Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” Optical Communications Telecommunication Engineering School of Engineering University of Rome La Sapienza Rome, Italy 2005-2006 Lecture #11, June 14 2006
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” Medium Access Control for Optical Communications Part II
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” COLLISION AVOIDANCE • Collision Avoidance (CA) mechanisms have been proposed in order to solve the hidden terminal problem. • CA-based protocols can be divided in: – Out-of-Band CA protocols : the Collision Avoidance is performed on the dedicated channel, separated (usually in frequency) from the data channel • Example: Busy Tone Multiple Access (BTMA) – In-band CA protocols : the Collision Avoidance is performed on the same channel used for data traffic • Example: Medium Access with Collision Avoidance (MACA)
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” IN-BAND CA: MEDIUM ACCESS WITH COLLISION AVOIDANCE (MACA) • MACA does not use Carrier Sensing, in the sense that terminals start transmitting a packet without spending any time sensing the channel • In MACA when a terminal has a data packet to send, it does not transmit directly the data packet, but instead starts a Collision Avoidance procedure with the intended destination, based on three steps: 1. Transmission of a Request-To-Send (RTS) packet from source to destination 2. Transmission of a Clear-To-Send (CTS) packet from destination to source, in response to the CTS 3. Transmission of the DATA packet from source to destination, after reception of the CTS • The procedure is called handshaking
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” MACA: REQUEST-TO-SEND • The RTS is emitted by the source S , and is received by the destination D and by all other terminals within transmission range of S (terminal A1 in figure) • The RTS includes: S D A2 – The ID of the source S A1 – The ID of the destination D – The expected duration of the DATA packet to be transmitted • The RTS has two goals: a. communicating to all terminals within range of S that a transmission is going to start b. trigger the destination D to emit a CTS message in reply to the RTS • After the reception of the RTS, A1 will expect to hear the CTS transmitted by D in reply to the RTS • If A1 does not hear the CTS within a given time, it can start transmitting in any moment, since it is out of transmission range of D
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” MACA: CLEAR-TO-SEND • The CTS is emitted by the destination D , and is received by the source S and by all other terminals within transmission range of D (terminal A2 in figure) S D A2 • The CTS includes: A1 – The IDs of S and D – The expected duration of the DATA packet to be transmitted • The CTS has two goals: a. communicating to all terminals within range of D that a reception is going to start b. trigger the source S to emit the DATA packet • Terminal A2 will hear the CTS and will know that a transmission is going to start, even if it did not hear the CTS transmitted by S • If A2 has packets to send, it will thus postpone the transmission until the transmission S->D is over • Since CS is not used, A2 understands how long it will have to wait by reading the content of the CTS
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” MACA: ADVANTAGES • The adoption of the RTS/CTS exchange relieves both the hidden and exposed terminal problems: – If CSMA was used with the same network topology presented in the previous slides, A1 would be an exposed terminal, while A2 would be a hidden terminal S D A2 A1 • Note that collision is still possible between RTS packets, but the effect of such collisions is much lower of DATA collisions for two reasons: 1. No DATA information is lost 2. RTS packets are usually very short (20 Bytes) and thus a collision keeps the channel busy for a short time
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” CSMA WITH COLLISION AVOIDANCE (CSMA-CA) • Although MACA was proposed as an alternative to CSMA protocols, CSMA and In- band Collision Avoidance can be combined in order to get the advantages provided by MACA and reduce the probability of having collisions on the RTS packets (CSMA- CA) • The CSMA-CA approach is adopted in the Distributed Foundation Wireless MAC (DFWMAC) adopted in the IEEE 802.11 standard (WiFi) • In DFWMAC the handshaking is formed of four steps. The four steps are: 1. RTS (Direction: S -> D) 2. CTS (D -> S) 3. Data (S -> D) 4. Acknowledge (Ack) (D -> S) • CSMA is adopted before transmitting the RTS packet
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” CSMA-CA PROPOSED FOR IR PROTOCOLS CSMA-CA is adopted in the IrLAN protocol, part of the IrDA stack IrDA protocol stack Infrared Link Management Protocol (IrLMP)
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” CSMA-CA PROPOSED FOR IR PROTOCOLS IrLAN provides compatibility between applications requiring an IEEE 802 network connection and an IrDA device The MAC adopted for IrLAN is based on a simplified version of basic IEEE 802.11 protocol Access to the medium is controlled through the use of time intervals between the transmission of frames (mandatory periods of idle on the transmission medium). They can be short (usually called Short InterFrame Spacing or SIFS following the IEEE 802.11 denominations), used between control frames, and long ( DIFS ). The receiving station calculates the checksum and determines whether the packet was received correctly. Upon receipt of a correct packet, the receiving station waits a SIFS interval and transmits a Positive Acknowledgment (ACK) frame back to the source station, indicating that the transmission was successful All stations hearing the data frame adjust their NAV (Network Allocation Vector) based on the duration field value, which includes the SIFS interval and the ACK following the data frame
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” CSMA-CA PROPOSED FOR IR PROTOCOLS RTS/CTS protocol: A station can reserve channel bandwidth prior to transmission of a data unit RTS and CTS control frames can be used to minimize the amount of bandwidth wasted when collisions occur. RTS and CTS are relatively small control frames. The source station with a data or management frame queued for transmission to a specified destination station transmits a RTS control frame. All stations in the Base Service Set (BSS) hearing the RTS packet wait for a CTS packet after a SIFS idle period has elapsed. Stations hearing the CTS packet look at the duration field and again update their NAV. .
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” CSMA-CA PROPOSED FOR IR PROTOCOLS DATA DATA RTS SOURCE SIFS SIFS SIFS CTS ACK ACK DEST. SIFS DIFS DIFS DIFS DIFS CW CW OTHER NAV NAV NAV (RTS) NAV (CTS) (A) (B) Timing diagram of a successful data frame transmission: without handshaking (A) using a RTS/CTS mechanism (B)
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” CSMA-CA PROPOSED FOR IR PROTOCOLS Large data units may require fragmentation to increase transmission reliability. Data units are compared to a adjustable parameter (similar to the Fragmentation_Threshold of IEEE 802.11). If the data unit-size exceeds the value of this threshold, it is broken into multiple fragments) When a data unit is fragmented, all fragments are transmitted sequentially and the channel is not released until the complete data unit has been transmitted successfully, or the source station fails to receive an acknowledgment for a transmitted fragment. The destination station positively acknowledges each successfully received fragment by sending a ACK back to the source station. The source station maintains control of the channel throughout the transmission of the data unit waiting only an SIFS period after receiving an ACK and transmitting the next fragment. When an ACK is not received for a previously transmitted frame, the source station halts transmission and re-contends for the channel. Upon gaining access to the channel, the source starts transmitting with the last unacknowledged fragment.
Departamento de Señales y Dipartimento INFOCOM comunicaciones Università degli studi di ULPGC Roma “La Sapienza” CSMA-CA PROPOSED FOR IR PROTOCOLS DIFS SIFS SIFS SIFS SIFS SIFS SIFS CW Fragment 0 Fragment 1 Fragment 2 SOURCE DEST. ACK 0 ACK 2 ACK 1 OTHER NAV (CTS) NAV (Fragment 2) NAV (Fragment 0) NAV (Fragment 1) OTHER NAV (ACK 1) NAV (ACK 0) Timing diagram of a successful fragmented data frame transmission
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