2.2 ETHERNET

This array of different connectors has, to some extent, grown out of the need to get more and more ports securely connected to the minimum real estate on the equipment. The choice of connector is often not made by the network operator since they are constrained by the choices made by the hardware vendors. However, you may find that there is a choice between fibre or copper. This is a choice which is often made on the basis of convenience of connecting different vendors’ devices.

2.1.3CHANNELIZED INTERFACES

A neat solution to the demand for increasing port density is the use of channelized interfaces. These interfaces use the fact that traditional telephone companies’ (telcos) transmission networks often carry lower bandwidth circuits in larger capacity aggregates. The transmission networks allow the adding and dropping of the lower capacity circuits onto the aggregates at arbitrary points on the network. It is, therefore, possible to take tributary circuits from a wide geographical area and aggregate them into a single aggregate. Traditionally, those tributary circuits would be broken back out before being presented to the ISP. However, with the availability of channelized ports for routers, telcos can now deliver the aggregate direct to the ISP and the ISP can support many circuits on a single physical port (e.g. 61 E1s on a single STM-1, 12 DS-3s on a single OC-12). Now, with a single line card on a router capable of carrying several channelized STM-1s, for example, it is possible for thousands of customers to be supported on a single device or even a single linecard. This raises serious questions about the reliability of a network device, when thousands of customers can lose service if it fails (see Chapter 1 for a more thorough discussion of this issue).

2.2ETHERNET

Ethernet, in all its forms, is considered to be a comparatively cheap transport media. Traditionally considered a Local Area Network media, it is now being extended into the Metro Area and even the Wide Area network through use of technologies such as WDM transport over fibre using Long Haul optics.

Ethernet has developed from a bus-based architecture into a collapsed bus based on hubs and subsequently into a star-based architecture based on switches. The bus architectures were constrained to distances of 185 metres for 10Base2 and 500 metres for 10Base5. These are (almost) never used in service provider networks today. With the introduction of hubs and switches, the distance between the node and the hub on each link was limited to 100 metres. Only with the advent of Fast Ethernet and Gigabit Ethernet over fibre did the distance over which Ethernet could be used increase to the current situation, where it is possible to use long-haul optics to drive point-to-point dark fibre up to around 70 km without amplification. Cheaper, short-haul optics can more commonly transmit up to 250 to 500 metres over multimode fibre, depending upon the properties of the fibre.

Fundamentally, Ethernet is a broadcast medium. This is exemplified by the original bus architecture of Ethernet. It uses Carrier Sense Multiple Access with Collision Detection (CSMA-CD) as a transmission brokerage mechanism. Any node wishing to transmit must first listen to the media to see if any other device is currently transmitting. If no transmission is heard, then the node goes ahead and transmits its traffic. If transmission is heard, the node waits a randomized time before listening again. If two nodes start transmitting simultaneously, then both will detect the collision and transmit a ‘scrambling’ message to ensure that every receiver is aware of the collision. They then wait for a randomized time before trying to transmit again.

On bus-based systems (including the collapsed bus in a hub), there is a genuine collision domain, which incorporates all attached devices. Since there is no way that two devices can simultaneously transmit and we can assume that no-one is interested in listening to their own transmissions, these bus-based networks are inherently half-duplex in nature. Switches reduce the collision domain to the single link between the switch and each attached node. In addition, each attached device is connected with separate transmit and receive paths, meaning that incoming packets can never collide with outgoing packets. Since the device at each end of the link is the only device transmitting on a particular wire, they can transmit without first checking and be certain that there will never be a collision.

In addition to the ability to transmit full duplex across a single link, switches provide the means to transmit many simultaneous parallel streams of data. This means that it is possible to obtain true throughput of several gigabits per second, often only constrained by the capacity of the link between line cards and the backplane of the switch.

The ability to transmit full duplex on each link provides the basis for full duplex transmission end to end. As the number of hosts in a single collision domain grows, the number of collisions will inevitably increase. As the number of collisions increases, the throughput of valid data decreases dramatically. Switches, therefore, have a vital part to play in the scalability of Ethernet-based networks.

All the benefits of full duplex communication associated with switches are also available when Ethernet is used in a point-to-point mode. The same distance constraints also apply, 100 metres with a crossover Unshielded Twisted Pair cable and between 500 metres and around 70 kilometres with fibre, depending on the optics driving the line.

2.2.1 ADDRESS RESOLUTION PROTOCOL (ARP)

In order to carry IP over Ethernet, it is necessary to map IP addresses to Ethernet MAC addresses. This is achieved by the use of ARP. ARP is a relatively easy protocol, which is based on two simple messages. The first message is an ARP Request, also known colloquially as ‘who has’. This message is sent by hosts wishing to map a known IP address to an unknown MAC address. A ‘who has’ message containing the known IP address is broadcast onto the segment. The second message type is an ARP Reply, also known colloquially as ‘I have’. This is unicast directly to the requester and contains the IP address, along with the MAC address of the responder. In addition to replying with its own details,