Telecom formulas

ATM service categories:

CBR	Constant Bit Rate
rt-VBR	Realtime Variable Bit Rate
nrt-VBR	Nonrealtime Variable Bit Rate
ABR	Available Bit Rate
UBR	Unspecified Bit Rate
GFR	Guaranteed Frame Rate

Traffic parameters

PCR/SCR/MCR Peak/Sustained/Minimum Cell Rate

CDVT Cell Delay Variation Tolerance

BT Burst Tolerance

Qos Parameters

CLR Cell Loss Ration

CDV Cell Delay Variation

CTD Cell Transfer Delay (Max, Mean)

CER Cell error ratio

ATM header  (total Virtual Circuit ID  = virtual path ID + virtual channel ID)

virtual path ID
virtual path ID	virtual channel ID
Virtual Channel ID
Virtual Channel ID	Payload type(3) / CLP(1)
crc-8 checksum

	realtime		non-realtime; no delay requirements
Attribute	CBR	rt-VBR	nrt-VBR	ABR	UBR
CLR	specified	specified	specified	specified	no
CDV/CTD MaxCTD	CDV + maxCTD	CDV + maxCTD	MeanCTD [?], larger than for rt	no	no
PCR, CDVT	specified	specified	specified	specified	specified
SCR, BT	N/A	specified	specified, larger BT than rt-VBR	N/A	N/A
MCR	N/A	N/A	N/A	specified	N/A
Congestion control	no	no	no	yes	no

Abbreviations are above

GCRA(T, τ): Compute theoretical arrival time, tat, for each cell, starting with tat=0 for 1st cell. Let t = actual a arrival time.

Case 1: t < tat - τ: too early: cell is nonconforming; do not change tat
Case 2: t ≥ tat - τ: conforming; tat_new = max(t, tat) + T

Effective bandwidth requirement (in cells per unit time) for a connection with average cell spacing T, maximum acceptable delay D and maximum queue M:

    max(1/T, M/D).

The bandwidth of 1/T is needed to keep up with the average cell spacing, and the bandwidth of M/D is needed to process a burst of size M within the time constraint D.