what is broadband? data rates >1.5 Mbps (T1) and data-rates=2Mbps (E1). What is this T1 and E1 means???

Question

DL data transfers in PDCP when DRB mapped on RLC AM mode.

36.323. (Section :- 5.1.2.1.2)

This section mentioned alogithm but I want to know how conditions are defined ?
eg.
- if received PDCP SN – Last_Submitted_PDCP_RX_SN > Reordering_Window or 0 <= Last_Submitted_PDCP_RX_SN – received PDCP SN < Reordering_Window: ------> This is actually seems to be condition when PDU is out of reorder window boundary.

• Is this due ti circular definitions for PDCP SN ?

• kindly suggest any document which will give some detail description about it.

Thanks .