LANs Media Access Control Step 1 in Sharing Resources

Lots of Strategies Ethernet Token Ring Token Bus ATM DSL FDDI Sonet …….

General Interest Qualitative understanding of the function Comparison/Contrast of a few strategies Design tradeoffs Ethernet as a common example –Numerous faces of the same technology –Performance considerations analytical practical –Lots of access/info available –Widely used

Physical Layer.. Not network! Does not perform routing Requires additional hardware and software to interconnect networks Not functionally different from other direct connections like a serial cable –EXCEPT - must share the LAN Application Presentation Session Transport Network Data Link Physical

Physical Characteristics Limited distance –typically a building or room in a building Data rates used to be in the 10sMbps but now can be much faster (typically 100sMpbs now) Different LANs support different types of traffic better Performance limits the number of machines – Low error rates!

CSMA-CD Derived from Aloha net Similar performance issues No central control Avoids overhead of allocation Doesn’t perform as well in highly loaded scenarios In practice typical demands do not saturate the network

CSMA-CD basic algorithm Look to see if someone is ON If so refrain and if not try If you see your own transmission uncorrupted as you read the line, assume no one else sent If not, collision from someone else acting as you have Wait a while and try again See Figure 3.4

Figure 3.1 A starts to send Data hasn’t yet arrived at B B sends B sees collision right away A sees collision much later a -> (frames on link) is small see collision quickly recover and repeat quickly 10 collisions at 5microsecs ea one 10-millisecond frame calculate the efficiency do a real value of a for a LAN

What about collisions? If you collide, assume at least one other trying “Flip a coin” and try again to reduce collision chances If again, could just have bad luck OR more than one other trying Flip a 4-sided coin and then try again After 16 collisions, drop the packet –Truncated Binary Exponential Backoff

Physical Considerations 10 Base 2200mthin coaxbus 10 Base 5500mthick coaxbus 10 Base T(X)100mtwisted pairsstar 10 Base F1000mFiberpt-pt hubs and busses work on the same principles but have a different physical appearance physical vs logical 10 mpbs, 100 mpbs, 1000 mbps

Frame Format Figure 3.3 –details more apparent with protocol info Part b –slot time time to be sure all is ok –max frame size –min frame size

Token Ring

Token premise One token (symbol sequence) on ring First one to grab it gets to hold it Must hold it in order to transmit When done, sender retransmits the token Date flows in a circle One bit delay at each node If you put it on, take it off Assume reasonable behavior

Grabbing the token …… trash 0 insert a 0 instead See Figure 6.12

Token Ring Issues Consider large and small “a” Recall LAN “a”s are small Head of message returns before tail sent. Sender removes and reinserts Automatic ACK (protocols) Management more complex –what if token is lost –how do you negotiate if one isn’t in charge (it is) –what if the one on charge dies –what if the cable is cut

Comparison ETHERNET TOKEN RING No bound on time for access YUK on periodic Degrades as traffic ^ Immediate access low loads Consider offered load no priority Each sends once so you can do a “worst-case” answer to how long to wait Can support periodic Limit to degradation Wait for token always Consider offered load priority possible (T Bus)

Desirables Low access delay Fast transmission Fairness Prioritization Robustness Large number of nodes Any distance Cheap

Performance Analysis (see notes) Maximum case