1. 3. Bluetooth usage models 4. L2CAP process 5. Example 6. Terminology
목 차
L2CAP 정의 및 역할
2. Layer 구성 및 interface 개요
3. Bluetooth usage models
4. L2CAP process
5. Example
6. Terminology
7. Summary
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2. 1. L2CAP 정의 및 역할
- L2CAP : Logical Link Control and Adaptation Protocol
Adapts higher-layer protocols to the lower-layer protocol
: handle MUX of higher-layer protocols and segmentation/re-assembly of large packets.
*higher-layer protocols : RFCOMM, SDP, PPP, TCP/IP, WAP …
*lower-layer protocol : HCI, Baseband (concerned with connection establishment, addressing, timing, etc…)
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3. Applications Audio MUX L2CAP L2CAP process HCI Baseband RF
2. Layer 구성 및 interface 개요
Applications ↑
Higher
Audio
RFCOMM
SDP
TCS
MUX
L2CAP
L2CAP process
HCI
Lower ↓
Baseband RF
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4. 2. Layer 구성 및 interface 개요 (continued)
1) Multiplexing
Baseband didn’t know/support the every protocol type
☞ L2CAP CAN !
2) Fragmentation/Recombination
L2CAP segmentation과 HCI/BB에서 처리할 수 있는 packet size가 다르므로
HCI/BB가 처리할 수 있게 packet control
3) Flow control, error control (optional)
Baseband can’t detect or it’s hard to detect
☞ L2CAP CAN !
4) Segmentation/Reassembly
(only used in retransmission, flow control)
Memory management, error correction by retransmission
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5. 2. Layer 구성 및 interface 개요 (continued)
* Fragmentation/Recombination
L2CAP
HCI
Transmitter
L2CAP
HCI
Receiver
PDU 생성
PDU 생성
HCI가 알아듣게 packet 구성
L2CAP이 알아듣게 packet 구성
<Fragmentation>
<Recombination>
* Segmentation/Reassembly : Only retransmission, flow control
e.g) RFCOMM
L2CAP
Segmentation
SDU 생성
Reassembly
SDU 가 여러 개의 segmentation으로 조각남
여러 개의 segmentation이 SDU로 합쳐짐
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6. 3. Bluetooth usage models
File transfer application
OBEX
RFCOMM
SDP
L2CAP
(1) File transfer
(2) Dial-up networking
Modem emulation
AT commands
RFCOMM
SDP
L2CAP
PPP
Headset application
AT commands
RFCOMM
SDP
L2CAP
(3) Headset
(4) Cordless phone
Cordless phone application
TCS BIN
Audio
L2CAP
SDP
Audio
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7. Makes use of ACL links (not support for *SCO links)
4. L2CAP process
Makes use of ACL links (not support for *SCO links)
e.g) comparison text with audio transmission
L2CAP channel (3 types of logical channels)
- Connectionless : unidirectional. half-duplex. Broadcast data.
- Connection-oriented : bidirectional. Full-duplex. QoS is assigned.
- Signaling : signaling messages
2 byte 2
≥ 2
0 ~ 65533
Length
CID
*PSM
Data
Connectionless PDU
0x0002 2 2
0 ~ 65535
Connection-oriented PDU
Length
CID
Data
0x0040~0xFFFF 2 2
Length
CID
≥1 command(s)
Signaling PDU
0x0001
*SCO packets are never retransmitted.
*PSM(Protocol/Service Multiplexer) : identify the higher-layer recipient for the data in packet
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8. 4. L2CAP process (continued)
* CID
: Local에서만 의미가 있는 ID로, 다수의 connection-oriented channels을 구분하고자 할 때 쓰임
0x0041
0x0041
0x0001
0x0042
0x0050
0x0002
0x0002
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9. * JPG file transmission
5. Example
* JPG file transmission
Conditions )
1. JPG binary data transmission and reception
2. 전반적인 흐름 이해를 목적
3. Higher layer : OBEX protocol
4. Connection-oriented channel 사용
5. Prototype
6. Master : transmit the binary file
Slave : receive the binary file
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10. L2CAP RFCOMM OBEX L2CAP RFCOMM OBEX
5. Example (continued)
Master
Slave
Transmit .jpg file
Receive .jpg file
L2CAP
RFCOMM
OBEX
File transfer App.
HCI
Baseband RF
L2CAP
RFCOMM
OBEX
File transfer App.
HCI
Baseband RF
size : 353 KB
mux
Length : 353 KB, CID : 0x0040, Data : 0x9CCB …
0x0161
0x0040
0xFFD8FFE0 …
Length
CID
Data
Fragmentation
Recombination
Connection handle
PB flag
BC flag
Length
L2CAP data
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11. 6. Terminology
* ACL(Asynchronous Connectionless) : Point-to-multipoint link. Master exchange packets with any slave.
Retransmission is applied.
* Channel : logical connection between two endpoints in peer devices
* CID(Channel ID) : local name that indicates channel endpoint
* Fragmentation : lower layer와의 transmission을 위해 만들어짐. HCI가 처리하도록 조각냄 (Recombination과 반대)
* L2CAP : Logical Link Control and Adaptation Protocol
* LMP(Link Manager Protocol) : responsible for link setup between bluetooth devices
* OBEX(Object Exchange protocol) : binary transmission. similar to HTTP
* PDU(Protocol Data Unit) : L2CAP packet data
* PSM(Protocol/Service Multiplexer) : identify the higher-layer recipient for the data in packet
* Reassembly : segment are assembled. Only used in retransmission, flow control (Segmentation과 반대)
* Recombination : HCI는 L2CAP가 처리할 수 있도록 조합함(Fragmentation과 반대)
* RFCOMM(Radio Frequency Communications) : cable replacement protocol
* SCO(Synchronous Connection Oriented) : Point-to-point link. allocate a fixed bandwidth. SCO packets are never retransmitted.
* SDU(Service Data Unit) : upper layer에서 생성된 data. SDU는 1개 이상의 segment로 구성된다.
* SDP(Service Discovery Protocol) : query device information, services, etc to enable connection between two or more bluetooth devices
* Segmentation : SDU is split into several segmentations.
Only used in Retransmission, flow control (Reassembly와 반대)
* TCS BIN : Telephony Control Specification-Binary
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12. 7. Summary
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