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A Professional’s Guide to Data Communication in a TCP/IP World phần 4

, thực hiện cùng nhau, xác định 4.094 mạch ảo có thể. Các gói dữ liệu khác nhau ở số lượng bit được gán cho số lượng gói tin này [P (S), và số lượng của gói tin người gửi hy vọng sẽ nhận đượcdịch vụ unicast điểm. Chuyển mạch gói các liên kết WAN chẳng hạn như X.25, | 66 Wide Area Networks User s stack User s IP datagram User-network interface UNI Figure 4.3 X.25 architecture. Executes bit stuffing to achieve bit-transparency . On the transmit side generates frame check sequences FCSs . On the receive side confirms FCSs. In the physical layer or X.25-1 layer the frame is transmitted over a logical channel virtual channel to the network node. Figure 4.4 shows packet header formats for two data packets and a control packet. All include a 4-bit group number and an 8-bit channel number that taken together define 4 094 possible virtual circuits. The data packets differ in the number of bits assigned to the number of this packet P S and the number of the packet the sender expects to receive P R . With 3 bits P S and P R 7 with 7 bits P S and Bits 7 6 5 4 3 2 1 0 1 Bytes 3 DATA packet 1 Q I D I 0 I 1 I I Group I I I Channel I P R M P S 0 User data DATA packet 2 Q I D 1 0 Group --r--r--r--T--T--T--T-- Channel Í - P S 0 y r. Fp r y I I I Ml User data 1 0 0 0 1 1 0 Group r - - T- -T--T--T-Channel 3 Packet type 1 Additional information CONTROL packet 1 4 Figure 4.4 Packet formats. 4.2 Nonbroadcast Multiple Access Links 67 P R 127. Using 3 bits the sender must wait for an acknowledgment after sending seven frames. Only after all seven have been acknowledged as good can the sender begin the next packet number cycle. Using 7 bits the sender can send up to 127 frames before waiting for an acknowledgment. Bits M D and Q support special functions. 4.2.1.2 Routing How frames are routed over a packet-switched network depends on the instructions given by the users. Three basic styles similar to the routing techniques employed in router driven networks can be distinguished Distributed routing On the basis of information about traffic conditions and equipment status network map port status each node decides which link the frame shall take to its destination. Centralized routing A primary and perhaps an alternate path is dedicated to a pair of .