2014 2013 Capital Primario (Pilar 1)

Think back to the OSI model for a second and recall that Layer 2, called the Data Link layer, has 2 sublayers called the Logical Link Layer, or LLC, and the Media Access Control layer, or MAC layer. While the LLC pretty much just acts as a translator from the Network layer to the MAC sublayer, the MAC sublayer does a lot of work – it understands everything about Ethernet, Token Ring, FDDI, etc, and know how to format the data into something that the Physical layer (Layer 1) can understand. That is where we are going to focus our attention for a few minutes, and this area is collectively referred to as Media Access technology.

The one thing that all media access technologies must share is the communication channel – only one computer can be transmitting at a time regardless of the protocol you use. The method to determine which computer should be talking at any given time is the while point of media access control, and there are several methods to choose from. One thing to keep in mind is that when a computer sends a frame on a physical cable, ALL computers connected to that cable receive it – after all we’re talking about an electronic signal and there is no way to control which direction that signal travels in or how long it will travel. On ring networks, as a computer receives signals, it may or may not forward that signal on to the next computer. With bus networks , usually the two ends of the single cable are capped with a device that absorb signals so they are not reflected back onto the wire. With star networks, a switch will usually absorb the signal. But the media access technologies we will discuss next are logical – so when we discuss passing a token around, remember that we are referring to a logical construct, not a physical signal.

Token Passing

Sometimes when kids get mad at each other, and start yelling and interrupting each other, the parent will sit them down and say something like ‘Here is a spoon – you may not talk until you are holding it.’ While it drives most kids crazy, it is a very effective mechanism to ensure that everyone gets a chance to speak. Well, computers are no different – they all like to talk at the same time. However, if we pass around a virtual spoon – let’s call it a token – only the computer who possesses the token can speak, and everyone gets their chance. This is called token passing and is used by Token Ring and FDDI networks. The token is a 24-bit control frame, and here is how it works:

• Computer A wants to say something to Computer C on the network and possesses the token • Computer A puts its message inside of the token and sends it out

• Computer B gets the token but ignores it because it is not address to it

• Computer C gets the token, sees that it is addressed to itself, extracts the message, flips a ‘I got it’ bit on the control frame, and sends it out again

• Computer A sees the token and that the ‘I got it’ bit has been flipped, it knows the message was received.

• Computer A sends an empty token, ready for some other computer to grab it

CSMA

Ethernet approaches media access in a completely different manner using something called carrier sense multiple access, or CSMA. CSMA has two flavors that deal with collision detection and collision

avoidance.

If there is no token to pass around, what would happen if two computers tried to transmit frames at the exact same time? They would both be adding electrical signals to the same wire, resulting in a garbled mess of electrons that make no sense. That situation is called a collision – two or more electronic signals collide and both are corrupted.

When a computer transmits a frame, that transmission is called a carrier, because it carries information. Computers who have something to say will wait until there is a lull of traffic on the network and then transmit its data. However, this does not prevent collisions since two computers can both see there is no traffic and transmit at the exact same time, resulting in a collision. If a network has implemented a protocol called carrier sense multiple access with collision detection, or CSMA/CD, both computers will recognize a collision has occurred, send out a jam signal, and then back off and wait for a random period of time before trying the entire process again. This is why the CSMA/CD has the ‘detection’ part in it. Ethernet uses CSMA/CD.

A slightly different approach is closer to a token passing method in which a computer listens for a lull, and instead of trying to transmit immediately, sends out a broadcast letting all other computers in the same network know that it would like to ‘speak’. All other computers will back off for a random period of time before transmitting data to ensure collision do not happen. This is called collision avoidance, and the entire protocol is called carrier sense multiple access with collision avoidance, or CSMA/CA. This method is used with Wi-Fi networks.

Carrier sensing methods such as CSMA/CA and CSMA/CD result in a faster network over token passing methods, until you have a very large number of nodes – the resulting collisions have an increasingly negative effect as the number of nodes increase. To offset this, we can create smaller

collision domains by segmented the network with switches, which do not forward frames outside of their network unless they are supposed to go to another network.

So, a collision domain stops at any layer 2 device, such as hubs, repeaters, switches, bridges or wireless access points. A broadcast domain stops at any layer 3 device such as routers. One side benefit to creating collision or broadcast domains is that it reduces the amount of data that any one computer has access to – restricting the amount of data an attacker can sniff from a single computer.

Polling

A third type of media access control is called polling, and is used primarily in mainframe environments. With this method, a primary station will periodically poll secondary devices on a network, either to monitor or to give permission to communicate on then network.

Ethernet

Ethernet is a network communication standard that is usually used within a star or bus topology. It was invented in the 1970s and is officially defined through the IEEE 802.3 standard. It is a contention-based technology, which means that all resources use the same shared medium. It exclusively uses the CSMA/ CD access method, use collision and broadcast domains, supports full-duplex communication and is compatible with coaxial, twisted pair or fiber-optic cabling. We have previously discussed some Ethernet cabling types, but let’s revisit them in a little more depth.

By far the most common cabling for Ethernet is UTP. Ethernet cables have 4 pairs of UTP, so it contains 8 different wires. Within each pair, one wire is used to transmit data and the other is used to receive data. Ethernet UTP cabling used RJ-45 connectors (regular telephone connectors are RJ-11).

Originally, everyone used 10Base-T, which supports speeds up to 10 Mbps over 100 meters, and is called Category 3.

As bandwidth demands increased, Category 5 Fast Ethernet (100Base-TX) was developed that ran over Cat5 cable and supported speeds up to 100 Mbps. Both 10 Mbps and 100 Mbps speeds can coexist in the same network if using 10/100 hubs and switches. Gigabit Ethernet (1000Base-T) allowed for speed up to 1000 Mbps on Cat5 wire by allowing simultaneous transmission in both directions on all pairs. Cat5E cable is normally used for this application.

Next, Category 6 10GBase-T was invented that supported 10 Gbps over Cat6 cabling, but had to do away with the venerable CSMA/CD technology. It has not seen the same wide-spread adoption as Gigabit Ethernet enjoyed due to the poorer cost-to-performance ratio.

Token Ring

IEEE 802.5 defines the Token Ring media access standard, which used to be popular but has been

eclipsed by Ethernet. It uses a Token Passing methodology and supports up to 16Mbps. Token Ring uses an active monitor mechanism to remove frames that are continuously circulating when the targeted computer is no longer online. When a computer encounters a problem, it can send a beacon frame, and other computers will attempt to work around the problem – this is called beaconing.

FDDI

IEEE 802.4 describes the fiber distributed data interface, or FDDI, was developed by ANSI and is a token passing media access technology intended for high-speed networks. It supports up to 100 Mbps and can most commonly be found on fiber network backbones. To provide for redundancy, it has a primary ring traveling clockwise with a secondary ring operating in a counterclockwise direction. The secondary ring is only used if the primary ring goes down, which is triggered by sensors. All nodes are connected to both rings so that if a break in one ring occurs, the two rings can be joined in real-time. FDDI can be run up to 100 kilometers and allows multiple tokens to be present at the same time, increasing throughput. FDDI-2 provided Quality of Service (QoS) capabilities. FDDI was primarily meant for MANs (metro area networks). A variant of FDDI, copper distributed data interface (CDDI) provides the same capabilities but over UTP, and is meant for LANs.

Devices that can connect to FDDI are categorized as:

• Single-attachment station (SAS) – attaches to only one ring through a concentrator • Dual-attachment station (DAS) – attaches to both rings through a concentrator • Single-attachment concentrator (SAC) – attaches an SAS device to the primary ring • Dual-attachment concentrator (DAC) – attaches DAS, SAS and SAC to both rings

Transmission Methods

A packet can be intended for three types of recipients within a specific network: • Unicast – a single computer

• Multicast – a group of computers • Broadcast – all computers

Unicast is the transmission method we are usually familiar with. Multicast is similar to a radio station broadcast with a single source and multiple recipients. So how does a computer on the other side of the country take part in a multicast with 20 routers in between them? Well, the receiving computer tells its router that it wishes to be multicast to, and that router tells the next router, and so forth until the request reaches the router connect directly to the machine with the content to multicast. All 20 routers now know to forward packets from the sender to the recipient. This is all done using the Internet Group Management Protocol (IGMP), which has gone through 3 versions – version 2 added the ability to be removed from a group and version 3 allows recipients to specify the multicast sources.

One final note: all of the above is true for IPv4 – IPv6 completely redid multicasting, but that is a whole other story not in scope for this discussion.