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10.2 DATA LINK LAYER (LAPD) 245Figure 10.2-5. Information field of unnumbered information frame with a TEI managementmessage. (From Rec. Q.921. Courtesy of ITU-T.)ID-check-request. This is a command from the network to check out an assignedTEI value.ID-check-response. This is the response by a portable TE to an ID-check-request.ID-remove. This is a command sent from the network, to remove a portable TEwith a specified TEI value from the DSL.The information field of the UI frames is shown in Fig. 10.2-5. The code in octet 1indicates a TEI management message. The message type code is in octet 4 (seeTable 10.2-2). The message includes the parameters Ri (reference number) and Ai(action indicator).We now describe the procedures for TEI assignment and TEI checking.The TEI assignment procedure is shown in Fig. 10.2-6(a). When a portable TE isplugged into a DSL, it automatically sends an ID-request. Since the TE does nothave a TEI, it generates a random reference number (Ri) to identify itself. A TEcan request the assignment of a particular TEI value by specifying it in the Aifield or can leave the choice to the network by setting Ai ¼ 127.For each attached DSL, the network (local exchange) maintains a list of portableTEI values (range 64–126) that are currently assigned. When receiving anTABLE 10.2-2 Message Type Codes in TEIManagement MessagesMessage NameMessage Type CodeID-request 0000 0001ID-assigned 0000 0010ID-denied 0000 0011ID-check-request 0000 0100ID-check -response 0000 0101ID-remove 0000 1010Source: Rec. Q.921. Courtesy of ITU-T.