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Unformatted text preview: 6.4 Ethernet 399 Point-to-point Bus Star Mesh Figure 6.14 Ethernet topologies. GFP Client-Specific Aspects A client-specific function is the mapping of client signals to a GFP frame using a frame mapped GFP (GFP-F) or a transparent mapped GFP (GFP-T). As we mentioned earlier, a GFP-F frame is an encapsulation of a client packet. Transparent mapped GFP is a little more complicated. First, note that it is applicable to fixed-length packets that are encoded by (8,10) line codes. Note that the line code of the client signal is unnecessary for GFP transport because GFP frames have their own frame synchronization. Thus, the GFP-T mapping will first extract the data bytes and control characters in the client signals. Then blocks are formed from 8 data bytes or control characters, and then superblocks are formed from 8 blocks. The superblock is transported in a GFP-F frame. Latency is low because a superblock does not have to wait for its entire client frame before being forwarded. 6.4 Ethernet Ethernet was created in the 1970s to be a packet-switched data link that connects computers and computer equipment over a single coaxial cable, that is, a bus. It is easy to understand, implement, manage, and maintain, and has led to low network costs. Ethernet has since evolved to include a variety of topologies including point- to-point, bus, star, and mesh as shown in Figure 6.14; and adapted to a variety of physical communication media, including coaxial cable, twisted pair copper cable, wireless media, and optical fiber. It has a wide range of rates. Typical rates today are 10 Mb/s, 100 Mb/s (Fast Ethernet), 1 Gb/s (Gigabit Ethernet or GbE), and 10 Gb/s (10-Gigabit Ethernet or 10 GbE). At the time of this writing 40 Gb/s and 100 Gb/s Ethernet are being developed. It was one of the first local-area network (LAN) technologies and has thrived to become the predominant LAN as well as a predominant link layer technology. 400 Client Layers of the Optical Layer Media Access Control In the original Ethernet, computers were attached to the network coaxial cable with a network interface card (NIC), and each NIC has a unique 6-byte Ethernet address that is assigned by the NIC manufacturer. A node can transmit a packet on the cable, and the transmission signal will be received by all the nodes. The coaxial cable was effectively a broadcast communication link. A problem with this configuration is that nodes transmitting at the same time can interfere with one another’s transmissions, causing a transmission collision . Since such collisions mean transmissions are not received properly, they are a waste of link bandwidth. Ethernet has a media access control (MAC) protocol to arbitrate transmissions between nodes. When a node has a packet to transmit, it listens to the link. When it detects that the link is idle (i.e., there are no transmissions), it transmits its packet and at the same time listens to the link. If it detects a collision, then it stops transmitting, avoiding further waste of bandwidth. Then it attempts to retransmit the packetavoiding further waste of bandwidth....
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- Spring '09