Each node has a dedicated connection to the network --disconnecting a single node does not bring down the rest of the nodes on the network
Network and cable administration are centralized
Disadvantages of star topologies:
More expensive to install -- require more cable and the additional cost of a hub
Maximum length of each spoke of the hub is limited to the allowed maximum length of the medium (for example, on a 10-Base-T network using UTP cable, the maximum distance from the hub to a host is 100m)
Breakdown of the hub causes breakdown of the entire system
The devices which are linked together by a common medium are collectively known as a subnet .
Data are sent in blocks called frames . A frame, in addition to data, contains some header information such as source and destination addresses, control data bits, error-checking bits, etc. Frame size, the number of bits, depends on the underlying protocol. Actual frame format depends on the protocol.
Both ALOHA and slotted ALOHA exhibit poor utilization and fail to take advantage of the fact that propagation delay is usually very small compared to frame transmission time for both packet radio and LANs.
Token passing protocols are much more complex than contention-based protocols. For example, the protocol must deal with:
What happens when a token gets lost
What happens if two or more tokens show up on the subnet
IEEE 802.5 Frame Format SD AC FC DA SA Data unit FCS ED FS 1 1 1 6 6 > 0 4 1 1 SD: starting delimiter SA: source address AC: access control FCS: frame check sequence FC: frame control ED: ending delimiter DA: destination address FS: frame status Bytes SD AC ED 1 1 1 A. General frame format B. Token frame format