X 25 Prof Choong Seon HONG Kyung Hee

X. 25 Prof. Choong Seon HONG Kyung Hee University 1

X. 25 패킷 교환망 (cont’d) DCE : Data Circuit Equipment DTE : Data Terminal Equipment Kyung Hee University 4

17. 1 X. 25 계층 구조 q X. 25 Layers in Relation to the OSI Layers Kyung Hee University 5

X. 25 계층 구조 (cont’d) § X. 25 계층구조 • 물리층 : ITU-T X. 21(X. 21 bis) • 데이터 링크층 : LAPB(link access procedure, balanced) • Frame layer • 네트워크층 : PLP(packet layer protocol) • X. 25 requires error detection and correction in both the data link and network layers Kyung Hee University 6

Frame Layer q Format of a Frame Kyung Hee University 7

Frame Layer (cont’d) q Addressing Kyung Hee University at the Frame Layer 8

Frame Layer (cont’d) q Three Phases of the Frame Layer Kyung Hee University 9

Packet Layer § Network layer in X. 25 is called the Packet Layer Protocol (PLP) § It is responsible for establishing the connection, transferring the data and terminating the connection § Virtual circuit : VCs in X. 25 are created at the network layer Kyung Hee University 10

Packet Layer (cont’d) q Frame layer and packet layer domain Kyung Hee University 11

Packet Layer (cont’d) q Virtual circuit in X. 25 Kyung Hee University 12

Packet Layer (cont’d) q Virtual Circuit Identifier (VCI) v논리채널번호(LCN: Logical Channel Number) v. LCN의 영역은 지역(Local)에 국한 사용 q Kyung Hee University 13

Packet Layer (cont’d) q LCN 할당 v최대 4096(212)까지 허용 vone-way LCN : simplex 통신에 사용 vtwo-way LCN : duplex 통신에 사용 Kyung Hee University 14

Packet Layer (cont’d) § PLP 패킷 Kyung Hee University 15

Packet Layer (cont’d) < PLP 패킷 필드 Ü GFI(General Format Identifier) : 4 bit 필드 Q D 0: PLP 1: 다른 상위 프로토콜 순서번호 0: Local DCE 1: Remote DTE 순서번호 크기 01: 3 bit 10: 7 bit Ü LCN(Logical Channel Number) : 12 bit 필드 w 4 bit : LGCN(Logical Group Channel Number) w 8 bit : LCN (Logical Channel Number) Ü PTI (Packet Type Identifier) w 패킷 유형 식별자 Kyung Hee University 16

Packet Layer (cont’d) q Categories of PLP packets Kyung Hee University 17

Packet Layer (cont’d) q Data Packets in PLP Layer v사용자 데이터 전송에 사용 Kyung Hee University 18

Packet Layer (cont’d) q RR, RNR, and REJ packets Kyung Hee University 19

Packet Layer (cont’d) q Other control packets Kyung Hee University 20

§제어 패킷 유형 Packet Layer (cont’d) DTE to DCE to DTE Call request Incoming call 000010 Call accepted Call connected 000011 Clear request Clear indication 000100 Clear confirm 000101 Interrupt 001000 Interrupt confirm 001001 Reset request Reset indication 000110 Reset confirm 000111 Restart request Restart indication 111110 Restart confirmation 111111 Registration request 111100 Registration confirm Kyung Hee University Code 111101 21

Packet Layer (cont’d) §제어 패킷 형식 Kyung Hee University Optional services - contractual call basis : incoming calls barred, outgoing calls barred, etc - per call basis : flow control negotiation, reverse charging, etc 22

Packet Layer (cont’d) §제어 패킷 형식 (cont’d) Reset : using an existing VC Restart : using a new VC Kyung Hee University 23

Packet Layer (cont’d) §연결 설정 Kyung Hee University 24

Packet Layer (cont’d) §연결 해제 Kyung Hee University 25

Other Protocol Related to X. 25 (cont’d) q Triple-X 프로토콜 v X. 3 l PAD : Packet Assembler/Disassembler l 문자중심(dumb) 단말 연결에 사용 v X. 28 : X. 25 기능이 없는 단말(dumb terminal)과 PAD간의 통신규약 v X. 29 : PAD와 원격 단말 간의 관계 정의 Kyung Hee University 27