Handbook for Sound Engineers

Digital Audio Interfacing and Networking 1489

tronics Engineers (IEEE) and are assigned a block of
MAC addresses for their use. Each manufacturer is
responsible to make sure that each and every device it
ships has a unique MAC address within that range.
When it has used up 90% of its addresses it can apply
for an additional block of addresses.
A MAC address is 48 bits, or 6 bytes long, which
allows for 281,474,976,710,656 unique addresses.
While Ethernet is extremely popular, we have not begun
to run out of possible MAC addresses.
An Ethernet packet starts with the MAC address of
the destination. That is followed by the MAC address of
the station sending the packet. Next come 2 bytes called
the EtherType number or protocol identifier, which
identify the protocol used for the payload. Again the
IEEE assigns these numbers. The protocol identifier
assigned for CobraNet®, for example, is 8819 in hexa-
decimal notation.
The data payload can range from a minimum size of
46 bytes to a maximum size of 1500 bytes. The protocol
identifier specifies the content of the payload and how it
is to be interpreted. Data of less than 46 bytes must be
extended or padded to 64 bytes, while data of more than
1500 bytes must be broken into multiple packets for
transmission.
The frame check sequence (FCS) is a 4 byte long
cyclic redundancy check (CRC) calculated by the trans-
mitting station based on the contents of the rest of the
Ethernet packet (destination address, source address,
protocol, and data fields). The receiving station also
calculates the FCS and compares it with the received
FCS. If they match, the data received is assumed to
have been received without corruption. There is a
99.9% probability that even a 1 bit error will be
detected.
As you can see, the smallest possible Ethernet packet
is 64 bytes long, and the longest is 1518 bytes long.

39.11.3 Network Diameter

The maximum allowable network diameter, Fig. 39-24,
that will permit Ethernet’s collision detection scheme to
work is dependent on:

• The minimum packet size (64 bytes),


• The data rate (these last two together determine the
  time duration of the minimum size packet), and
  
  
  • The quality of the cable (which determines the speed
    of propagation down the cable).
  
  
  
  

39.11.4 Ethernet Varieties

In 1980 the IEEE standardized Ethernet as IEEE 802.3.

This initial standard was based on the use of 10 mm

50 : coaxial cable. Many variations quickly appeared.

• 10Base5—This was the original Ethernet, also called
  thicknet or thick Ethernet because of the large diam-
  eter coaxial cable used. It ran at 10 MBit/s, baseband,
  with a maximum segment size of 500 m (hence
  10Base5).


• 10Base2—Designed as a less expensive Ethernet, it
  was called thinnet or thin Ethernet due to the thinner
  RG-58 50: coaxial cable used. It ran at 10 MBit/s,
  baseband, with a maximum segment length of 200 m.


• 1Base2—A slower variant of thinnet. It ran at
  1 MBit/s, baseband, with a maximum segment length
  of 200 m.


• 10Broad36—Very rare, this ran over a RF cable plant
  similar to cable TV distribution systems, and was
  built with cable TV distribution components.

All of these variants suffer from a common problem.

Since they use a shared media that had to physically

connect to every station in the network, a problem at

any point along the backbone could disable the entire

network. Clearly a different approach was needed to

protect the shared media from disruption.

In 1990 a new technology called 10Base-T was

introduced to solve these problems. Instead of the

vulnerable shared media being strung all over the entire

facility, it was concentrated into a box called a repeater

hub, Fig. 39-25. The media was still shared, but

protected. It had an allowable network diameter of

2000 m, and used the same packet structure. Each

station was connected to the hub with twisted pair cable,

with two pairs used. One pair carried the signal from the

station to the hub, while the other pair carried the signal

from the hub to the station. Category 3 (Cat3)

unshielded twisted pair (UTP) was used with trans-

former isolation at both ends of each pair. The

maximum length of a single cable run was restricted to

Figure 39-23. Ethernet packet format.

Destination

address

(6 bytes)

Source

Address

(6 bytes)

Protocol

(2 bytes)

Payload

(46–1500

bytes)

FCS

(4 bytes)

Figure 39-24. Ethernet maximum allowable network

diameter.

TT

node node node node