Browse Prior Art Database

CRC Over WLAN Layer 2 Header

IP.com Disclosure Number: IPCOM000008668D
Original Publication Date: 2002-Jul-02
Included in the Prior Art Database: 2002-Jul-02
Document File: 3 page(s) / 49K

Publishing Venue

Motorola

Related People

Randy L. Ekl: AUTHOR [+3]

Abstract

The Air Interface of WLAN systems is capable of very high throughputs. Current 802.11a systems can operate at 54 Mbps at close range. However, the throughput of the system drops quickly as the distance increases. 802.11a drops to 6 Mbps as a raw throughput rate, and 802.11b can operate as low as 1 Mbps. Since there is limited bandwidth, there needs to be mechanisms to make the air interface more efficient. Further, in current WLAN systems, a CRC is calculated over the entire packet and placed at the end of the Layer 2 message. Figure 1 shows the MAC frame structure, with the CRC. The CRC encompasses both the L2 header (from Frame Control to QoS Control, inclusive) and the L2 payload. However, it is not always important that all of the payload bits be correct. If the payload delivery is of an unconfirmed type, there is no need to retry the packet if there is an error in the payload. Retrying packets of type unconfirmed adds to making the WLAN air interface even more inefficient.

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This is the abbreviated version, containing approximately 50% of the total text.

CRC Over WLAN Layer 2 Header

By Randy L. Ekl, Joseph E. Phillips, and Mike Retzer

 
 

Introduction & Problem

The Air Interface of WLAN systems is capable of very high throughputs.  Current 802.11a systems can operate at 54 Mbps at close range.  However, the throughput of the system drops quickly as the distance increases.  802.11a drops to 6 Mbps as a raw throughput rate, and 802.11b can operate as low as 1 Mbps.  Since there is limited bandwidth, there needs to be mechanisms to make the air interface more efficient.

Further, in current WLAN systems, a CRC is calculated over the entire packet and placed at the end of the Layer 2 message.  Figure 1 shows the MAC frame structure, with the CRC.  The CRC encompasses both the L2 header (from Frame Control to QoS Control, inclusive) and the L2 payload.  However, it is not always important that all of the payload bits be correct.  If the payload delivery is of an unconfirmed type, there is no need to retry the packet if there is an error in the payload.  Retrying packets of type unconfirmed adds to making the WLAN air interface even more inefficient.

There are two specific cases for payload that is unconfirmed. First, voice or video payload can be unconfirmed; a next voice or video payload will come along in x msec.  A second case for unconfirmed payload is if the payload contains multiple fields, and if there are bit errors in one field, the rest of the fields may still be intact.

If a payload bit is incorrect in the current systems, the MAC CRC will not match, the packet will be discarded at the destination, an Ack will not be generated, and the packet will be retried by the source.  This takes additional time, as well as additional bandwidth and additional processing.  Furthermore, often, the payload is sent as a L2 Multicast message.  In this case, if the calculated CRC does not match the transmitted CRC, the packet will be discarded and not retried (since the source is not expecting an Ack).  In this case, the destination loses the information.  This is especially disastrous for multicast voice & video packets.

Figure 1: MAC Frame format (Bytes)
[Data frame shown for example]

Frm Cntr

Frm Cntr

Duration

Duration

Addr 1

Addr 1

Addr 1

Addr 1

Addr 1

Addr 1

Addr 2

Addr 2

Addr 2

Addr 2

Addr 2

Addr 2

Addr 3

Addr 3

Addr 3

Addr 3

Addr 3

Addr 3

Seq Cntr

Seq Cntr

Addr 4

Addr 4

Addr 4

Addr 4

Addr 4

Addr 4

QoS Cntr

QoS Cntr

Payload

Payload

Payload

……

……

……

……

……

……

……

……

……

CRC

CRC

CRC

CRC

Proposed Solution

The general concept for the solution to this problem is to perform the CRC over only the part of the frame that is required to be confirmed.  This may be just the MAC header, it may be the MAC header plus the start of the L2 Payload, or it may be additional multiple fields in the payload.  A mechanism is therefore needed to indicate over which fi...