Relationship between Ethernet Frame/Packet Size and Cable Length

In one of the previous blog entry, shared my understanding about the minimum Ethernet Frame Size. In this entry, sharing my understanding about the relationship between the Minimum Ethernet Frame/Packet Size and the Maximum Ethernet Cable Length.

Now we are going to find the maximum cable length for the old 10Mbps (10Base5) Ethernet network:

Minimum Ethernet Frame size = 64bytes = 512bits

Propagation Delay/ Signal speed on cable = 200m/us

Time to transmit one bit in 10Mbps = (1000ms * 1000us * 1000ns) / 10000000 bits = 100ns

Time to transmit the minimum Frame = (100ns * 512bits) = 51.2us

Length of the cable = speed * min frame TX time = 200m * 51us = 10200m = 10Km.

The maximum Ethernet cable length is 10Km. Is it right?? NO!!. Why?? The 51.2us is the Round Trip Delay(RTD). So the 10Km is Round Trip Distance and the actual length is 10Km/2 = 5Km.

The new maximum cable length is 5KM. Is it right?? NO!!. Why?? To answer this Qus, we need to see more details: The maximum length of the one segment of old 10Base5 cable is only 500m. To make an 1Km cable, 2 segments can be connected (500 + 500) using a repeater. The repeater device in the middle will also introduce some delay. Old repeater will add 3us delay in one direction and bi-directional delay is 6us.

To make a 5Km cable, we need 10 cable segments and 9 repeaters. The repeaters alone add nearly (9*6)= 54us delay in the path, which is more then the time to transmit the minimum frame. For us, the total round trip delay should be with-in 51us. The available options are, reduce the total length of the cable and reduce the number of repeaters in the middle. In the total delay, delay introduced by repeaters should not be greater then 50%. So the new formula is (25us + 25us) = 50us.

No of repeaters that introduce 25us delay = 25/6 = 4 Repeaters

Cable length that introduce 25us RTD = (25 * 200m) / 2 = 2500m = 2.5Km

The maximum cable length for the 10Mbps Ethernet network is 2.5Km, which can be made using 5 cable segments and 4 repeaters.

In the recent years, Ethernet device speed increased by 10 and 100 times. Due to this, the sender transmits the frame faster. As per CSMA/CA, the collision can be detected only before the end of the packet transmission. So if we maintain the same frames size and cable length, then a station will transmit a frame too fast (very much less then 50us), and miss the collision occurs at the other end of the cable. To avoid this, we have to,

  • maintain the maximum cable length and increase the minimum frame size OR
  • maintain the same slot time and decrease the maximum cable length OR
  • increase the slot time and decrease the maximum cable length.

For example, in 100Mbps network, the cable length is reduced to 500m and in 1Gbps network, minimum Ethernet frame/packet size is increased to 256 or 512 bytes.

Let see the calculation behind the length reduction in 100Mbps network:

Time to transmit one bit in 100Mbps = (1000ms * 1000us * 1000ns) / 100000000 bits = 10ns

Time to transmit the minimum Frame = (10ns * 512bits) = 5.12us

Length of the cable = speed * min frame TX time = 200m * 5us = 1000m = 1Km (Round Trip Length).

The one way length is 500m. But the practical limit is around  250m.

Effect of Duplex Mode in Cable Length:

The old Ethernet network was using the coaxial cable and all systems were connected in that cable directly. In this transmission and reception is on the same medium, but not at the same time. Because it is not an collision free environment and work in Half-Duplex mode, CSMA/CD will help to detect the collisions.

The new Ethernet device with IEEE 802.3x standard are having capability to transmit and receive in a independent point to point link (PC to PC or PC to Hub/Switch) on CAT UTP cables. This made the link as a collision free and need of CSMA/CD protocol is removed. This mode of transmission is called Full-Duplex. In this, throughput of the link is doubled, time wasted due to the collision is removed and cable length can also be increased.

Let see how the cable length can be increased in Full-Duplex mode:

In Half-Duplex mode, after transmission of every frame, device should be change to receive mode. So there was a hard limit in the timing requirements, to ensure collision propagated to all systems within the time required to transmit 512 bits. In Full-Duplex mode this limitation is removed, because Tx and Rx independent and system can sustain for some more delay. So the cable length can be extended upto 1 or 1.5Km.

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