Knowledge Base - Estimating Internet data transfer & call capacity

Sometimes you need to know how much data you can transfer across a network - this might be to estimate the time to perform a file transfer or to determine how many simultaneous phone calls you can potentially support on a given connection.

We will start with the results - and then explain how we got the information below. Here's a summary of data transfer potential in Gigabytes (1,000,000,000 bytes) over various time periods as well as the number of simultaneous calls possible with various typical connection speeds:

Explanation of the data above...

DSL vs. Fiber

DSL (Digital Subscriber Line) is a technology delivering network data over telephone wire. The speeds noted are nominal - they can vary above or below these standardized speeds based on the distance from the head end equipment. Fiber experiences no such distance loss, and speeds are typically delivered as quoted. Additionally, with Fiber the upstream speed is normally the same as the downstream speed. Latency (the time it takes information to get from one end to the other) is also typically lower with Fiber. Contact our sales team for your best possible solution.

The bit (b) vs. the Byte (B), and GB vs. GiB

There is a lot of background that goes into the above information. For more information on the bits & bytes, please see this KB. For more information on Megabytes vs. Mebibytes, please see this KB. 

Ethernet (underneath TCP/IP)

Networks are made of layers of information. Because we are looking at data transfer it makes sense to start at a layer close to the hardware. 802.3 frames have:

• Preamble 7 Bytes
• Start 1 Bytes
• Mac Address Destination 6 Bytes
• Mac Address Source 6 Bytes
• 802.1Q tag 4 Bytes (optional)
• Type or length 2 Bytes
• Payload 46 to 1500 Bytes (42 Bytes minimum if 802.1Q tagged)
• CRC (error check code) 4 Bytes
• Interpacket gap 12 Bytes

This means a single data packet on the network typically has 38 or 42 Bytes of overhead and a 1500 Byte payload. That is what is known as an Ethernet packet.

TCP Packets

Underneath an Ethernet packet (contained inside the 1500 Byte payload noted above) is a TCP Packet. TCP adds from 20 to 60 Bytes of header information - the remaining portion of the packet can be used for data. For more information see https://en.wikipedia.org/wiki/Transmission_Control_Protocol. Each packet sent results in a 20 to 60 B packet sent back as an acknowledgement. If a packet is missed, it will be resent, and re-acknowledged. This is something to keep in mind - packet loss results in loss of efficiency and retransmission.

For our efficiency calculations, we will work with the simplest / best case:

• NO VLAN tag (38 Byte Ethernet overhead)
• 1500 Bytes IP packet
• No TCP options (20 Bytes TCP header) note that these packets are part of the 1500 Bytes total.
• Speeds are measured in metric (1Mb = 1,000,000 bits)
• If the entire packet is 1538 Bytes, the net data would be 1480 Bytes though typically the maximum size is 1460 Bytes
• 1480/1538 = 95%
• 1,000,000 / 8 / 1538 = 81 packets * 1460B = 118,260 Bytes transmitted.
• 118,260 Bytes/s * 3600 seconds = 425 MB / hour

The trouble with networks is that there are a lot of things that can affect efficiency such as the addition of TCP options (which take space in the packets) or retransmission due to errors on the line, etc. The use of L2TP or VPN's can increase overhead significantly. In all these calculations, we are also ignoring the added overhead of any protocol for encryption or communication – like the overheads of “logging in”, etc.

For comparison, consider a less ideal case (still not the worst! Just a smaller packet size):

• NO VLAN tag (38 Bytes Ethernet overhead)
• 500 Bytes IP packet
• If the entire packet is 538 Bytes, the net data would be 480 Bytes
• 480/538 = 89.22%
• 1,000,000 / 8 / 538 = 232 packets * 480 Bytes = 111,360 Bytes transmitted.
• 111,360 Bytes / second * 3600 seconds = 400 MB / hour
• That’s a 6% reduction in speed over the ideal case above.
• TCP options settings, connection bonding, encryption and protocol overhead can all reduce this performance further.

For the table above, we used 0.4GB / Mb / hour.

encryption

HTTPS

HTTP

ATA

BES

LAN

MAC

NIC

RFO

TCP

VPN

IP

I.T