Consider the case of a hub and spoke topology with NBMA network (figure-1).
FIGURE-1: Bandwidth Provisioning
The hub is provisioned with 768kbps whereas spokes are configured with 384kbps of bandwidth. The total accumulated bandwidth required by spokes is 3 times 384 equals to 1152kbps (1.1Mbps). In case, all spokes start to transmit at the 384kbps, there will be considerable packet loss.
The best tool to compensate for this problem is to add more bandwidth. But some other efficiency tools such as TCP header compression can compensate for this issue. Queuing can affect bandwidth for certain types of applications such as Voice over IP by prioritizing.
Delay is the time difference between the sender and receiver. For example: you sent a packet at 1:00 pm in and packet received at the destination around 1:02pm. The delay is around 2 minutes. Delay can be fixed or variable.
Fixed delay includes:
- Serialization delay: how long it takes to put the bit on wire.
- Propagation delay: is the time taken by a single bit to get from one end of the link to the other.
Variable delays can be:
- Queuing delay: packet experiences queuing dealy when they must wait in the queue, till other packets are processed to be sent.
- Forwarding/Processing delay: it refers to the processing time between frame is fully processed and when it is put in the output queue.
- Shaping delay: applies to traffic shaping. This is an additional delay when a packet must wait in the queue to avoid policing at the other end.
- Compression delay: occurs if the packet must be compressed before sending and de-compressed at the receiver end.
Tools that affect the Delay problem: bandwidth, queuing (or scheduling), link fragmentation and interleaving, and compression.
It is a variation in arrival rate. Consecutive packets that experience different amounts of delays. Data application typically expect some jitter and performance don’t degrade. However, real-time traffic such as VoIP is sensitive to both delay and jitter.
Tools that can affect Jitter include: bandwidth, queuing, link fragmentation and interleaving, compression, and traffic shaping.
Router drop, lose, or discard packets for many reasons. For example: CRC failures. But major traffic is dropped due to full buffer. Tools that affect packet loss include: random early detection
- Classification and Marking
Traffic must be classified and marked for a certain class of service. For example: web traffic is different than RTP traffic, this is classification. Web traffic is assigned IP precedence 2 and RTP traffic is assigned IP Precedence 5.
- Queuing or Scheduling
Allow traffic to schedule according to classification and marking. For example: RTP is always service first as it was assigned Precedence 5 and bandwidth will always be reserved for this traffic. Email traffic can be processed normally.
Traffic Policing and Shaping
Traffic shaping is an egress function; means it is done for outbound traffic. Usually done to avoid packet loss and delay problems. Policing is (usually) an ingress function and it is used to solve problems related to network capacity and traffic engineering.
It helps to elevate the queue being filled rapidly. It uses techniques such Random Early Detection (RED) and Explicit Congestion Notification (ECN). When RED is used, problems such as TCP global synchronization can be avoided. RED solves this problem by randomly dropping some traffic (based on certain criteria such as IP Precedence). The analogy is simple: drop some random traffic, before congestion occurs, instead of dropping all traffic.
ECN achieves the same goal without dropping traffic. When ECN is enabled, the router signals the host to slow down its traffic rate. This is done by setting ECN bits in IP header.
Link Efficiency Mechanisms
Tools include fragmentation and interleaving and payload and header compression techniques. They directly attack the serialization delay. Fragmentation divides larger packets into smaller chunks. Interleaving allows inserting these smaller packets into the queue to be processed first. For example: VoIP packets are smaller in size. If a file transfer of 1500 bytes packet is already taking place, the 20-byte VoIP packet must wait till FTP packet is completely processed. Fragmentation will allow dividing the FTP packet into smaller chunks and interleaving will then insert the VoIP packet in between.
Compression allows more bits to be sent in the same bandwidth, For example: RTP header compression will compression IP/RTP/UDP header from 40-bytes to 2-bytes, thus saving bandwidth.