Line status

Difference between off hook and being on call

When you go off hook on a line and while the dial tone plays or when you try to dial out,  Call waiting will not take effect.

Call waiting takes effect when you are actively on a call.

QoS on 6500

Congestion avoidance on 6500

Tail drop advanced	WRED
Drops packets with a certain CoS when a certain threshold is reached When 50% of queue is filled, drop CoS 0 and 1 When 60% is filled, drop CoS 2 and 3	Defines a min and max threshold CoS 0 and 1 has a min threshold of 50 and max of 70. When queue reaches 50, WRED starts dropping packets in random and when it reaches 70 will drop all packets

Scheduling

WRR – Weighted Round Robin	Strict Priority
Queues emptied in round robin fashion based on the weights By default, has 2 queues. Queue 1 is served 70% of the time and Queue 2 is served 30% of the time	Always the priority queue will be emptied first. After each packet from the other queues are send, the priority queue will be checked and emptied

Available queue configurations on 6500

Map CoS to queue thresholds	Cos 0 maps to Queue 1 Threshold 1 Cos 1 maps to Queue 1 threshold 2	set qos map 2q2t tx 1 1 cos 0 set qos map 2q2t tx 1 2 cos 1
Define the threshold levels for tail drop or WRED	Threshold 1 is 80 Threshold 2 is 100	set qos drop-threshold 2q2t tx queue 1 80 100 – tail drop set qos wred 2q2t tx queue 1 80 100 — WRED
Queue serving time	Queue 1 served 5/260 Queue 2 served 255/260	set qos wrr 2q2t 5 255
Assigning buffer length	Queue 1 has 80 (low priority queue is assigned more buffer) Queue 2 has 20	set qos txq-ratio 2q2t 80 20

Congestion avoidance

Congestion avoidance

	RED	WRED	FWRED
Packet drop	Random	Based on IP Precedence Packets with higher IP Precedence are less likely to be dropped than packets with lower precedence	Based on the flow.
Statistical drop	none	Drops more packets from larger users than smaller	Flows that respond to packet drops are protected against those that do not
Non-IP Traffic	no special consideration	considered to belong to IP Precedence 0
Command		random-detect random-detect exponential-weighing-constant random-detect precedence	random-detect random-detect flow random-detect flow average-depth-factor random-detect flow count
RSVP Aware	no	yes

Fragmentation

Only in links less than 768 kbps

Frame-relay fragmentation voice-adaptive: Enables fragmentation only when there are voice packets in the LLQ PQ or when H323 messages are processed.

FRF.12	Frame relay does not distinguish between Voice and data packets It fragments packets that are larger than the configured fragment size frame-relay fragment Set the size to be larger than the voice packet size
FRF.11 Annex C	Frame relay distinguishes between voice and data packets Only data packets are fragmented
PPP multilink	ppp multilink ppp multilink interleave ppp multilink fragment-delay

Frame relay traffic shaping

Frame relay

 

1. Setting the CIR or MinCIR to the access rate should be avoided because those values does not consider the overhead bytes of flag and CRC and hence would lead to oversubscription of the links and cause packet drops. Shaping at access rates is not recommended. At a max, you should shape at 95% of the access rate.
2. Frame relay traffic shaping is enabled on the main interface and it applies to all DLCI’s on that interface. You can apply different traffic shaping values using the map class statements. If you do not apply any shaping to one DLCI, then it would match the default map class with a CIR of 56000

 

CIR	Average rate Higher than guaranteed rate Lower than access rate Allows for bursting When using VoIP/VoFR, must be same as the guaranteed rate. Same as MinCIR Default: 56000 bps
MinCIR	Guaranteed rate Default: half of CIR
Bc	Amount of data send per Tc For Data = CIR/8 so Tc = 125 ms For Voice = CIR/100 so Tc = 10ms
Be	Amount of excess data to be send during the first Tc interval once credit is build up Used only if CIR is less than Access rate For voice, set Be to 0 Default = 0
BECN	When it receives a BECN, the traffic rate is dropped by 25% For each BECN (one per time interval), the rate is dropped by 25%, until it reaches the MinCIR value It must allow 16 intervals of no BECN, before it traffic rate is increased The rate is increased by the byte limit/16. Hence it takes much longer to get back to CIR level

Erlang calculation

	Use	Assumption	Input	Output
Erlang B	To find the number of trunks	Blocked calls are cleared/re-routed and never to return on the same trunks	Traffic volume in Erlang Percentage of Blocked calls	Number of lines
Erlang Extended B	To find the number of trunks	Blocked calls are retried and will come back to the same trunks	Traffic volume in Erlang Percentage of Blocked calls Recall factor	Number of lines
Erlang C	To find the number of agents required in call center		Calls per hour Duration of call Average delay of call	Number of agents

1 Erlang = 1 call for 1 hour or 3600 seconds

1 CSS = 1 call for 100 seconds

1 Erlang = 36 CSS

SIP Basics

SIP Server functions

1. SIP Proxy
- does call routing, name lookup, address translation

- forwards SIP messages between endpoints

- if Record Route is enabled, then all messages will be send back and forth via the server. End points do not communicate with each other directly.
2. SIP Registrar

- clients register their address
3. SIP Redirect

- does call routing, name lookup, address translation

SIP loop prevention

When the SIP server gets an INVITE, it checks whether its own address is contained in the VIA field. If yes, the message will not be processed.