better than 10-12

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GBE Physical Layer

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Fast Ethernet vs. 1000base-T

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Summary

• Ethernet is running at 1000Mbps, and 10Gbps is available…

• Is competing with ATM and FC

• Both shared and full-duplex links

• Fully compatible with current Ethernet

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VLAN: Virtual LAN

• Virtual LANs (VLANs) can be viewed as a group of devices on different physical LAN segments which can communicate with each other as if they were all on the same physical LAN segment.

• Switches using VLANs create the same division of the network into separate broadcast domains but do not have the latency problems of a router.

• IEEE 802.1Q

81 00 User priority CFI VLAN identifier

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Traditional LAN routers segment the network and provide logical structure, but are slow, complicated and expensive.

Picture is from Intel

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Standard switches are much faster than routers and provide dedicated bandwidth where needed, but are vulnerable to broadcast storms.

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VLANs allow highly flexible, efficient network segmentation, enabling users and resources to be grouped logically, without regard to physical location.

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Pros and Cons

• PROs:– Flexible network segmentation: logical

– Simple management: management console

– Increased performance: limiting broadcast traffic

– Better use of server resources: member of multiple VLANs

– Enhanced network security: virtual bounderis can only be crossed through a router.

• CONs:– Broadcast limitations – Device limitations – Port constraints